Tissue-based T cell activation and viral RNA persist for up to 2 years after SARS-CoV-2 infection.

The mechanisms of postacute medical conditions and unexplained symptoms after SARS-CoV-2 infection [Long Covid (LC)] are incompletely understood. There is growing evidence that viral persistence, immune dysregulation, and T cell dysfunction may play major roles. We performed whole-body positron emission tomography imaging in a well-characterized cohort of 24 participants at time points ranging from 27 to 910 days after acute SARS-CoV-2 infection using the radiopharmaceutical agent [18F]F-AraG, a selective tracer that allows for anatomical quantitation of activated T lymphocytes. Tracer uptake in the postacute COVID-19 group, which included those with and without continuing symptoms, was higher compared with prepandemic controls in many regions, including the brain stem, spinal cord, bone marrow, nasopharyngeal and hilar lymphoid tissue, cardiopulmonary tissues, and gut wall. T cell activation in the spinal cord and gut wall was associated with the presence of LC symptoms. In addition, tracer uptake in lung tissue was higher in those with persistent pulmonary symptoms specifically. Increased T cell activation in these tissues was also observed in many individuals without LC. Given the high [18F]F-AraG uptake detected in the gut, we obtained colorectal tissue for in situ hybridization of SARS-CoV-2 RNA and immunohistochemical studies in a subset of five participants with LC symptoms. We identified intracellular SARS-CoV-2 single-stranded spike protein-encoding RNA in rectosigmoid lamina propria tissue in all five participants and double-stranded spike protein-encoding RNA in three participants up to 676 days after initial COVID-19, suggesting that tissue viral persistence could be associated with long-term immunologic perturbations.

[18F]F-AraG imaging reveals association between neuroinflammation and brown- and bone marrow adipose tissue.

Brown and brown-like adipose tissues have attracted significant attention for their role in metabolism and therapeutic potential in diabetes and obesity. Despite compelling evidence of an interplay between adipocytes and lymphocytes, the involvement of these tissues in immune responses remains largely unexplored. This study explicates a newfound connection between neuroinflammation and brown- and bone marrow adipose tissue. Leveraging the use of [18F]F-AraG, a mitochondrial metabolic tracer capable of tracking activated lymphocytes and adipocytes simultaneously, we demonstrate, in models of glioblastoma and multiple sclerosis, the correlation between intracerebral immune infiltration and changes in brown- and bone marrow adipose tissue. Significantly, we show initial evidence that a neuroinflammation-adipose tissue link may also exist in humans. This study proposes the concept of an intricate immuno-neuro-adipose circuit, and highlights brown- and bone marrow adipose tissue as an intermediary in the communication between the immune and nervous systems. Understanding the interconnectedness within this circuitry may lead to advancements in the treatment and management of various conditions, including cancer, neurodegenerative diseases and metabolic disorders.

Rapid Biphasic Decay of Intact and Defective HIV DNA Reservoir During Acute Treated HIV Disease.

Antiretroviral therapy (ART) is not a cure. Upon ART cessation, virus rapidly rebounds from latently-infected cells ("the HIV reservoir"). The reservoir is largely stabilized at the time of ART initiation and then decays slowly. Here, leveraging >500 longitudinal samples from 67 people with HIV (PWH) treated during acute infection, we developed a novel mathematical model to predict reservoir decay using the intact proviral DNA assay (IPDA) from peripheral CD4+ T cells. Nonlinear generalized additive models adjusted for initial CD4+ T count, pre-ART viral load, and timing of ART initiation demonstrated rapid biphasic decay of intact DNA (week 0-5: t1/2 ~0.71 months; week 5-24: t1/2 ~3.9 months) that extended out to 1 year of ART, with similar trends for defective DNA. Predicted reservoir decay were faster for participants individuals with earlier timing of ART initiation, higher initial CD4+ T cell count, and lower pre-ART viral load. These estimates are ~5-fold faster than prior reservoir decay estimates among chronic-treated PWH. Thus, these data add to our limited understanding of host viral control at the earliest stages of HIV reservoir stabilization, potentially informing future HIV cure efforts aimed at diverse, global population of PWH initiating ART at varying stages of disease.

Prevalence, Patterns, and Predictors of SARS-CoV-2 RNA and Culturable Virus in Tears of a Case-Ascertained Household Cohort.

