Seropositivity in blood donors and pregnant women during 9-months of SARS-CoV-2 transmission in Stockholm, Sweden

In Sweden, social restrictions to contain SARS-CoV-2 have to date primarily relied upon voluntary adherence to a set of recommendations and strict lockdowns/regulations have not been enforced, potentially affecting viral dissemination. To understand the levels of past SARS-CoV-2 infection in the Stockholm population before the start of mass vaccinations, healthy blood donors and pregnant women (n=5,100) were sampled at random between 14th March 2020-28th February 2021. All individuals (n=200/sampling week) were screened for anti-SARS-CoV-2 spike (S) trimer- and RBD-specific IgG responses and the results were compared with those from historical controls (n=595). Data were modelled using a probabilistic Bayesian framework that considered individual responses to both viral antigens. We found that after a steep rise at the start of the pandemic, the seroprevalence trajectory increased more steadily (over summer) in approach to the winter second-wave of infections, approaching 15% of all adults surveyed by mid-December 2020. The population seropositivity rate again increased more rapidly as cases rose over the winter period. By the end of February 2021, [~]19% ([~]one-in-five) in this study group tested seropositive. Notably, 96% of random seropositive samples screened (n=56), displayed virus neutralizing responses, with titers comparable to those engendered by recently approved mRNA vaccines, supporting that milder infections generally provoke a competent B cell response. These data offer baseline information about the level of seropositivity in this group of active adults in the Stockholm metropolitan area following a full year of SARS-CoV-2 transmission and prior to the introduction of vaccines. Structured abstractO_ST_ABSObjectivesC_ST_ABSSweden did not enforce social lockdown in response to the SARS-CoV-2 pandemic. Therefore, we sought to determine the proportion of seropositive healthy, active adults in Stockholm, the countrys most populous region. Random sampling (of blood donors and pregnant women) was carried out during the first year following virus emergence in the country and prior to vaccination of the general adult population - allowing for an estimate of seroprevalence in response to natural infection. DesignIn this cross-sectional prospective study, otherwise-healthy blood donors (n=2,600) and pregnant women(n=2,500) were sampled at random for consecutive weeks (at four intervals) between 14th March and 28th February 2021. Sera from all participants and a cohort of historical controls (n=595) were screened for IgG responses against trimers of the SARS-CoV-2 spike (S) glycoprotein and the smaller receptor-binding domain (RBD). As a complement to standard analytical approaches, a probabilistic (cut-off-independent) Bayesian framework that assigns likelihood of past infection was used to analyze data over time. The study was carried out in accordance with Swedish Ethical Review Authority: registration number 2020-01807. SettingHealthy participant samples were selected from their respective pools at random through Karolinska University Hospital. ParticipantsNone of the participants were symptomatic at sampling. No additional metadata was available from the samples. ResultsBlood donors and pregnant women showed a similar seroprevalence. After a steep rise at the start of the pandemic, the seroprevalence trajectory increased steadily in approach to the winter second-wave of infections, approaching 15% of all individuals surveyed by 13th December 2020. By the end of February 2021, when deaths were in decline and at low levels following their winter peak, 19% of the population tested seropositive. Notably, 96% of seropositive healthy donors screened (n=56) developed neutralizing antibody responses at titers comparable to, or higher than those observed in clinical trials of SARS-CoV-2 spike mRNA vaccination, supporting that mild infection engenders a competent B cell response. ConclusionsThese data indicate that in the year since the start of community transmission, seropositivity levels in metropolitan Stockholm had reached approximately one-in-five persons, providing important baseline seroprevalence information prior to the start of vaccination.

SARS-CoV-2 protein subunit vaccination elicits potent neutralizing antibody responses

The outbreak and spread of SARS-CoV-2 (Severe Acute Respiratory Syndrome coronavirus 2), the cause of coronavirus disease 2019 (COVID-19), is a current global health emergency and a prophylactic vaccine is needed urgently. The spike glycoprotein of SARS-CoV-2 mediates entry into host cells, and thus is a target for neutralizing antibodies and vaccine design. Here we show that adjuvanted protein immunization with SARS-CoV-2 spike trimers, stabilized in prefusion conformation 1, results in potent antibody responses in mice and rhesus macaques with neutralizing antibody titers orders of magnitude greater than those typically measured in serum from SARS-CoV-2 seropositive humans. Neutralizing antibody responses were observed after a single dose, with exceptionally high titers achieved after boosting. Furthermore, neutralizing antibody titers elicited by a dose-sparing regimen in mice were similar to those obtained from a high dose regimen. Taken together, these data strongly support the development of adjuvanted SARS-CoV-2 prefusion-stabilized spike protein subunit vaccines.

Disease-associated antibody phenotypes and probabilistic seroprevalence estimates during the emergence of SARS-CoV-2

Serological studies are critical for understanding pathogen-specific immune responses and informing public health measures1,2. Here, we evaluate tandem IgM, IgG and IgA responses in a cohort of individuals PCR+ for SARS-CoV-2 RNA (n=105) representing different categories of disease severity, including mild and asymptomatic infections. All PCR+ individuals surveyed were IgG-positive against the virus spike (S) glycoprotein. Elevated Ab levels were associated with hospitalization, with IgA titers, increased circulating IL-6 and strong neutralizing responses indicative of intensive care status. Additional studies of healthy blood donors (n=1,000) and pregnant women (n=900), sampled weekly during the initial outbreak in Stockholm, Sweden (weeks 14-25, 2020), demonstrated that anti-viral IgG titers differed over 1,000-fold between seroconverters, highlighting the need for careful evaluation of assay cut-offs for individual measurements and accurate estimates of seroprevalence (SP). To provide a solution to this, we developed probabilistic machine learning approaches to assign likelihood of past infection without setting an assay cut-off, allowing for more quantitative individual and population-level Ab measures. Using these tools, that considered responses against both S and RBD, we report SARS-CoV-2 S-specific IgG in 6.8% of blood donors and pregnant women two months after the peak of spring COVID-19 deaths, with the SP curve and country death rate following similar trajectories.

