Developing transmissible vaccines for animal infections.

Intrinsically safe designs and a staged transparent development process will be essential.

Recombinant transmissible vaccines will be intrinsically contained despite the ability to superinfect.

INTRODUCTION: Transmissible vaccines offer a novel approach to suppressing viruses in wildlife populations, with possible applications against viruses that infect humans as zoonoses - Lassa, Ebola, rabies. To ensure safety, current designs propose a recombinant vector platform in which the vector is isolated from the target wildlife population. Because using an endemic vector creates the potential for preexisting immunity to block vaccine transmission, these designs focus on vector viruses capable of superinfection, spreading throughout the host population following vaccination of few individuals. AREAS COVERED: We present original theoretical arguments that, regardless of its R0 value, a recombinant vaccine using a superinfecting vector is not expected to expand its active infection coverage when released into a wildlife population that already carries the vector. However, if superinfection occurs at a high rate such that individuals are repeatedly infected throughout their lives, the immunity footprint in the population can be high despite a low incidence of active vaccine infections. Yet we provide reasons that the above expectation is optimistic. EXPERT OPINION: High vaccine coverage will typically require repeated releases or release into a population lacking the vector, but careful attention to vector choice and vaccine engineering should also help improve transmissible vaccine utility.

Severe disease during both primary and secondary dengue virus infections in pediatric populations.

Dengue is a global epidemic causing over 100 million cases annually. The clinical symptoms range from mild fever to severe hemorrhage and shock, including some fatalities. The current paradigm is that these severe dengue cases occur mostly during secondary infections due to antibody-dependent enhancement after infection with a different dengue virus serotype. India has the highest dengue burden worldwide, but little is known about disease severity and its association with primary and secondary dengue infections. To address this issue, we examined 619 children with febrile dengue-confirmed infection from three hospitals in different regions of India. We classified primary and secondary infections based on IgM:IgG ratios using a dengue-specific enzyme-linked immunosorbent assay according to the World Health Organization guidelines. We found that primary dengue infections accounted for more than half of total clinical cases (344 of 619), severe dengue cases (112 of 202) and fatalities (5 of 7). Consistent with the classification based on binding antibody data, dengue neutralizing antibody titers were also significantly lower in primary infections compared to secondary infections (P ≤ 0.0001). Our findings question the currently widely held belief that severe dengue is associated predominantly with secondary infections and emphasizes the importance of developing vaccines or treatments to protect dengue-naive populations.

Prevention of respiratory virus transmission by resident memory CD8+ T cells.

An ideal vaccine both attenuates virus growth and disease in infected individuals and reduces the spread of infections in the population, thereby generating herd immunity. Although this strategy has proved successful by generating humoral immunity to measles, yellow fever and polio, many respiratory viruses evolve to evade pre-existing antibodies1. One approach for improving the breadth of antiviral immunity against escape variants is through the generation of memory T cells in the respiratory tract, which are positioned to respond rapidly to respiratory virus infections2-6. However, it is unknown whether memory T cells alone can effectively surveil the respiratory tract to the extent that they eliminate or greatly reduce viral transmission following exposure of an individual to infection. Here we use a mouse model of natural parainfluenza virus transmission to quantify the extent to which memory CD8+ T cells resident in the respiratory tract can provide herd immunity by reducing both the susceptibility of acquiring infection and the extent of transmission, even in the absence of virus-specific antibodies. We demonstrate that protection by resident memory CD8+ T cells requires the antiviral cytokine interferon-γ (IFNγ) and leads to altered transcriptional programming of epithelial cells within the respiratory tract. These results suggest that tissue-resident CD8+ T cells in the respiratory tract can have important roles in protecting the host against viral disease and limiting viral spread throughout the population.

The menstrual cycle regulates migratory CD4 T-cell surveillance in the female reproductive tract via CCR5 signaling.

Despite their importance for immunity against sexually transmitted infections, the composition of female reproductive tract (FRT) memory T-cell populations in response to changes within the local tissue environment under the regulation of the menstrual cycle remains poorly defined. Here, we show that in humans and pig-tailed macaques, the cycle determines distinct clusters of differentiation 4 T-cell surveillance behaviors by subsets corresponding to migratory memory (TMM) and resident memory T cells. TMM displays tissue-itinerant trafficking characteristics, restricted distribution within the FRT microenvironment, and distinct effector responses to infection. Gene pathway analysis by RNA sequencing identified TMM-specific enrichment of genes involved in hormonal regulation and inflammatory responses. FRT T-cell subset fluctuations were discovered that synchronized to cycle-driven CCR5 signaling. Notably, oral administration of a CCR5 antagonist drug blocked TMM trafficking. Taken together, this study provides novel insights into the dynamic nature of FRT memory CD4 T cells and identifies the menstrual cycle as a key regulator of immune surveillance at the site of STI pathogen exposure.

