Monoclonal antibody levels and protection from COVID-19

Multiple monoclonal antibodies have been shown to be effective for both prophylaxis and therapy for SARS-CoV-2 infection. Here we aggregate data from randomized controlled trials assessing the use of monoclonal antibodies in preventing symptomatic SARS-CoV-2 infection. We use data on changes in the in vivo concentration of monoclonal antibodies, and the associated protection from COVID-19, over time to model the dose-response relationship of monoclonal antibodies for prophylaxis. We estimate that 50% protection from COVID-19 is achieved with a monoclonal antibody concentration of 54-fold of the in vitro IC50 (95% CI: 16 - 183). This relationship provides a quantitative tool allowing prediction of the prophylactic efficacy and duration of protection for new monoclonal antibodies administered at different doses and against different SARS-CoV-2 variants. Finally, we compare the relationship between neutralization titer and protection from COVID-19 after either monoclonal antibody treatment or vaccination. We find no evidence for a difference between the 50% protective titer for monoclonal antibodies and vaccination.

Broad immunity to SARS-CoV-2 variants of concern mediated by a SARS-CoV-2 receptor-binding domain protein vaccine

The SARS-CoV-2 global pandemic has fuelled the generation of vaccines at an unprecedented pace and scale. However, many challenges remain, including: the emergence of vaccine-resistant mutant viruses, vaccine stability during storage and transport, waning vaccine-induced immunity, and concerns about infrequent adverse events associated with existing vaccines. Here, we report on a protein subunit vaccine comprising the receptor-binding domain (RBD) of the ancestral SARS-CoV-2 spike protein, dimerised with an immunoglobulin IgG1 Fc domain. These were tested in conjunction with three different adjuvants: a TLR2 agonist R4-Pam2Cys, an NKT cell agonist glycolipid -Galactosylceramide, or MF59(R) squalene oil-in-water adjuvant. Each formulation drove strong neutralising antibody (nAb) responses and provided durable and highly protective immunity against lower and upper airway infection in mouse models of COVID-19. We have also developed an RBD-human IgG1 Fc vaccine with an RBD sequence of the highly immuno-evasive beta variant (N501Y, E484K, K417N). This beta variant RBD vaccine, combined with MF59(R) adjuvant, induced strong protection in mice against the beta strain as well as the ancestral strain. Furthermore, when used as a third dose booster vaccine following priming with whole spike vaccine, anti-sera from beta-RBD-Fc immunised mice increased titres of nAb against other variants including alpha, delta, delta+, gamma, lambda, mu, and omicron BA.1 and BA.2. These results demonstrated that an RBD-Fc protein subunit/MF59(R) adjuvanted vaccine can induce high levels of broad nAbs, including when used as a booster following prior immunisation of mice with whole ancestral-strain Spike vaccines. This vaccine platform offers a potential approach to augment some of the currently approved vaccines in the face of emerging variants of concern, and it has now entered a phase I clinical trial.