PURPOSE: To investigate the prevalence, patterns, and predictors of SARS-CoV-2 RNA and culturable virus in tears of a case-ascertained household cohort. DESIGN: Prospective, longitudinal case-ascertained household cohort identified through convenience sampling. METHODS: This analysis was restricted to individuals who were non-hospitalized, symptomatic, and tested positive for SARS-CoV-2 by nasal RT-PCR. Tears and anterior nasal biospecimens were serially collected throughout the acute period. Tears specimens were collected by the study staff using Schirmer test strips, and nasal specimens were self-collected. For both, SARS-CoV-2 RNA was quantified using qRT-PCR, and culturable virus was detected using presence of cytopathic effect (CPE) in tissue culture; positive CPE was confirmed by a qRT-PCR step. A series of cross-sectional unadjusted analyses were performed investigating the relationship between different sociodemographic determinants and biological factors associated with tears RNA positivity. RESULTS: Among the 83 SARS-CoV-2 infected participants, 10 (12%) had at least one RNA-positive tears specimen. Amongst these 10, 5 (50%) had concurrent presence of culturable virus, at a median of 7 days postsymptom onset (IQR: 4-7 days) (absolute range: 4-8 days). CONCLUSIONS: In this longitudinal cohort, we found evidence of culturable virus in the tears of a small proportion of nonhospitalized SARS-CoV-2 infected individuals. Current public health infection precautions do not account for transmission via tears, so these findings may improve our understanding of potential sources of SARS-CoV-2 transmission and contribute to developing future guidelines.

Systems analysis of innate and adaptive immunity in Long COVID.

Since the onset of the COVID-19 pandemic, significant progress has been made in developing effective preventive and therapeutic strategies against severe acute SARS-CoV-2 infection. However, the management of Long COVID (LC), an infection-associated chronic condition that has been estimated to affect 5-20% of individuals following SARS-CoV-2 infection, remains challenging due to our limited understanding of its mechanisms. Although LC is a heterogeneous disease that is likely to have several subtypes, immune system disturbances appear common across many cases. The extent to which these immune perturbations contribute to LC symptoms, however, is not entirely clear. Recent advancements in multi-omics technologies, capable of detailed, cell-level analysis, have provided valuable insights into the immune perturbations associated with LC. Although these studies are largely descriptive in nature, they are the crucial first step towards a deeper understanding of the condition and the immune system's role in its development, progression, and resolution. In this review, we summarize the current understanding of immune perturbations in LC, covering both innate and adaptive immune responses, and the cytokines and analytes involved. We explore whether these findings support or challenge the primary hypotheses about LC's underlying mechanisms. We also discuss the crosstalk between various immune system components and how it can be disrupted in LC. Finally, we emphasize the need for more tissue- and subtype-focused analyses of LC, and for enhanced collaborative efforts to analyze common specimens from large cohorts, including those undergoing therapeutic interventions. These collective efforts are vital to unravel the fundaments of this new disease, and could also shed light on the prevention and treatment of the larger family of chronic illnesses linked to other microbial infections.

Mild HIV-specific selective forces overlaying natural CD4+ T cell dynamics explain the clonality and decay dynamics of HIV reservoir cells.

The latent reservoir of HIV persists for decades in people living with HIV (PWH) on antiretroviral therapy (ART). To determine if persistence arises from the natural dynamics of memory CD4+ T cells harboring HIV, we compared the clonal dynamics of HIV proviruses to that of memory CD4+ T cell receptors (TCRß) from the same PWH and from HIV-seronegative people. We show that clonal dominance of HIV proviruses and antigen-specific CD4+ T cells are similar but that the field's understanding of the persistence of the less clonally dominant reservoir is significantly limited by undersampling. We demonstrate that increasing reservoir clonality over time and differential decay of intact and defective proviruses cannot be explained by mCD4+ T cell kinetics alone. Finally, we develop a stochastic model of TCRß and proviruses that recapitulates experimental observations and suggests that HIV-specific negative selection mediates approximately 6% of intact and 2% of defective proviral clearance. Thus, HIV persistence is mostly, but not entirely, driven by natural mCD4+ T cell kinetics.

Long COVID manifests with T cell dysregulation, inflammation and an uncoordinated adaptive immune response to SARS-CoV-2.