A translational multiplex serology approach to profile the prevalence of anti-SARS-CoV-2 antibodies in home-sampled blood

The COVID-19 pandemic has posed a tremendous challenge for the global community. We established a translational approach combining home blood sampling by finger-pricking with multiplexed serology to assess the exposure to the SARS-CoV-2 virus in a general population. The developed procedure determines the immune response in multiplexed assays against several spike (S, here denoted SPK), receptor binding domain (RBD) and nucleocapsid (NCP) proteins in eluates from dried capillary blood. The seroprevalence was then determined in two study sets by mailing 1000 blood sampling kits to random households in urban Stockholm during early and late April 2020, respectively. After receiving 55% (1097/2000) of the cards back within three weeks, 80% (878/1097) were suitable for the analyses of IgG and IgM titers. The data revealed diverse pattern of immune response, thus seroprevalence was dependent on the antigen, immunoglobulin class, stringency to include different antigens, as well as the required analytical performance. Applying unsupervised dimensionality reduction to the combined IgG and IgM data, 4.4% (19/435; 95% CI: 2.4%-6.3%) and 6.3% (28/443; 95% CI: 4.1%-8.6%) of the samples clustered with convalescent controls. Using overlapping scores from at least two SPK antigens, prevalence rates reached 10.1% (44/435; 95% CI: 7.3%-12.9%) in study set 1 and 10.8% (48/443; 95% CI: 7.9%-13.7%). Measuring the immune response against several SARS-CoV-2 proteins in a multiplexed workflow can provide valuable insights about the serological diversity and improve the certainty of the classification. Combining such assays with home-sampling of blood presents a viable strategy for individual-level diagnostics and towards an unbiased assessment of the seroprevalence in a population and may serve to improve our understanding about the diversity of COVID-19 etiology. One Sentence SummaryA multiplexed serology assay was developed to determine antibodies against SARS-CoV-2 proteins in home-sampled dried blood spots collected by finger pricking.

SARS-CoV-2 exposure, symptoms and seroprevalence in health care workers

BackgroundSARS-CoV-2 may pose an occupational health risk to health care workers, but the prevalence of infections in this population is unknown. We examined the seroprevalence of SARS-CoV-2 antibodies among health care workers at a large acute care hospital in Stockholm, Sweden. We determined correlations between seroprevalence, self-reported symptoms and occupational exposure to SARS-CoV-2. Methods and findingsAll employees at Danderyd Hospital (n=4375) were invited to participate in a cross-sectional study. 2149 employees from all hospital departments were enrolled in the study between April 14th and May 8th 2020. Study participants completed a questionnaire consisting of symptoms compatible with SARS-CoV-2 infection since January 2020 and occupational exposure to patients infected with SARS-CoV-2. IgG antibodies against SARS-CoV-2 were analyzed using a multiplex assay evaluated to have 99.4% sensitivity and 99.1% specificity. The over-all seroprevalence among 2149 participants was 19.1% (n=410). There was no difference in age or sex between seropositive and seronegative participants. The symptoms with the strongest correlation to seroprevalence were anosmia and ageusia, with odds ratios of 28.4 (p=2.02*10^-120) and 19.2 (p=1.67*10^-99) respectively. Seroprevalence was strongly associated with patient-related work (OR 2.9, p=4.24*10^-8), covid-19 patient contact (OR 1.43, p=0.003), and occupation as assisting nurse (OR 3.67, p=2.16*10^-9). ConclusionThese results demonstrate that anosmia and ageusia should be included in screening guidance and in the recommendations of self-isolation to reduce further spread of SARS-CoV-2. The results furthermore imply an occupational health risk for SARS-CoV-2 infection among hospital workers. Continued measures are warranted to assure healthcare worker safety and reduce transmission from health care settings to the community during the covid-19 outbreak.

Detection of asymptomatic SARS-CoV-2 exposed individuals by a sensitive S-based ELISA.

The current SARS-CoV-2 pandemic has highlighted a need for easy and safe blood sampling in combination with accurate serological methodology. Venipuncture is usually performed by trained staff at health care centers. Long travel distances may introduce a bias of testing towards relatively large communities with close access to health care centers. Rural regions may thus be overlooked. Here, we demonstrate a sensitive method to measure antibodies to the S-protein of SARS-CoV-2. We adapted and optimized this assay for clinical use together with capillary blood sampling to meet the geographical challenges of serosurveillance. Finally, we tested remote at-home capillary blood sampling together with centralized assessment of S-specific IgG in a rural region of northern Scandinavia that encompasses 55,185 sq kilometers. We conclude that serological assessment from capillary blood sampling gives comparable results as analysis of venous blood. Importantly, at-home sampling enabled citizens living in remote rural areas access to centralized and sensitive laboratory antibody tests.