Neutralising antibodies predict protection from severe COVID-19

BackgroundVaccine protection from COVID-19 has been shown to decline with time-since-vaccination and against SARS-CoV-2 variants. Protection against severe COVID-19 is higher than against symptomatic infection, and also appears relatively preserved over time and against variants. Although protection from symptomatic SARS-CoV-2 infection is strongly correlated with neutralising antibody titres, this relationship has been less well described for severe COVID-19. Here we analyse whether neutralising antibody titre remains predictive of protection against severe COVID-19 in the face of waning neutralising antibody levels and emerging variants. MethodsWe extracted data from 15 studies reporting on protection against a range of SARS-CoV-2 clinical endpoints ( any infection, symptomatic infection and severe COVID-19). We then estimated the concurrent neutralising antibody titres using existing parameters on vaccine potency, neutralising antibody decay, and loss of recognition of variants and investigated the relationship between neutralising antibody titre and vaccine effectiveness against severe COVID-19. FindingsPredicted neutralising antibody titres are strongly correlated with vaccine effectiveness against symptomatic and severe COVID-19 (Spearman{rho} = 0.94 and 0.63 respectively, p<.001 for both). The relationship between neutralisation titre and protection is consistent with previous estimates, with 76% (127 of 167) of reported values of protection against severe COVID-19 across a range of vaccines and variants lie within the 95% confidence intervals of the previously published model. InterpretationNeutralising antibody titres are predictive of vaccine effectiveness against severe COVID-19 including in the presence of waning immunity and viral variants. FundingNational Health and Medical Research Council (Australia), Medical Research Future Fund (Australia). O_TEXTBOXEvidence before this study COVID-19 vaccine effectiveness against symptomatic SARS-CoV-2 infection has been shown to be strongly predicted by neutralising antibody titres. However, there has not been sufficient data on vaccine efficacy against severe disease to determine whether the correlation between neutralising antibody titres and protection is maintained for severe COVID-19. Indeed, the apparent faster waning of vaccine efficacy against symptomatic (compared to severe) COVID-19 has led some to hypothesise about a decoupling of protection from symptomatic and severe disease. It is therefore important to identify whether neutralising antibody titre remains a correlate of protection against severe COVID-19 in real-world scenarios of waning immunity and SARS-CoV-2 variants. We searched PubMed between inception and March 2, 2022 for studies (Key search terms: (SARS-CoV-2 OR COVID-19) AND (followup OR waning OR duration OR durable) AND (protection OR efficacy OR effectiveness)) and also monitored other public sources of information such as Twitter. We identified 15 studies that reported data on vaccine effectiveness against (i) a defined clinical endpoint (ii) for an identifiable variant, (ii) for a single vaccine (or vaccine type), (iii) over an identified time since vaccination, and (iv) for which data was either provided in or readily extractable from the original publication. Added value of this study Previous work has identified that neutralising antibodies correlate strongly with protection from symptomatic COVID-19 disease for both ancestral virus and for variants of concern. Here we examine published data on vaccine effectiveness to determine if this correlation remains valid for predicting protection against severe COVID-19. Our work shows that vaccine effectiveness against severe COVID-19 is strongly correlated with neutralising antibody titres for different vaccines, variants, and over the first six months after vaccination. The relationship between vaccine effectiveness and protection against severe COVID-19 is consistent with a previously published analysis and indicates that the 50% protective titre for protection against severe COVID-19 is lower than that associated with protection from symptomatic infection. Implications of all of the available evidence In the face of increased exposure and immunity to numerous SARS-CoV-2 variants it is becoming increasingly important to be able to predict not only protection against symptomatic infection, but also protection against severe COVID-19. Here we show that neutralising antibody titres remain predictive of vaccine effectiveness against severe COVID-19 in the face of waning immunity and SARS-CoV-2 variants. This is consistent with a low level of neutralising antibodies being associated with protection from severe COVID-19. This work will be of use in providing early estimates of protection against severe COVID-19 for new SARS-CoV-2 antigenic variants, informing optimal booster timing, and will support future vaccine development by allowing prediction of vaccine protection conferred against severe outcomes. C_TEXTBOX

Correlates of protection, thresholds of protection, and immunobridging in SARS-CoV-2 infection

Several studies show neutralizing antibody levels are an important correlate of immune protection from COVID-19 and have estimated the relationship between neutralizing antibodies and protection. However, a number of these studies appear to yield quite different estimates of the level of neutralizing antibodies required for protection. Here we show that after normalization of antibody titers current studies converge on a consistent relationship between antibody levels and protection from COVID-19.

