Immunological memory to <scp>SARS‐CoV</scp>‐2 infection and <scp>COVID</scp>‐19 vaccines

Protective immunity provided by vaccines is predicated on the existence of immunological memory: the capacity of the adaptive immune system to not only recognize a novel pathogen but to also remember it. Only in the past few decades have the cellular and molecular sources of immunological memory been defined, and much remains to be determined. The three main branches of the adaptive immune system are B cells (the source of antibodies, “Abs”), CD4 T cells, and CD8 T cells. Immune memory is encoded in four main compartments of adaptive immunity: memory CD8 T cells, memory CD4 T cells, memory B cells (B Mem ), and circulating Abs 1 (Figure 1 ). There is evidence of roles for B cells (including Abs), CD4 T cells, and CD8 T cells in protective immunity to SARS‐CoV‐2, and thus, it is important to study immune memory to SARS‐CoV‐2 and COVID‐19 vaccines to understand protective immunity against COVID‐19. Because of the size and scope of immunological studies of SARS‐CoV‐2 in humans, the large number of first‐time infections, the large number of first‐time vaccinations, and the diversity of COVID‐19 vaccines developed in a short period of time, there are now more data on human antigen‐specific immune responses to SARS‐CoV‐2 than any other acute pathogen. As a result, immune memory to SARS‐CoV‐2 is now a benchmark in human immunology for understanding antigen‐specific T cell and B cell memory. Figure 1 Components of immune memory. Virus‐specific CD4 T cells, CD8 T cells, Abs, and B Mem cells constitute the four major components of immune memory to a viral infection Immune memory to SARS‐CoV‐2 can be generated by infection (classically referred to as “natural immunity”), vaccination, or hybrid immunity. Hybrid immunity is the combination of infection‐induced immunity and vaccine‐induced immunity. 2 Each of these causes of immune memory is discussed in each section of this review. Overall, immune memory from prior infection, vaccination, or hybrid immunity each have distinctive characteristics. Previous infection can generate robust immune memory, 3 , 4 including memory CD8 T cell, CD4 T cell, B Mem , durable Abs, and local immune memory (Figure 2 ). Epidemiological data on protective immunity in previously infected individuals are consistent with the immune memory measurements. Multiple large studies observe that prior infection provides approximately 80%–95% protection against symptomatic COVID‐19 reinfections for 8+ months, for SARS‐CoV‐2 ancestral strain and the Alpha through Delta VOCs, 5 , 6 , 7 , 8 , 9 , 10 , 11 and significant protection against disease with Omicron. 12 , 13 , 14 Figure 2 Kinetics of immune memory to SARS‐CoV‐2 infection and COVID‐19 vaccines. Schematics of immune memory components against SARS‐CoV‐2. (A) Memory CD8 T cells, (B) memory CD4 T cells, (C) memory T FH cells, (D) neutralizing antibodies, and (E) B Mem cells. For T cell memory, with vaccines memory is to spike, and with infection, memory is to the entire virus. For B cell memory, spike‐specific is shown in all cases. "Inf" = SARS‐CoV‐2 infected. "Hybrid" = Hybrid immunity, infected and then vaccinated. "mRNA" = Moderna mRNA‐1273 or Pfizer/BioNTech BNT162b2, a 3 dose regimen. "NVX" = Novavax NVX‐CoV2373, given as the 2‐dose regimen in the main clinical trials. "J&amp;J" = Janssen Ad26.COV2.S, given as the 1‐dose approved by EUAs. Lines are color coded by vaccine. CD8 T cell % indicates the % of individuals with detectable CD8 T cell memory at 3–6 months. Scales are non‐quantitative, but the antibody scale approximates log10 and the cellular scales approximate log2. For hybrid immunity, in this schematic, the vaccination occurs at approximately 6 months, indicated by the blue triangle. For mRNA vaccines, in this schematic, the 1st two dose are given at d1 and d21‐28, with the 3rd dose ("booster") given at approximately 8 months, indicated by the red arrows COVID‐19 vaccines, focusing for the moment on 2‐dose mRNA vaccines (Moderna mRNA‐1273, Pfizer/BioNTech BNT162b2), clearly provide high levels of protective immunity against SARS‐CoV‐2 ancestral strain and the Alpha through Delta VOCs. 9 , 15 , 16 However, the high levels of vaccine immunity against detectable SARS‐CoV‐2 infections wanes over a period of months. 