Long COVID (LC) occurs after at least 10% of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections, yet its etiology remains poorly understood. We used 'omic" assays and serology to deeply characterize the global and SARS-CoV-2-specific immunity in the blood of individuals with clear LC and non-LC clinical trajectories, 8 months postinfection. We found that LC individuals exhibited systemic inflammation and immune dysregulation. This was evidenced by global differences in T cell subset distribution implying ongoing immune responses, as well as by sex-specific perturbations in cytolytic subsets. LC individuals displayed increased frequencies of CD4+ T cells poised to migrate to inflamed tissues and exhausted SARS-CoV-2-specific CD8+ T cells, higher levels of SARS-CoV-2 antibodies and a mis-coordination between their SARS-CoV-2-specific T and B cell responses. Our analysis suggested an improper crosstalk between the cellular and humoral adaptive immunity in LC, which can lead to immune dysregulation, inflammation and clinical symptoms associated with this debilitating condition.

Association of nirmatrelvir for acute SARS-CoV-2 infection with subsequent Long COVID symptoms in an observational cohort study.

Oral nirmatrelvir/ritonavir is approved as treatment for acute COVID-19, but the effect of treatment during acute infection on risk of Long COVID is unknown. We hypothesized that nirmatrelvir treatment during acute SARS-CoV-2 infection reduces risk of developing Long COVID and rebound after treatment is associated with Long COVID. We conducted an observational cohort study within the Covid Citizen Science (CCS) study, an online cohort study with over 100 000 participants. We included vaccinated, nonhospitalized, nonpregnant individuals who reported their first SARS-CoV-2 positive test March-August 2022. Oral nirmatrelvir/ritonavir treatment was ascertained during acute SARS-CoV-2 infection. Patient-reported Long COVID symptoms, symptom rebound and test-positivity rebound were asked on subsequent surveys at least 3 months after SARS-CoV-2 infection. A total of 4684 individuals met the eligibility criteria, of whom 988 (21.1%) were treated and 3696 (78.9%) were untreated; 353/988 (35.7%) treated and 1258/3696 (34.0%) untreated responded to the Long COVID survey (n = 1611). Among 1611 participants, median age was 55 years and 66% were female. At 5.4 ± 1.3 months after infection, nirmatrelvir treatment was not associated with subsequent Long COVID symptoms (odds ratio [OR]: 1.15; 95% confidence interval [CI]: 0.80-1.64; p = 0.45). Among 666 treated who answered rebound questions, rebound symptoms or test positivity were not associated with Long COVID symptoms (OR: 1.34; 95% CI: 0.74-2.41; p = 0.33). Within this cohort of vaccinated, nonhospitalized individuals, oral nirmatrelvir treatment during acute SARS-CoV-2 infection and rebound after nirmatrelvir treatment were not associated with Long COVID symptoms more than 90 days after infection.

Plasma-based antigen persistence in the post-acute phase of SARS-CoV-2 infection.

BACKGROUND: Persistent symptoms among some persons who develop COVID-19 has led to the hypothesis that SARS-CoV-2 may, in some form or location, persist for long periods following acute infection. Several studies have shown data in this regard but are limited by non-representative and small study populations, short duration since acute infection, and lack of a true-negative comparator group to assess assay specificity. METHODS: We evaluated adults with RNA-confirmed COVID-19 at multiple time points following acute infection (pandemic-era participants) and adults with specimens collected prior to 2020 (pre-pandemic era). Using once-thawed plasma, we employed the Simoa® (Quanterix) single molecule array detection platform to measure SARS-CoV-2 spike, S1, and nucleocapsid antigens. RESULTS: Compared to 250 pre-pandemic participants who had 2% assay positivity, detection of any SARS-CoV-2 antigen was significantly more frequent among 171 pandemic-era participants at three different time periods in the post-acute phase of infection. The absolute difference in SARS-CoV-2 plasma antigen prevalence was +11% (95% CI: +5.0% to +16%) at 3.0-6.0 months post-onset of COVID-19; +8.7% (95% CI: +3.1% to +14%) at 6.1 to 10.0 months; and +5.4% (95% CI: +0.42% to +10%) at 10.1-14.1 months. Hospitalization for acute COVID-19 and, among the non-hospitalized, worse self-reported health during acute COVID-19 were associated with greater post-acute phase antigen detection. CONCLUSIONS: Compared to uninfected persons, there is an excess prevalence of SARS-CoV-2 antigenemia in SARS-CoV-2-infected individuals up to 14 months after acute COVID-19. These findings motivate an urgent research agenda regarding the short-term and long-term clinical manifestations of this viral persistence.