Determinants of passive antibody effectiveness in SARS-CoV-2 infection

BackgroundA large number of studies have been carried out involving passive antibody administration for the treatment and prophylaxis of COVID-19 and have shown variable efficacy. However, the determinants of treatment effectiveness have not been identified. Here we aimed to aggregate all available data on randomised controlled trials of passive antibody treatment for COVID-19 to understand how the dose and timing affect treatment outcome. MethodsWe analysed published studies of passive antibody treatment from inception to 7 January 2022 that were identified after searching various databases such as MEDLINE, Pubmed, ClinicalTrials.gov. We extracted data on treatment, dose, disease stage at treatment, and effectiveness for different clinical outcomes from these studies. To compare administered antibody levels between different treatments, we used data on in vitro neutralisation of pseudovirus to normalise the administered dose of antibody. We used a mixed-effects regression model to understand the relationship between disease stage at treatment and effectiveness. We used a logistic model to analyse the relationship between administered antibody dose (normalised to the mean convalescent titre) and outcome, and to predict efficacy of antibodies against different Omicron subvariants. FindingsWe found that clinical stage at treatment was highly predictive of the effectiveness of both monoclonal antibodies and convalescent plasma therapy in preventing progression to subsequent stages (p<0.0001 and p=0.0089, respectively, chi-squared test). We also analysed the dose-response curve for passive antibody treatment of ambulant COVID-19 patients to prevent hospitalisation. Using this quantitative dose-response relationship, we predict that a number of existing monoclonal antibody treatment regimens should maintain clinical effectiveness in infection with currently circulating Omicron variants. InterpretationEarly administration of passive antibody therapy is crucial to achieving high efficacy in preventing clinical progression. A dose-response curve was derived for passive antibody therapy administered to ambulant symptomatic subjects to prevent hospitalisation. For many of the monoclonal antibody regimens analysed, the administered doses are estimated to be between 7 and >1000 fold higher than necessary to achieve 90% of the maximal efficacy against the ancestral (Wuhan-like) virus. This suggests that a number of current treatments should maintain high efficacy against Omicron subvariants despite reduction in in vitro neutralisation potency. This work provides a framework for the rational assessment of future passive antibody prophylaxis and treatment strategies for COVID-19. FundingThis work is supported by an Australian government Medical Research Future Fund awards GNT2002073 and MRF2005544 (to MPD, SJK), MRF2005760 (to MPD), an NHMRC program grant GNT1149990 (SJK and MPD), and the Victorian Government (SJK). SJK is supported by a NHMRC fellowship. DC, MPD, ZKM and EMW are supported by NHMRC Investigator grants and ZKM and EMW by an NHMRC Synergy grant (1189490). DSK is supported by a University of New South Wales fellowship. KLC is supported by PhD scholarships from Monash University, the Haematology Society of Australia and New Zealand and the Leukaemia Foundation. TT, HW and CB are members of the National COVID-19 Clinical Evidence Taskforce which is funded by the Australian Government Department of Health. Research in contextO_ST_ABSEvidence before this studyC_ST_ABSWe identified randomised controlled trials (RCTs) evaluating the effectiveness of SARS-CoV-2-specific neutralising monoclonal antibodies, hyperimmune immunoglobulin and convalescent plasma in the treatment of participants with a confirmed diagnosis of COVID-19 and in uninfected participants with or without potential exposure to SARS-CoV-2. The RCTs were identified from published searches conducted by the Cochrane Haematology living systematic review teams. A total of 37 randomised controlled trials (RCT) of passive antibody administration for COVID-19 were identified. This included 12 trials on monoclonal antibodies, 21 trials of convalescent plasma treatment, and 4 trials of hyperimmune globulin. These trials involved treatment of individuals either prophylactically or at different stages of infection including post-exposure prophylaxis, symptomatic infection, and hospitalisation. The level of antibody administered ranged from a 250 ml volume of convalescent plasma through to 8 grams of monoclonal antibodies. Data for analysis was extracted from the original publications including dose and antibody levels of antibody administered, disease stage and timing of administration, primary outcome of study and whether they reported on our prespecified outcomes of interest, which include protection against symptomatic infection, hospitalisation, need for invasive mechanical ventilation (IMV) and death (all-cause mortality at 30 days). Added value of this studyOur study included data across all 37 RCTs of passive antibody interventions for COVID-19 and aggregated the studies by the stage of infection at initiation of treatment. We found that prophylactic administration or treatment in earlier stages of infection had significantly higher effectiveness than later treatment. We also estimated the dose-response relationship between administered antibody dose and protection from progression from symptomatic ambulant COVID-19 to hospitalisation. We used this relationship to predict the efficacy of different monoclonal antibody treatment regimes against the Omicron subvariants BA.1, BA.2, and BA.4/5. We also used this dose-response relationship to estimate the maximal efficacy of monoclonal antibody therapy in the context of pre-existing endogenous neutralising antibodies. Implications of all the available evidenceThis work identifies that both prophylactic therapy and treatment in the early stages of symptomatic infection can achieve significant protection from infection or hospitalisation respectively. The dose-response relationship provides a quantitative means to predict the change in efficacy of different monoclonal antibodies against new variants and in semi-immune populations based on in vitro neutralisation data. We predict a number of existing monoclonal antibodies will be effective for preventing severe outcomes when administered early in BA.4/5 infections. It is likely that these therapies will provide little protection in individuals with high levels of endogenous neutralising antibodies, such as healthy individuals who have recently received a third dose of an mRNA vaccine.