Platinum-based chemotherapy attenuates the effector response of CD8 T cells to concomitant PD-1 blockade.

PURPOSE: Combination of chemotherapy (CT) with programmed cell death (PD)-1 blockade is a front-line treatment for lung cancer. However, it remains unknown whether and how CT affects the response of exhausted CD8 T cells to PD-1 blockade. EXPERIMENTAL DESIGN: We used the well-established mouse model of T cell exhaustion with chronic lymphocytic choriomeningitis virus (LCMV) infection to assess the effect of CT (cisplatin+pemetrexed) on T cell response to PD-1 blockade, in the absence of the impact of CT on antigen release and presentation observed in tumor models. RESULTS: When concomitantly administered with PD-1 blockade, CT affected the differentiation path of LCMV-specific CD8 T cells from stem-like to transitory effector cells, thereby reducing their expansion and production of interferon (IFN)-γ. After combination treatment, these restrained effector responses resulted in impaired viral control, compared to PD-1 blockade alone. The sequential combination strategy, where PD-1 blockade followed CT, proved to be superior to the concomitant combination, preserving the proliferative response of exhausted CD8 T cells to PD-1 blockade. Our findings suggest that the stem-like CD8 T cells themselves are relatively unaffected by CT partly because they are quiescent and maintained by slow self-renewal at the steady state. However, upon the proliferative burst mediated by PD-1 blockade, the accelerated differentiation and self-renewal of stem-like cells may be curbed by concomitant CT, ultimately resulting in impaired overall CD8 T cell effector functions. CONCLUSIONS: In a translational context, we provide a proof-of-concept to consider optimizing the timing of chemo-immunotherapy strategies for improved CD8 T cell functions.

When does humoral memory enhance infection?

Antibodies and humoral memory are key components of the adaptive immune system. We consider and computationally model mechanisms by which humoral memory present at baseline might increase rather than decrease infection load; we refer to this effect as EI-HM (enhancement of infection by humoral memory). We first consider antibody dependent enhancement (ADE) in which antibody enhances the growth of the pathogen, typically a virus, and typically at intermediate 'Goldilocks' levels of antibody. Our ADE model reproduces ADE in vitro and enhancement of infection in vivo from passive antibody transfer. But notably the simplest implementation of our ADE model never results in EI-HM. Adding complexity, by making the cross-reactive antibody much less neutralizing than the de novo generated antibody or by including a sufficiently strong non-antibody immune response, allows for ADE-mediated EI-HM. We next consider the possibility that cross-reactive memory causes EI-HM by crowding out a possibly superior de novo immune response. We show that, even without ADE, EI-HM can occur when the cross-reactive response is both less potent and 'directly' (i.e. independently of infection load) suppressive with regard to the de novo response. In this case adding a non-antibody immune response to our computational model greatly reduces or completely eliminates EI-HM, which suggests that 'crowding out' is unlikely to cause substantial EI-HM. Hence, our results provide examples in which simple models give qualitatively opposite results compared to models with plausible complexity. Our results may be helpful in interpreting and reconciling disparate experimental findings, especially from dengue, and for vaccination.

Poor immunogenicity upon SARS-CoV-2 mRNA vaccinations in autoimmune SLE patients is associated with pronounced EF-mediated responses and anti-BAFF/Belimumab treatment.