The breadth of the neutralizing antibody response to original SARS-CoV-2 infection is linked to the presence of Long COVID symptoms.

The associations between longitudinal dynamics and the breadth of SARS-CoV-2 neutralizing antibody (nAb) response with various Long COVID phenotypes before vaccination are not known. The capacity of antibodies to cross-neutralize a variety of viral variants may be associated with ongoing pathology and persistent symptoms. We measured longitudinal neutralizing and cross-neutralizing antibody responses to pre- and post-SARS-CoV-2 Omicron variants in participants infected early in the COVID-19 pandemic, before widespread rollout of SARS-CoV-2 vaccines. Cross-sectional regression models adjusted for clinical covariates and longitudinal mixed-effects models were used to determine the impact of the breadth and rate of decay of neutralizing responses on the development of Long COVID symptoms, as well as Long COVID phenotypes. We identified several novel relationships between SARS-CoV-2 antibody neutralization and the presence of Long COVID symptoms. Specifically, we show that, although nAb responses to the original, infecting strain of SARS-CoV-2 were not associated with Long COVID in cross-sectional analyses, cross-neutralization ID50 levels to the Omicron BA.5 variant approximately 4 months following acute infection was independently and significantly associated with greater odds of Long COVID and with persistent gastrointestinal and neurological symptoms. Longitudinal modeling demonstrated significant associations in the overall levels and rates of decay of neutralization capacity with Long COVID phenotypes. A higher proportion of participants had antibodies capable of neutralizing Omicron BA.5 compared with BA.1 or XBB.1.5 variants. Our findings suggest that relationships between various immune responses and Long COVID are likely complex but may involve the breadth of antibody neutralization responses.

Impact of extended-course oral nirmatrelvir/ritonavir (Paxlovid) in established Long COVID: Case series and research considerations.

Background: Prior case series suggest that a 5-day course of oral Paxlovid (nirmatrelvir/ritonavir) benefits some people with Long COVID, within and/or outside of the context of an acute reinfection. To the best of our knowledge, there have been no prior case series of people with Long COVID who have attempted longer courses of nirmatrelvir/ritonavir. Methods: We documented a case series of 13 individuals with Long COVID who initiated extended courses (>5 days; range: 7.5-30 days) of oral nirmatrelvir/ritonavir outside (n=11) of and within (n=2) the context of an acute SARS-CoV-2 infection. Participants reported on symptoms and health experiences before, during, and after their use of nirmatrelvir/ritonavir. Results: Among those who took a long course of nirmatrelvir/ritonavir outside of the context of an acute infection, some experienced a meaningful reduction in symptoms, although not all benefits persisted; others experienced no effect on symptoms. One participant reported intense stomach pain that precluded her from continuing her course. Among the two participants who took a long course of nirmatrelvir/ritonavir within the context of an acute reinfection, both eventually returned to their pre-re-infection baseline. Discussion: Long courses of nirmatrelvir/ritonavir may have meaningful benefits for some people with Long COVID but not others. We encourage researchers to study who, how, and why nirmatrelvir/ritonavir benefits some and what course length is most effective, with the goal of informing clinical recommendations for using nirmatrelvir/ritonavir and/or other antivirals as a potential treatment for Long COVID.

Association of SARS-CoV-2 Infection and Cardiopulmonary Long COVID With Exercise Capacity and Chronotropic Incompetence Among People With HIV.