Anti-PEG antibodies boosted in humans by SARS-CoV-2 lipid nanoparticle mRNA vaccine

Humans commonly have low level antibodies to poly(ethylene) glycol (PEG) due to environmental exposure. Lipid nanoparticle (LNP) mRNA vaccines for SARS-CoV-2 contain small amounts of PEG but it is not known whether PEG antibodies are enhanced by vaccination and what their impact is on particle-immune cell interactions in human blood. We studied plasma from 130 adults receiving either the BNT162b2 (Pfizer-BioNTech) or mRNA-1273 (Moderna) mRNA vaccines, or no SARS-CoV-2 vaccine for PEG-specific antibodies. Anti-PEG IgG was commonly detected prior to vaccination and was significantly boosted a mean of 13.1-fold (range 1.0 to 70.9) following mRNA-1273 vaccination and a mean of 1.78-fold (range 0.68 to 16.6) following BNT162b2 vaccination. Anti-PEG IgM increased 68.5-fold (range 0.9 to 377.1) and 2.64-fold (0.76 to 12.84) following mRNA-1273 and BNT162b2 vaccination, respectively. The rise in PEG-specific antibodies following mRNA-1273 vaccination was associated with a significant increase in the association of clinically relevant PEGylated LNPs with blood phagocytes ex vivo. PEG antibodies did not impact the SARS-CoV-2 specific neutralizing antibody response to vaccination. However, the elevated levels of vaccine-induced anti-PEG antibodies correlated with increased systemic reactogenicity following two doses of vaccination. We conclude that PEG-specific antibodies can be boosted by LNP mRNA-vaccination and that the rise in PEG-specific antibodies is associated with systemic reactogenicity and an increase of PEG particle-leukocyte association in human blood. The longer-term clinical impact of the increase in PEG-specific antibodies induced by lipid nanoparticle mRNA-vaccines should be monitored.

Dynamics of immune recall following SARS-CoV-2 vaccination or breakthrough infection

Vaccination against SARS-CoV-2 results in protection from acquisition of infection as well as improved clinical outcomes even if infection occurs, likely reflecting a combination of residual vaccine-elicited immunity and the recall of immunological memory. Here, we define the early kinetics of spike-specific humoral and T cell immunity after vaccination of seropositive individuals, and after breakthrough infection in vaccinated individuals. Intensive and early longitudinal sampling reveals the timing and magnitude of recall, with the phenotypic activation of B cells preceding an increase in neutralizing antibody titres. In breakthrough infections, the delayed kinetics of humoral immune recall provides a mechanism for the lack of early control of viral replication but likely underpins accelerated viral clearance and the protective effects of vaccination against severe COVID-19.

SARS-CoV-2 variants: levels of neutralisation required for protective immunity

A number of SARS-CoV-2 variants of concern (VOC) have been identified that partially escape serum neutralisation activity elicited by current vaccines. Recent studies have also shown that vaccines demonstrate reduced protection against symptomatic infection with SARS-CoV-2 variants. Here we integrate published data on in vitro neutralisation and clinical protection to understand and predict vaccine efficacy against existing SARS-CoV-2 variants. We find that neutralising activity against the ancestral SARS-CoV-2 is highly predictive of neutralisation of the VOC, with all vaccines showing a similar drop in neutralisation to the variants. Neutralisation levels remain strongly correlated with protection from infection with SARS-CoV-2 VOC (r=0.81, p=0.0005). We apply an existing model relating in vitro neutralisation to protection (parameterised on data from ancestral virus infection) and find this remains predictive of vaccine efficacy against VOC once drops in neutralisation to the VOC are taken into account. Modelling of predicted vaccine efficacy against variants over time suggests that protection against symptomatic infection may drop below 50% within the first year after vaccination for some current vaccines. Boosting of previously infected individuals with existing vaccines (which target ancestral virus) has been shown to significantly increase neutralising antibodies. Our modelling suggests that booster vaccination should enable high levels of immunity that prevent severe infection outcomes with the current SARS-CoV-2 VOC, at least in the medium term.