Novel mRNA vaccines have resulted in a reduced number of SARS-CoV-2 infections and hospitalizations. Yet, there is a paucity of studies regarding their effectiveness on immunocompromised autoimmune subjects. In this study, we enrolled subjects naïve to SARS-CoV-2 infections from two cohorts of healthy donors (HD, n=56) and systemic lupus erythematosus (SLE, n=69). Serological assessments of their circulating antibodies revealed a significant reduction of potency and breadth of neutralization in the SLE group, only partially rescued by a 3rd booster dose. Immunological memory responses in the SLE cohort were characterized by a reduced magnitude of spike-reactive B and T cell responses that were strongly associated with poor seroconversion. Vaccinated SLE subjects were defined by a distinct expansion and persistence of a DN2 spike-reactive memory B cell pool and a contraction of spike-specific memory cTfh cells, contrasting with the sustained germinal center (GC)-driven activity mediated by mRNA vaccination in the healthy population. Among the SLE-associated factors that dampened the vaccine responses, treatment with the monoclonal antibody anti-BAFF/Belimumab (a lupus FDA-approved B cell targeting agent) profoundly affected the vaccine responsiveness by restricting the de novo B cell responses and promoting stronger extra-follicular (EF)-mediated responses that were associated with poor immunogenicity and impaired immunological memory. In summary, this study interrogates antigen-specific responses and characterized the immune cell landscape associated with mRNA vaccination in SLE. The identification of factors associated with reduced vaccine efficacy illustrates the impact of SLE B cell biology on mRNA vaccine responses and provides guidance for the management of boosters and recall vaccinations in SLE patients according to their disease endotype and modality of treatment.

Vaccine models predict rules for updating vaccines against evolving pathogens such as SARS-CoV-2 and influenza in the context of pre-existing immunity.

Currently, vaccines for SARS-CoV-2 and influenza viruses are updated if the new vaccine induces higher antibody-titers to circulating variants than current vaccines. This approach does not account for complex dynamics of how prior immunity skews recall responses to the updated vaccine. We: (i) use computational models to mechanistically dissect how prior immunity influences recall responses; (ii) explore how this affects the rules for evaluating and deploying updated vaccines; and (iii) apply this to SARS-CoV-2. Our analysis of existing data suggests that there is a strong benefit to updating the current SARS-CoV-2 vaccines to match the currently circulating variants. We propose a general two-dose strategy for determining if vaccines need updating as well as for vaccinating high-risk individuals. Finally, we directly validate our model by reanalysis of earlier human H5N1 influenza vaccine studies.

Modeling suggests that multiple immunizations or infections will reveal the benefits of updating SARS-CoV-2 vaccines.

When should vaccines to evolving pathogens such as SARS-CoV-2 be updated? Our computational models address this focusing on updating SARS-CoV-2 vaccines to the currently circulating Omicron variant. Current studies typically compare the antibody titers to the new variant following a single dose of the original-vaccine versus the updated-vaccine in previously immunized individuals. These studies find that the updated-vaccine does not induce higher titers to the vaccine-variant compared with the original-vaccine, suggesting that updating may not be needed. Our models recapitulate this observation but suggest that vaccination with the updated-vaccine generates qualitatively different humoral immunity, a small fraction of which is specific for unique epitopes to the new variant. Our simulations suggest that these new variant-specific responses could dominate following subsequent vaccination or infection with either the currently circulating or future variants. We suggest a two-dose strategy for determining if the vaccine needs updating and for vaccinating high-risk individuals.

Antibody Response to COVID-19 mRNA Vaccine in Patients With Lung Cancer After Primary Immunization and Booster: Reactivity to the SARS-CoV-2 WT Virus and Omicron Variant.

PURPOSE: To examine COVID-19 mRNA vaccine-induced binding and neutralizing antibody responses in patients with non-small-cell lung cancer (NSCLC) to SARS-CoV-2 614D (wild type [WT]) strain and variants of concern after the primary 2-dose and booster vaccination. METHODS: Eighty-two patients with NSCLC and 53 healthy volunteers who received SARS-CoV-2 mRNA vaccines were included in the study. Blood was collected longitudinally, and SARS-CoV-2-specific binding and neutralizing antibody responses were evaluated by Meso Scale Discovery assay and live virus Focus Reduction Neutralization Assay, respectively. RESULTS: A majority of patients with NSCLC generated binding and neutralizing antibody titers comparable with the healthy vaccinees after mRNA vaccination, but a subset of patients with NSCLC (25%) made poor responses, resulting in overall lower (six- to seven-fold) titers compared with the healthy cohort (P = < .0001). Although patients age > 70 years had lower immunoglobulin G titers (P = < .01), patients receiving programmed death-1 monotherapy, chemotherapy, or a combination of both did not have a significant impact on the antibody response. Neutralizing antibody titers to the B.1.617.2 (Delta), B.1.351 (Beta), and in particular, B.1.1.529 (Omicron) variants were significantly lower (P = < .0001) compared with the 614D (WT) strain. Booster vaccination led to a significant increase (P = .0001) in the binding and neutralizing antibody titers to the WT and Omicron variant. However, 2-4 months after the booster, we observed a five- to seven-fold decrease in neutralizing titers to WT and Omicron viruses. CONCLUSION: A subset of patients with NSCLC responded poorly to the SARS-CoV-2 mRNA vaccination and had low neutralizing antibodies to the B.1.1.529 Omicron variant. Booster vaccination increased binding and neutralizing antibody titers to Omicron, but antibody titers declined after 3 months. These data highlight the concern for patients with cancer given the rapid spread of SARS-CoV-2 Omicron variant.