Background Postacute sequelae of COVID-19 (PASC) and HIV are both associated with reduced exercise capacity, but whether SARS-CoV-2 or PASC are associated with exercise capacity among people with HIV (PWH) is unknown. We hypothesized that PWH with PASC would have reduced exercise capacity from chronotropic incompetence. Methods and Results We conducted cross-sectional cardiopulmonary exercise testing within a COVID recovery cohort that included PWH with and without prior SARS-CoV-2 infection and people without HIV with prior SARS-CoV-2 infection (controls). We evaluated associations of HIV, SARS-CoV-2, and PASC with exercise capacity (peak oxygen consumption) and chronotropy (adjusted heart rate reserve). We included 83 participants (median age, 54 years; 35% women; 37 PWH): 23 out of 37 (62%) PWH and all 46 controls had prior SARS-CoV-2 infection, and 11 out of 23 (48%) PWH and 28 out of 46 (61%) without HIV had PASC. Peak oxygen consumption was reduced among PWH versus controls (80% predicted versus 99%, P=0.005), a difference of 5.5 mL/kg per minute (95% CI, 2.7-8.2; P<0.001). Chronotropic incompetence was more prevalent among PWH (38% versus 11%, P=0.002), with lower adjusted heart rate reserve (60% versus 83%, P<0.0001) versus controls. Among PWH, SARS-CoV-2 coinfection and PASC were not associated with exercise capacity. Chronotropic incompetence was more common among PWH with PASC: 7 out of 11 (64%) with PASC versus 7 out of 26 (27%) without PASC (P=0.04). Conclusions Exercise capacity and chronotropy are lower among PWH compared with individuals with SARS-CoV-2 infection without HIV. Among PWH, SARS-CoV-2 infection and PASC were not strongly associated with reduced exercise capacity. Chronotropic incompetence may be a common underrecognized mechanism of exercise intolerance among PWH, especially those with cardiopulmonary PASC.

SARS-CoV-2 reservoir in post-acute sequelae of COVID-19 (PASC).

Millions of people are suffering from Long COVID or post-acute sequelae of COVID-19 (PASC). Several biological factors have emerged as potential drivers of PASC pathology. Some individuals with PASC may not fully clear the coronavirus SARS-CoV-2 after acute infection. Instead, replicating virus and/or viral RNA-potentially capable of being translated to produce viral proteins-persist in tissue as a 'reservoir'. This reservoir could modulate host immune responses or release viral proteins into the circulation. Here we review studies that have identified SARS-CoV-2 RNA/protein or immune responses indicative of a SARS-CoV-2 reservoir in PASC samples. Mechanisms by which a SARS-CoV-2 reservoir may contribute to PASC pathology, including coagulation, microbiome and neuroimmune abnormalities, are delineated. We identify research priorities to guide the further study of a SARS-CoV-2 reservoir in PASC, with the goal that clinical trials of antivirals or other therapeutics with potential to clear a SARS-CoV-2 reservoir are accelerated.

Viral Determinants of Acute COVID-19 Symptoms in a Nonhospitalized Adult Population in the Pre-Omicron Era.

Background: The influence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA level and presence of infectious virus on symptom occurrence is poorly understood, particularly among nonhospitalized individuals. Methods: The study included 85 nonhospitalized, symptomatic adults, who were enrolled from September 2020 to November 2021. Data from a longitudinal cohort studied over 28 days was used to analyze the association of individual symptoms with SARS-CoV-2 viral RNA load, or the presence or level of infectious (culturable) virus. Presence of infectious virus and viral RNA load were assessed daily, depending on specimen availability, and amount of infectious virus was assessed on the day of maximum RNA load. Participants were surveyed for the start and end dates of 31 symptoms at enrollment and at days 9, 14, 21, and 28; daily symptom presence was determined analytically. We describe symptoms and investigate their possible association with viral determinants through a series of single or pooled (multiple days across acute period) cross-sectional analyses. Results: There was an association between viral RNA load and the same-day presence of many individual symptoms. Additionally, individuals with infectious virus were more than three times as likely to have a concurrent fever than individuals without infectious virus, and more than two times as likely to have concurrent myalgia, chills, headache, or sore throat. Conclusions: We found evidence to support the association of viral RNA load and infectious virus on some, but not all symptoms. Fever was most strongly associated with the presence of infectious virus; this may support the potential for symptom-based isolation guidance for COVID-19.

Performance of Blood-Based Nucleocapsid Antigen Tests for Diagnosis of Severe Acute Respiratory Syndrome Coronavirus 2 Infection and Infectious Viral Shedding: A Systematic Review.