Tear antibodies to SARS-CoV-2: implications for transmission

ObjectivesSARS-CoV-2 can be transmitted by aerosols and the ocular surface may be an important route of transmission. Little is known about protective antibody responses to SARS-CoV-2 in tears after infection or vaccination. We analysed SARS-CoV-2 specific IgG and IgA responses in human tears after either COVID-19 infection or vaccination. MethodsWe recruited 16 subjects with COVID-19 infection an average of 7 months previously and 15 subjects before and 2 weeks after Comirnaty (Pfizer-BioNtech) vaccination. Plasma, saliva and basal tears were collected. Pre-pandemic plasma, saliva and basal tears from 11 individuals were included as healthy controls. Antibody responses to 5 SARS-CoV-2 antigens were measured via multiplex. ResultsIgG antibodies to Spike and Nucleoprotein were detected in tears, saliva and plasma from subjects with prior SARS-CoV-2 infection in comparison to uninfected controls. While RBD-specific antibodies were detected in plasma, minimal RBD-specific antibodies were detected in tears and saliva. In contrast, high levels of IgG antibodies to Spike and RBD, but not Nucleoprotein, were induced in tears, saliva and plasma of subjects receiving 2 doses of the Comirnaty vaccine. Increased levels of IgA1 and IgA2 antibodies to SARS-CoV-2 antigens were detected in plasma following infection or vaccination, but were unchanged in tears and saliva. ConclusionBoth infection and vaccination induce SARS-CoV-2-specific IgG antibodies in tears. RBD-specific IgG antibodies in tears were induced by vaccination but were not present 7 months post-infection. This suggests neutralising antibodies may be low in the tears late following infection.

Immunological dysfunction persists for 8 months following initial mild-moderate SARS-CoV-2 infection

A proportion of patients surviving acute COVID-19 infection develop post-COVID syndrome (long COVID) encompassing physical and neuropsychiatric symptoms lasting longer than 12 weeks. Here we studied a prospective cohort of individuals with long COVID compared to age/gender matched subjects without long COVID (from the ADAPT study), healthy donors and individuals infected with other non-SARS CoV2 human coronaviruses (the ADAPT-C study). We found highly activated innate immune cells and an absence of subsets of un-activated naive T and B cells in peripheral blood of long COVID subjects, that did not reconstitute over time. These activated myeloid cells may contribute to the elevated levels of type I (IFN-{beta}) and III interferon (IFN-{lambda}1) that remained persistently high in long COVID subjects at 8 months post-infection. We found positive inter-analyte correlations that consisted of 18 inflammatory cytokines in symptomatic long COVID subjects that was not observed in asymptomatic COVID-19 survivors. A linear classification model was used to exhaustively search through all 20475 combinations of the 29 analytes measured, that had the strongest association with long COVID and found that the best 4 analytes were: IL-6, IFN-{gamma}, MCP-1 (CCL2) and VCAM-1. These four inflammatory biomarkers gave an accuracy of 75.9%, and an F1 score of 0.759, and have also previously been associated with acute severe disease. In contrast, plasma ACE2 levels, while elevated in the serum of people previously infected with SARS-CoV-2 were not further elevated in subjects with long COVID symptoms. This work defines immunological parameters associated with long COVID and suggests future opportunities to prevention and treatment.

A point-of-care lateral flow assay for neutralising antibodies against SARS-CoV-2

As vaccines against SARS-CoV-2 are now being rolled out, a better understanding of immunity to the virus; whether through infection, or passive or active immunisation, and the durability of this protection is required. This will benefit from the ability to measure SARS-CoV-2 immunity, ideally with rapid turnaround and without the need for laboratory-based testing. Current rapid point-of-care (POC) tests measure antibodies (Ab) against the SARS-CoV-2 virus, however, these tests provide no information on whether the antibodies can neutralise virus infectivity and are potentially protective, especially against newly emerging variants of the virus. Neutralising Antibodies (NAb) are emerging as a strong correlate of protection, but most current NAb assays require many hours or days, samples of venous blood, and access to laboratory facilities, which is especially problematic in resource-limited settings. We have developed a lateral flow POC test that can measure levels of RBD-ACE2 neutralising antibodies from whole blood, with a result that can be determined by eye (semi-quantitative) or on a small instrument (quantitative), and results show high correlation with microneutralisation assays. This assay also provides a measure of total anti-RBD antibody, thereby providing evidence of exposure to SARS-CoV-2, regardless of whether NAb are present in the sample. By testing samples from immunised macaques, we demonstrate that this test is equally applicable for use with animal samples, and we show that this assay is readily adaptable to test for immunity to newly emerging SARS-CoV-2 variants. Accordingly, the COVID-19 NAb-test test described here can provide a rapid readout of immunity to SARS-CoV-2 at the point of care.