17 , 18 Immunity provided by 2‐dose ChAdOx1 (AstraZeneza/Oxford ChAdOx1‐nCoV‐19 AZD1222 “ChAdOx1”) is somewhat lower and wanes faster than the mRNA vaccines. 19 With hybrid immunity, neutralizing Ab (nAb) titers and breadth of recognition of SARS‐CoV‐2 variants are dramatically higher in previously infected individuals receiving at least one dose of a COVID‐19 RNA vaccine 2 (Figure 2 ). Hybrid immunity from vaccination plus subsequent infection (breakthrough infection) also results in similarly robust immune responses. 20 , 21 Multiple epidemiological studies have now validated those immunological findings, by observing that hybrid immunity results in more robust protection against COVID‐19 than either previous infection immun

Spike‐specific CD8 T cell responses are detected in approximately 70%–90% of individuals weeks after receiving 2‐dose mRNA COVID‐19 vaccines, 76 , 77 , 78 , 79 and memory CD8 T cells are detectable in approximately 41%–65% of individuals at 6 months after the 2nd dose (7 months from 1st dose). 76 , 77 , 78 , 80 A low dose (25 μg) of mRNA‐1273 was found to generate memory CD8 T cells at similar frequencies as previous infection, comparing 6 months after the 2nd dose to 6 months after infection, indicating similar spike‐specific CD8 T cell responses between mRNA vaccination and infection. A separate study also found similar spike‐specific CD8 T cell responses at earlier times. 81 Among individuals with detectable CD8 T cell memory to mRNA vaccines months after immunization, the magnitude of the memory is generally observed to be low, 76 , 77 , 79 , 80 , 82 both in comparison with spike‐specific CD4 T cell memory, and memory to influenza. 56 There was initial confusion about whether both BNT162b2 and mRNA‐1273 mRNA COVID‐19 vaccines generated CD8 T cell responses. 43 , 83 , 84 More recently, multiple groups have observed similar CD8 T cell responses to both vaccines, 77 including in head‐to‐head comparisons. 77 , 79 , 82 Methodological differences measuring antigen‐specific T cells can result in different findings, as studies not detecting memory CD8 T cells usually utilized a less than optimal short stimulation ICS protocol, or less sensitive ELISPOT formats. The mRNA vaccine‐elicited memory CD8 T cells detected predominantly have a T EM surface phenotype, consistently express IFNγ, and have proliferative capacity. 44 , 79 The first 6 months are likely to be the period with the fastest decline in T cell memory. 36 The observation of approximately twofold declines at 6 months in spike‐specific CD8 T cell memory from peak cytokine‐positive CD8 T cell frequencies is encouraging evidence that CD8 T cell memory to mRNA vaccines is long‐lived and may last many years 76 , 78 , 79 (Figure 2 ). Memory CD4 T cells exhibit similar kinetics, as discussed in the CD4 T cell section below. The adenoviral vector vaccines ChAdOx1 and Ad26.COV2.S elicit spike‐specific CD8 T cell responses in 51%–64% of individuals in immunogenicity clinical trials, 85 though the response rate drops to 24%–36% in individuals &gt; 65 years old. 85 Stable CD8 T cell memory to Ad26.COV2.S is observed to 8 months. 86 Some comparisons between mRNA vaccines and adenoviral vector vaccines are available for CD8 T cell memory. Similar (within approximately twofold) spike‐specific CD8 T cell responses to mRNA vaccines and adenoviral vector vaccines are observed approximately 1 month after immunization, including the 1‐dose Ad26.COV2.S, 77 , 79 , 82 , 87 2‐dose Ad26.COV2.S, 87 or 2‐dose ChAdOx1. 88 , 89 , 90 Two studies that assessed CD8 T cell memory at 5+ months determined that mRNA‐1273 elicited larger spike‐specific CD8 T cell memory that Ad26.COV2.S. 77 , 79 Two others studies reported the opposite, 91 , 92 with a clinical trial reporting 45/57 Ad26.COV2.S subjects and only 20/116 BNT162b2 or mRNA‐1273 subjects positive for CD8 T cell memory. 92 In PBMC IFNγ ELISPOT assays, T cell memory 2–4 months after ChAdOx1 and BNT162b2 appears to be equivalent, though CD8 and CD4 T cells were not distinguished 93 , 94 (bulk PBMC IFNγ ELISPOT assay signal comes from a mixture of CD8 T cells and CD4 T cells. 43 ). A similar observation was made for Ad.COV2.S. 95 In sum, CD8 T cell memory to mRNA and adenoviral vector COVID‐19 vaccines appears to be similar in magnitude and % responders (Figure 2 ), but conclusions vary depending on the study. Additional head‐to‐head studies or memory are warranted, including examination of CD8 T cell functionality. Memory CD8 T cells elicited by mRNA vaccines recognize diverse spike epitopes. 50 , 51 , 77 Memory CD8 T cells largely have conserved recognition of variants, including Omicron. 77 , 81 , 87 , 96 , 97 , 98 Mix &amp; match adenoviral vector + mRNA vaccine approaches may increase CD8 T cell responses 88 , 89 , 95 and thereby may alter CD8 T cell memory. Perplexingly, lower CD8 T cell responses were reported to vaccine extended dose intervals, with the caveat that minimal CD8 T cells were measurable in any group. 99