Which 'imperfect vaccines' encourage the evolution of higher virulence?

Background and objectives: Theory suggests that some types of vaccines against infectious pathogens may lead to the evolution of variants that cause increased harm, particularly when they infect unvaccinated individuals. This theory was supported by the observation that the use of an imperfect vaccine to control Marek's disease virus in chickens resulted in the virus evolving to be more lethal to unvaccinated birds. This raises the concern that the use of some other vaccines may lead to similar pernicious outcomes. We examine that theory with a focus on considering the regimes in which such outcomes are expected. Methodology: We evaluate the plausibility of assumptions in the original theory. The previous theory rested heavily on a particular form of transmission-mortality-recovery trade-off and invoked other assumptions about the pathways of evolution. We review alternatives to mortality in limiting transmission and consider evolutionary pathways that were omitted in the original theory. Results: The regime where the pernicious evolutionary outcome occurs is narrowed by our analysis but remains possible in various scenarios. We propose a more nuanced consideration of alternative models for the within-host dynamics of infections and for factors that limit virulence. Our analysis suggests imperfect vaccines against many pathogens will not lead to the evolution of pathogens with increased virulence in unvaccinated individuals. Conclusions and implications: Evolution of greater pathogen mortality driven by vaccination remains difficult to predict, but the scope for such outcomes appears limited. Incorporation of mechanistic details into the framework, especially regarding immunity, may be requisite for prediction accuracy. Lay Summary: A virus of chickens appears to have evolved high mortality in response to a vaccine that merely prevented disease symptoms. Theory has predicted this type of evolution in response to a variety of vaccines and other interventions such as drug treatment. Under what circumstances is this pernicious result likely to occur? Analysis of the theory in light of recent changes in our understanding of viral biology raises doubts that medicine-driven, pernicious evolution is likely to be common. But we are far from a mechanistic understanding of the interaction between pathogen and host that can predict when vaccines and other medical interventions will lead to the unwanted evolution of more virulent pathogens. So, while the regime where a pernicious result obtains may be limited, caution remains warranted in designing many types of interventions.

Humoral Responses Against SARS-CoV-2 and Variants of Concern After mRNA Vaccines in Patients With Non-Hodgkin Lymphoma and Chronic Lymphocytic Leukemia.

PURPOSE: Patients with non-Hodgkin lymphoma including chronic lymphocytic leukemia (NHL/CLL) are at higher risk of severe SARS-CoV-2 infection. We investigated vaccine-induced antibody responses in patients with NHL/CLL against the original SARS-CoV-2 strain and variants of concern including B.1.167.2 (Delta) and B.1.1.529 (Omicron). MATERIALS AND METHODS: Blood from 121 patients with NHL/CLL receiving two doses of vaccine were collected longitudinally. Antibody binding against the full-length spike protein, the receptor-binding, and N-terminal domains of the original strain and of variants was measured using a multiplex assay. Live-virus neutralization against Delta, Omicron, and the early WA1/2020 strains was measured using a focus reduction neutralization test. B cells were measured by flow cytometry. Correlation between vaccine response and clinical factors was determined. RESULTS: Mean anti-SARS-CoV-2 spike immunoglobulin G-binding titers were 85-fold lower in patients with NHL/CLL compared with healthy controls, with seroconversion occurring in only 67% of patients. Neutralization titers were also lower and correlated with binding titers (P < .0001). Treatment with anti-CD20-directed therapies within 1 year resulted in 136-fold lower binding titers. Peripheral blood B-cell count also correlated with vaccine response. At 3 months from last anti-CD20-directed therapy, B-cell count ≥ 4.31/µL blood around the time of vaccination predicted response (OR 7.46, P = .04). Antibody responses also correlated with age. Importantly, neutralization titers against Delta and Omicron were reduced six- and 42-fold, respectively, with 67% of patients seropositive for WA1/2020 exhibiting seronegativity for Omicron. CONCLUSION: Antibody binding and live-virus neutralization against SARS-CoV-2 and its variants of concern including Delta and Omicron were substantially lower in patients with NHL/CLL compared with healthy vaccinees. Anti-CD20-directed therapy < 1 year before vaccination and number of circulating B cells strongly predict vaccine response.