Simultaneous evaluation of antibodies that inhibit SARS-CoV-2 RBD variants with a novel competitive multiplex assay

The SARS-CoV-2 Receptor Binding Domain (RBD) is both the principal target of neutralizing antibodies, and one of the most rapidly evolving domains, which can result in the emergence of immune escape mutations limiting the effectiveness of vaccines and antibody therapeutics. To facilitate surveillance, we developed a rapid, high-throughput, multiplex assay able to assess the inhibitory response of antibodies to 24 RBD natural variants simultaneously. We demonstrate that immune escape can occur through two mechanisms, antibodies that fail to recognize mutations, along with antibodies that have reduced inhibitory capacity due to enhanced variant RBD-ACE2 affinity. A competitive approach where antibodies simultaneously compete with ACE2 for binding to the RBD may therefore more accurately reflect the physiological dynamics of infection. We describe the enhanced affinity of RBD variants N439K, S477N, Q493L, S494P and N501Y to the ACE2 receptor, and demonstrate the ability of this assay to bridge a major gap for SARS-CoV-2 research; informing selection of complementary monoclonal antibody candidates and the rapid identification of immune escape to emerging RBD variants following vaccination or natural infection.

What level of neutralising antibody protects from COVID-19? .

Both previous infection and vaccination have been shown to provide potent protection from COVID-19. However, there are concerns that waning immunity and viral variation may lead to a loss of protection over time. Predictive models of immune protection are urgently needed to identify immune correlates of protection to assist in the future deployment of vaccines. To address this, we modelled the relationship between in vitro neutralisation levels and observed protection from SARS-CoV-2 infection using data from seven current vaccines as well as convalescent cohorts. Here we show that neutralisation level is highly predictive of immune protection. The 50% protective neutralisation level was estimated to be approximately 20% of the average convalescent level (95% CI = 14-28%). The estimated neutralisation level required for 50% protection from severe infection was significantly lower (3% of the mean convalescent level (CI = 0.7-13%, p = 0.0004). Given the relationship between in vitro neutralization titer and protection, we then used this to investigate how waning immunity and antigenic variation might affect vaccine efficacy. We found that the decay of neutralising titre in vaccinated subjects over the first 3-4 months after vaccination was at least as rapid as the decay observed in convalescent subjects. Modelling the decay of neutralisation titre over the first 250 days after immunisation predicts a significant loss in protection from SARS-CoV-2 infection will occur, although protection from severe disease should be largely retained. Neutralisation titres against some SARS-CoV-2 variants of concern are reduced compared to the vaccine strain and our model predicts the relationship between neutralisation and efficacy against viral variants. Our analyses provide an evidence-based prediction of SARS-CoV-2 immune protection that will assist in developing vaccine strategies to control the future trajectory of the pandemic.

Adaptive immunity to human coronaviruses is widespread but low in magnitude

Endemic human coronaviruses (hCoV) circulate worldwide but cause minimal mortality. Although seroconversion to hCoV is near ubiquitous during childhood, little is known about hCoV-specific T cell memory in adults. We quantified CD4 T cell and antibody responses to hCoV spike antigens in 42 SARS-CoV-2 uninfected individuals. T cell responses were widespread within conventional memory and cTFH compartments but did not correlate with IgG titres. SARS-CoV-2 cross-reactive T cells were observed in 48% of participants and correlated with HKU1 memory. hCoV-specific T cells exhibited a CCR6+ central memory phenotype in the blood, but were enriched for frequency and CXCR3 expression in human lung draining lymph nodes. Overall, hCoV-specific humoral and cellular memory are independently maintained, with a shared phenotype existing among coronavirus-specific CD4 T cells. This understanding of endemic coronavirus immunity provides insight into the homeostatic maintenance of immune responses that are likely to be critical components of protection against SARS-CoV-2.