Data on the performance of blood-based nucleocapsid antigen tests for diagnosing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and infectious viral shedding are limited. To address this knowledge gap, we conducted a systematic review to assess the performance of blood-based nucleocapsid (N) antigen tests in diagnosing SARS-CoV-2 infection and identifying infectiousness. This review was registered on PROSPERO (registration no. CRD42022339635). We comprehensively searched PubMed, Embase, Web of Science, and the Coronavirus Research Database for relevant studies published through 27 February 2023. Each study's risk of bias was evaluated using the Quality Assessment of Diagnostic Accuracy Studies 2 (QUADAS-2) tool. Our findings indicate that the performance of the N-antigen test is influenced by factors such as assay type, sampling timing, and illness severity. Sensitive assays provide suitable methods for viable screening and laboratory diagnostic tests in different clinical and research settings during the early phase of illness.

Multimodal Molecular Imaging Reveals Tissue-Based T Cell Activation and Viral RNA Persistence for Up to 2 Years Following COVID-19.

The etiologic mechanisms of post-acute medical morbidities and unexplained symptoms (Long COVID) following SARS-CoV-2 infection are incompletely understood. There is growing evidence that viral persistence and immune dysregulation may play a major role. We performed whole-body positron emission tomography (PET) imaging in a cohort of 24 participants at time points ranging from 27 to 910 days following acute SARS-CoV-2 infection using a novel radiopharmaceutical agent, [18F]F-AraG, a highly selective tracer that allows for anatomical quantitation of activated T lymphocytes. Tracer uptake in the post-acute COVID group, which included those with and without Long COVID symptoms, was significantly higher compared to pre-pandemic controls in many anatomical regions, including the brain stem, spinal cord, bone marrow, nasopharyngeal and hilar lymphoid tissue, cardiopulmonary tissues, and gut wall. Although T cell activation tended to be higher in participants imaged closer to the time of the acute illness, tracer uptake was increased in participants imaged up to 2.5 years following SARS-CoV-2 infection. We observed that T cell activation in spinal cord and gut wall was associated with the presence of Long COVID symptoms. In addition, tracer uptake in lung tissue was higher in those with persistent pulmonary symptoms. Notably, increased T cell activation in these tissues was also observed in many individuals without Long COVID. Given the high [18F]F-AraG uptake detected in the gut, we obtained colorectal tissue for in situ hybridization SARS-CoV-2 RNA and immunohistochemical studies in a subset of participants with Long COVID symptoms. We identified cellular SARS-CoV-2 RNA in rectosigmoid lamina propria tissue in all these participants, ranging from 158 to 676 days following initial COVID-19 illness, suggesting that tissue viral persistence could be associated with long-term immunological perturbations.

A common allele of HLA is associated with asymptomatic SARS-CoV-2 infection.

Studies have demonstrated that at least 20% of individuals infected with SARS-CoV-2 remain asymptomatic1-4. Although most global efforts have focused on severe illness in COVID-19, examining asymptomatic infection provides a unique opportunity to consider early immunological features that promote rapid viral clearance. Here, postulating that variation in the human leukocyte antigen (HLA) loci may underly processes mediating asymptomatic infection, we enrolled 29,947 individuals, for whom high-resolution HLA genotyping data were available, in a smartphone-based study designed to track COVID-19 symptoms and outcomes. Our discovery cohort (n = 1,428) comprised unvaccinated individuals who reported a positive test result for SARS-CoV-2. We tested for association of five HLA loci with disease course and identified a strong association between HLA-B*15:01 and asymptomatic infection, observed in two independent cohorts. Suggesting that this genetic association is due to pre-existing T cell immunity, we show that T cells from pre-pandemic samples from individuals carrying HLA-B*15:01 were reactive to the immunodominant SARS-CoV-2 S-derived peptide NQKLIANQF. The majority of the reactive T cells displayed a memory phenotype, were highly polyfunctional and were cross-reactive to a peptide derived from seasonal coronaviruses. The crystal structure of HLA-B*15:01-peptide complexes demonstrates that the peptides NQKLIANQF and NQKLIANAF (from OC43-CoV and HKU1-CoV) share a similar ability to be stabilized and presented by HLA-B*15:01. Finally, we show that the structural similarity of the peptides underpins T cell cross-reactivity of high-affinity public T cell receptors, providing the molecular basis for HLA-B*15:01-mediated pre-existing immunity.