Persistence of Virus-Specific Antibody after Depletion of Memory B Cells.

Humoral immunity is a major component of the adaptive immune response against viruses and other pathogens with pathogen-specific antibody acting as the first line of defense against infection. Virus-specific antibody levels are maintained by continual secretion of antibody by plasma cells residing in the bone marrow. This raises the important question of how the virus-specific plasma cell population is stably maintained and whether memory B cells are required to replenish plasma cells, balancing their loss arising from their intrinsic death rate. In this study, we examined the longevity of virus-specific antibody responses in the serum of mice following acute viral infection with three different viruses: lymphocytic choriomeningitis virus (LCMV), influenza virus, and vesicular stomatitis virus (VSV). To investigate the contribution of memory B cells to the maintenance of virus-specific antibody levels, we employed human CD20 transgenic mice, which allow for the efficient depletion of B cells with rituximab, a human CD20-specific monoclonal antibody. Mice that had resolved an acute infection with LCMV, influenza virus, or VSV were treated with rituximab starting at 2 months after infection, and the treatment was continued for up to a year postinfection. This treatment regimen with rituximab resulted in efficient depletion of B cells (>95%), with virus-specific memory B cells being undetectable. There was an early transient drop in the antibody levels after rituximab treatment followed by a plateauing of the curve with virus-specific antibody levels remaining relatively stable (half-life of 372 days) for up to a year after infection in the absence of memory B cells. The number of virus-specific plasma cells in the bone marrow were consistent with the changes seen in serum antibody levels. Overall, our data show that virus-specific plasma cells in the bone marrow are intrinsically long-lived and can maintain serum antibody titers for extended periods of time without requiring significant replenishment from memory B cells. These results provide insight into plasma cell longevity and have implications for B cell depletion regimens in cancer and autoimmune patients in the context of vaccination in general and especially for COVID-19 vaccines. IMPORTANCE Following vaccination or primary virus infection, virus-specific antibodies provide the first line of defense against reinfection. Plasma cells residing in the bone marrow constitutively secrete antibodies, are long-lived, and can thus maintain serum antibody levels over extended periods of time in the absence of antigen. Our data, in the murine model system, show that virus-specific plasma cells are intrinsically long-lived but that some reseeding by memory B cells might occur. Our findings demonstrate that, due to the longevity of plasma cells, virus-specific antibody levels remain relatively stable in the absence of memory B cells and have implications for vaccination.

The changing epidemiology of SARS-CoV-2.

We have come a long way since the start of the COVID-19 pandemic-from hoarding toilet paper and wiping down groceries to sending our children back to school and vaccinating billions. Over this period, the global community of epidemiologists and evolutionary biologists has also come a long way in understanding the complex and changing dynamics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19. In this Review, we retrace our steps through the questions that this community faced as the pandemic unfolded. We focus on the key roles that mathematical modeling and quantitative analyses of empirical data have played in allowing us to address these questions and ultimately to better understand and control the pandemic.

Antibody response to SARS-CoV-2 mRNA vaccine in lung cancer patients: Reactivity to vaccine antigen and variants of concern.