Decay of Fc-dependent antibody functions after mild to moderate COVID-19

The capacity of antibodies to engage with innate and adaptive immune cells via the Fc region is important in preventing and controlling many infectious diseases, and is likely critical in SARS-CoV-2 infection. The evolution of such antibodies during convalescence from COVID-19 is largely unknown. We developed novel assays to measure Fc-dependent antibody functions against SARS-CoV-2 spike (S)-expressing cells in serial samples from a cohort of 53 subjects primarily with mild-moderate COVID-19, out to a maximum of 149 days post-infection. We found that S-specific antibodies capable of engaging dimeric Fc{gamma}RIIa and Fc{gamma}RIIIa decayed linearly over time. S-specific antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent phagocytosis (ADP) activity within plasma declined linearly as well, in line with the decay of S-specific IgG. Although there was significant decay in S-specific plasma ADCC and ADP activity, they remained readily detectable by all assays in 94% of our cohort at the last timepoint studied, in contrast with neutralisation activity which was only detectable in 70% of our cohort by the last timepoint. Our results suggest that Fc effector functions such as ADCC and ADP could contribute to the durability of SARS-CoV-2 immunity, particularly late in convalescence when neutralising antibodies have waned. Understanding the protective potential of antibody Fc effector functions is critical for defining the durability of immunity generated by infection or vaccination.

Plasma ACE2 activity is persistently elevated following SARS-CoV-2 infection: implications for COVID-19 pathogenesis and consequences

COVID-19 causes persistent endothelial inflammation, lung and cardiovascular complications. SARS-CoV-2 utilises the catalytic site of full-length membrane-bound angiotensin converting enzyme 2 (ACE2) for cell entry causing downregulation of tissue ACE2. We reported downregulation of cardiac ACE2 is associated with increased plasma ACE2 activity. In this prospective observational study in recovered COVID-19 patients, we hypothesised that SARS-CoV-2 infection would be associated with shedding of ACE2 from cell membranes and increased plasma ACE2 activity. MethodsWe measured plasma ACE2 catalytic activity using a validated, sensitive quenched fluorescent substrate-based assay in a cohort of Australians aged [≥]18 years (n=66) who had recovered from mild, moderate or severe SARS-CoV-2 infection (positive result by PCR testing) and age and gender matched uninfected controls (n=70). Serial samples were available in 23 recovered SARS-CoV-2 patients. ResultsPlasma ACE2 activity at a median of 35 days post-infection [interquartile range 30-38 days] was 97-fold higher in recovered SARS-CoV-2 patients compared to controls (5.8 [2-11.3] vs. 0.06 [0.02-2.2] pmol/min/ml, p<0.0001). There was a significant difference in plasma ACE2 activity according to disease severity (p=0.033), with severe COVID-19 associated with higher ACE2 activity compared to mild disease (p=0.027). Men (n=39) who were SARS-CoV-2 positive had higher median plasma ACE2 levels compared to women (n=27) (p<0.0001). We next analysed whether an elevated plasma ACE2 activity level persisted following SARS-CoV-2 infection in subjects with blood samples at 63 [56-65] and 114 [111-125] days post infection. Plasma ACE2 activity remained persistently elevated in almost all subjects, with no significant differences between timepoints in post-hoc comparisons (p>0.05). DiscussionThis is the first description that plasma ACE2 activity is elevated after COVID-19 infection, and the first with longitudinal data indicating plasma ACE2 activity remains elevated out to a median of 114 days post-infection. Larger studies are now needed to determine if persistent elevated plasma ACE2 activity identifies people at risk of prolonged illness following COVID-19.

Prime-boost protein subunit vaccines against SARS-CoV-2 are highly immunogenic in mice and macaques

SARS-CoV-2 vaccines are advancing into human clinical trials, with emphasis on eliciting high titres of neutralising antibodies against the viral spike (S). However, the merits of broadly targeting S versus focusing antibody onto the smaller receptor binding domain (RBD) are unclear. Here we assessed prototypic S and RBD subunit vaccines in homologous or heterologous prime-boost regimens in mice and non-human primates. We find S is highly immunogenic in mice, while the comparatively poor immunogenicity of RBD was associated with limiting germinal centre and T follicular helper cell activity. Boosting S-primed mice with either S or RBD significantly augmented neutralising titres, with RBD-focussing driving moderate improvement in serum neutralisation. In contrast, both S and RBD vaccines were comparably immunogenic in macaques, eliciting serological neutralising activity that generally exceed levels in convalescent humans. These studies confirm recombinant S proteins as promising vaccine candidates and highlight multiple pathways to achieving potent serological neutralisation.