Memory CD4 T cells are important in the control and clearance of viral infections, both directly and by the effects exerted in the support and amplification of antibody responses. Several different subsets of CD4 T cells can differentiate in antigen‐specific responses to infections. This heterogeneity is manifested at the level of different memory subsets, each associated with distinctive patterns of cytokine secretion, transcription factors, and differentiation profiles (T H 1, T H 2, T H 17, T FH, and others 102 ). This heterogeneity is further amplified by a diverse array of functional roles. T FH cells play a key role in orchestrating the development and maturation of antibody responses, 103 , 104 while Th1 and cytotoxic CD4 T cells (CD4‐CTL) can exert direct antiviral functions. 105 , 106 , 107 Multiple lines of evidence suggest that CD4 T cell is a relevant and valuable component of the overall adaptive immune response to SARS‐CoV‐2. 22 , 33 , 108 Protective effects of CD4 T cells against COVID‐19 are fully reviewed in the companion article [Goldblatt et al., this volume]. 24 3.1 CD4 T cell memory to SARS‐CoV ‐2 infection Dan et al. found that antibody, CD4 T cell, CD8 T cell, and B cell memory responses were durable over 8 months after infection, with 95% of the subjects still retaining multiple measurable memory responses, including memory CD4 T cells. 3 Notably, memory CD4 T cells are detectable in 93% of COVID‐19 cases 1 month after infection 3 and still 92% at &gt; 6 months post‐infection. 3 Multiple studies have reported similar memory CD4 T cell findings, 4 , 39 , 40 , 109 with a notable large longitudinal study. 4 The estimated t 1/2 of memory SARS‐CoV‐2‐specific CD4 T cells is 94–207 days during the first 8 months, 3 , 4 with the t 1/2 likely increasing substantially over time, based on a study determining a t 1/2 of 377 days for memory SARS‐CoV‐2‐specific CD4 T cells at 6–15 months post‐infection 110 (Figure 2 ), as well as similar data on CD8 T cell memory against a different virus. 36 These SARS‐CoV‐2 infection data are consistent with T cell memory to SARS‐CoV being detected 17 years post‐infection. 37 , 111 , 112 , 113 SARS‐CoV‐2‐specific memory CD4 T cells after SARS‐CoV‐2 infection predominantly are T H 1, T FH , and CD4‐CTL cells. 3 , 4 , 110 Eight months post‐infection, the memory CD4 T cells predominantly have central memory (T CM ) and effector memory (T EM ) surface phenotypes. 3 , 4 Virus‐specific T H 2 cells and T H 17 cells are generally not detectable. 4 , 26 , 38 Regarding the T H 1 memory cells, they are a stably maintained population, 4 predominantly expressing CD40L and IFNγ, 4 with significant expression of TNF and some expression of IL‐2. 4 , 29 , 56 , 100 , 110 , 114 The CD4‐CTL cells express CD40L and granzyme B (GzB). 4 , 79 , 115 , 116 CD4‐CTL cells are of interest because SARS‐CoV‐2‐infected epithelial cells upregulate class II expression, 117 and CD8 T cell responses appear to be low in many individuals, leaving open the possibly that memory CD4‐CTL may compensate. 117 , 118 Memory T FH cells are generated after SARS‐CoV‐2 infection and are stably maintained after a brief decline 3 , 110 , 119 (Figure 2 ). The memory T FH cells are highly functional, as they greatly enhance nAb responses to COVID‐19 vaccines. 2 , 120 , 121 Some memory T FH cells express CCR6, which is associated with lung homing. 3 , 119 Other memory T FH cells express CXCR3, which is associated with rapid anamnestic antibody responses, 104 , 122 while the CXCR3 neg memory T FH population is associated with higher quality germinal center and antibody responses. 