PURPOSE: We investigated SARS-CoV-2 mRNA vaccine-induced binding and live-virus neutralizing antibody response in NSCLC patients to the SARS-CoV-2 wild type strain and the emerging Delta and Omicron variants. METHODS: 82 NSCLC patients and 53 healthy adult volunteers who received SARS-CoV-2 mRNA vaccines were included in the study. Blood was collected longitudinally, and SARS-CoV-2-specific binding and live-virus neutralization response to 614D (WT), B.1.617.2 (Delta), B.1.351 (Beta) and B.1.1.529 (Omicron) variants were evaluated by Meso Scale Discovery (MSD) assay and Focus Reduction Neutralization Assay (FRNT) respectively. We determined the longevity and persistence of vaccine-induced antibody response in NSCLC patients. The effect of vaccine-type, age, gender, race and cancer therapy on the antibody response was evaluated. RESULTS: Binding antibody titer to the mRNA vaccines were lower in the NSCLC patients compared to the healthy volunteers (P=<0.0001). More importantly, NSCLC patients had reduced live-virus neutralizing activity compared to the healthy vaccinees (P=<0.0001). Spike and RBD-specific binding IgG titers peaked after a week following the second vaccine dose and declined after six months (P=<0.001). While patients >70 years had lower IgG titers (P=<0.01), patients receiving either PD-1 monotherapy, chemotherapy or a combination of both did not have a significant impact on the antibody response. Binding antibody titers to the Delta and Beta variants were lower compared to the WT strain (P=<0.0001). Importantly, we observed significantly lower FRNT50 titers to Delta (6-fold), and Omicron (79-fold) variants (P=<0.0001) in NSCLC patients. CONCLUSIONS: Binding and live-virus neutralizing antibody titers to SARS-CoV-2 mRNA vaccines in NSCLC patients were lower than the healthy vaccinees, with significantly lower live-virus neutralization of B.1.617.2 (Delta), and more importantly, the B.1.1.529 (Omicron) variant compared to the wild-type strain. These data highlight the concern for cancer patients given the rapid spread of SARS-CoV-2 Omicron variant.

Dynamics and turnover of memory CD8 T cell responses following yellow fever vaccination.

Understanding how immunological memory lasts a lifetime requires quantifying changes in the number of memory cells as well as how their division and death rates change over time. We address these questions by using a statistically powerful mixed-effects differential equations framework to analyze data from two human studies that follow CD8 T cell responses to the yellow fever vaccine (YFV-17D). Models were first fit to the frequency of YFV-specific memory CD8 T cells and deuterium enrichment in those cells 42 days to 1 year post-vaccination. A different dataset, on the loss of YFV-specific CD8 T cells over three decades, was used to assess out of sample predictions of our models. The commonly used exponential and bi-exponential decline models performed relatively poorly. Models with the cell loss following a power law (exactly or approximately) were most predictive. Notably, using only the first year of data, these models accurately predicted T cell frequencies up to 30 years post-vaccination. Our analyses suggest that division rates of these cells drop and plateau at a low level (0.1% per day, ∼ double the estimated values for naive T cells) within one year following vaccination, whereas death rates continue to decline for much longer. Our results show that power laws can be predictive for T cell memory, a finding that may be useful for vaccine evaluation and epidemiological modeling. Moreover, since power laws asymptotically decline more slowly than any exponential decline, our results help explain the longevity of immune memory phenomenologically.

Transition to endemicity: Understanding COVID-19.

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its associated disease, coronavirus disease 2019 (COVID-19), has caused a devastating pandemic worldwide. Here, we explain basic concepts underlying the transition from an epidemic to an endemic state, where a pathogen is stably maintained in a population. We discuss how the number of infections and the severity of disease change in the transition from the epidemic to the endemic phase and consider the implications of this transition in the context of COVID-19.

Longitudinal analysis shows durable and broad immune memory after SARS-CoV-2 infection with persisting antibody responses and memory B and T cells.

Ending the COVID-19 pandemic will require long-lived immunity to SARS-CoV-2. Here, we evaluate 254 COVID-19 patients longitudinally up to 8 months and find durable broad-based immune responses. SARS-CoV-2 spike binding and neutralizing antibodies exhibit a bi-phasic decay with an extended half-life of >200 days suggesting the generation of longer-lived plasma cells. SARS-CoV-2 infection also boosts antibody titers to SARS-CoV-1 and common betacoronaviruses. In addition, spike-specific IgG+ memory B cells persist, which bodes well for a rapid antibody response upon virus re-exposure or vaccination. Virus-specific CD4+ and CD8+ T cells are polyfunctional and maintained with an estimated half-life of 200 days. Interestingly, CD4+ T cell responses equally target several SARS-CoV-2 proteins, whereas the CD8+ T cell responses preferentially target the nucleoprotein, highlighting the potential importance of including the nucleoprotein in future vaccines. Taken together, these results suggest that broad and effective immunity may persist long-term in recovered COVID-19 patients.