Evolution of immunity to SARS-CoV-2

The durability of infection-induced SARS-CoV-2 immunity has major implications for public health mitigation and vaccine development. Animal studies1,2 and the scarcity of confirmed re-infection3 suggests immune protection is likely, although the durability of this protection is debated. Lasting immunity following acute viral infection requires maintenance of both serum antibody and antigen-specific memory B and T lymphocytes and is notoriously pathogen specific, ranging from life-long for smallpox or measles4, to highly transient for common cold coronaviruses (CCC)5. Neutralising antibody responses are a likely correlate of protective immunity and exclusively recognise the viral spike (S) protein, predominantly targeting the receptor binding domain (RBD) within the S1 sub-domain6. Multiple reports describe waning of S-specific antibodies in the first 2-3 months following infection7-12. However, extrapolation of early linear trends in decay might be overly pessimistic, with several groups reporting that serum neutralisation is stable over time in a proportion of convalescent subjects8,12-17. While SARS-CoV-2 specific B and T cell responses are readily induced by infection6,13,18-24, the longitudinal dynamics of these key memory populations remains poorly resolved. Here we comprehensively profiled antibody, B and T cell dynamics over time in a cohort recovered from mild-moderate COVID-19. We find that binding and neutralising antibody responses, together with individual serum clonotypes, decay over the first 4 months post-infection, as expected, with a similar decline in S-specific CD4+ and circulating T follicular helper (cTFH) frequencies. In contrast, S-specific IgG+ memory B cells (MBC) consistently accumulate over time, eventually comprising a significant fraction of circulating MBC. Modelling of the concomitant immune kinetics predicts maintenance of serological neutralising activity above a titre of 1:40 in 50% of convalescent subjects to 74 days, with probable additive protection from B and T cells. Overall, our study suggests SARS-CoV-2 immunity after infection is likely to be transiently protective at a population level. SARS-CoV-2 vaccines may require greater immunogenicity and durability than natural infection to drive long-term protection.

Immunogenic profile of SARS-CoV-2 spike in individuals recovered from COVID-19

The rapid global spread of SARS-CoV-2 and resultant mortality and social disruption have highlighted the need to better understand coronavirus immunity to expedite vaccine development efforts. Multiple candidate vaccines, designed to elicit protective neutralising antibodies targeting the viral spike glycoprotein, are rapidly advancing to clinical trial. However, the immunogenic properties of the spike protein in humans are unresolved. To address this, we undertook an in-depth characterisation of humoral and cellular immunity against SARS-CoV-2 spike in humans following mild to moderate SARS-CoV-2 infection. We find serological antibody responses against spike are routinely elicited by infection and correlate with plasma neutralising activity and capacity to block ACE2/RBD interaction. Expanded populations of spike-specific memory B cells and circulating T follicular helper cells (cTFH) were detected within convalescent donors, while responses to the receptor binding domain (RBD) constitute a minor fraction. Using regression analysis, we find high plasma neutralisation activity was associated with increased spike-specific antibody, but notably also with the relative distribution of spike-specific cTFH subsets. Thus both qualitative and quantitative features of B and T cell immunity to spike constitute informative biomarkers of the protective potential of novel SARS-CoV-2 vaccines.

Distinct systems serology features in children, elderly and COVID patients

SARS-CoV-2, the pandemic coronavirus that causes COVID-19, has infected millions worldwide, causing unparalleled social and economic disruptions. COVID-19 results in higher pathogenicity and mortality in the elderly compared to children. Examining baseline SARS-CoV-2 cross-reactive coronavirus immunological responses, induced by circulating human coronaviruses, is critical to understand such divergent clinical outcomes. The cross-reactivity of coronavirus antibody responses of healthy children (n=89), adults (n=98), elderly (n=57), and COVID-19 patients (n=19) were analysed by systems serology. While moderate levels of cross-reactive SARS-CoV-2 IgG, IgM, and IgA were detected in healthy individuals, we identified serological signatures associated with SARS-CoV-2 antigen-specific Fc{gamma} receptor binding, which accurately distinguished COVID-19 patients from healthy individuals and suggested that SARS-CoV-2 induces qualitative changes to antibody Fc upon infection, enhancing Fc{gamma} receptor engagement. Vastly different serological signatures were observed between healthy children and elderly, with markedly higher cross-reactive SARS-CoV-2 IgA and IgG observed in elderly, whereas children displayed elevated SARS-CoV-2 IgM, including receptor binding domain-specific IgM with higher avidity. These results suggest that less-experienced humoral immunity associated with higher IgM, as observed in children, may have the potential to induce more potent antibodies upon SARS-CoV-2 infection. These key insights will inform COVID-19 vaccination strategies, improved serological diagnostics and therapeutics.