123 , 124 Germinal centers are discussed in the B cell memory section. In terms of anatomical location of memory T cells, memory CD4 T cells are detectable in the bone marrow, spleen, lung, and multiple lymph nodes (LNs) for 6+ months after infection. 62 Memory T FH cells are observed in LNs. 62 CD4 T RM cells were present at substantial frequencies in lungs 62 and BAL 63 (Figure 3 ). Less is known regarding SARS‐CoV‐2‐specific CD4 T RM in the URT and oral cavity. Many SARS‐CoV‐2 antigens are recognized by human memory CD4 T cells in previously infected individuals, 38 with an estimated median of 19 epitopes per individual. 50 Recognized class II epitopes are distributed throughout the SARS‐CoV‐2 ORFeome, but structural proteins (spike, nucleocapsid, and M) are relatively immunodominant. 4 , 38 , 50 , 125 CD4 T cell epitopes are in general well conserved between variants. 51 , 52 , 53 SARS‐CoV‐2 CD4 T cell specificities have been recently reviewed elsewhere. 54 , 55 Human antiviral immune memory may be influenced by variables such as viral factors related to the infection event such as viral dose and tissue distribution, and host factors such as age, sex, and general health of the host. 126 A distinctive feature of SARS‐CoV‐2 infection and COVID‐19 disease is the wide range of clinical outcomes, ranging from fully asymptomatic infection to s

COVID‐19 vaccines can elicit robust CD4 T cell memory. Spike‐specific CD4 T cell responses are detected in close to 100% of individuals weeks after receiving 2‐dose mRNA COVID‐19 vaccines, 76 , 77 , 78 , 79 , 80 and memory CD4 T cells are detectable in approximately 100% of individuals at 6 months after the 2nd dose (7 months from 1st dose). 76 , 77 , 78 , 80 mRNA‐1273 generated spike‐specific memory CD4 T cell frequencies higher than seen in previously infected individuals, 79 while BNT162b2 generate spike‐specific memory CD4 T cell frequencies similar to infection. 79 , 80 A dose of mRNA‐1273 similar to that of BNT162b2 generated spike‐specific memory CD4 T cells at frequencies comparable to previous infection, 76 indicating that differences between memory CD4 T cells after the two mRNA vaccines most likely predominantly relate to the different doses of the two vaccines. Reductions in memory CD4 T cell frequencies over 6 months were modest, and half‐lives of memory CD4 T cells after mRNA COVID‐19 vaccines appear to be at least as long as after infection 76 , 79 , 80 (Figure 2 ). Memory CD4 T cells are generated after a single dose of mRNA vaccine or Ad26.COV2.S that maintained at least several months. 79 , 86 , 95 , 99 In the context of vaccine interval extension protocols, IL‐2 + memory CD4 T cells were increases in vaccinees who waited longer before the 2nd mRNA vaccine dose. 99 After vaccination, memory T H 1 cells are stably maintained, 77 , 78 , 79 , 80 , 97 and express CD40L, IFNγ, TNF, and IL‐2. 78 , 79 Memory T FH cells represent approximately 25% of CD4 T cell memory after mRNA immunization, 79 and the abundance of the cT FH cells is associated with the magnitude of the nAb response. 76 , 79 , 120 , 121 , 152 Vaccine‐elicited memory T FH cells in blood are stably maintained with minimal decline over 6+ months, 79 though the cT FH may change phenotype during the first months. 80 , 110 , 153 Active germinal center T FH (GC‐T FH ) cells are found in LNs for at least 6 months and appear to be critical for maintaining germinal centers and development of nAbs after vaccination. 152 , 153 Durable T FH cell memory in blood was observed for mRNA vaccines, Ad26.COV2.S, and NVX‐CoV2373. 79 Memory CD4‐CTL cells are also generated in response to several COVID‐19 vaccines and are stably maintained for at least 6 months. 79 Overall, each major subset of memory CD4 T cells is maintained for at least 6 months after vaccination with BNT162b2, mRNA‐1273, Ad26.COV2.S, and NVX‐CoV2373, with kinetics that indicate the memory CD4 T cells will be substantially maintained for years (Figure 2 ). After ChAdOx1‐nCoV‐19 immunization, polyfunctional T H 1 memory CD4 T cells are induced. 154 , 155 Similar (within approximately twofold) spike‐specific CD4 T cell responses to mRNA vaccines and ChAdOx1‐nCoV‐19 are observed approximately 1 month after 2 doses. 88 Immunization with 2 doses of the inactivated SARS‐CoV‐2 alum and imidazoquinolin adjuvanted vaccine BBV152 (Covaxin) generates memory CD4 T cell responses comparable to that seen in infected individuals, stably maintained for over 6 months. 156 Limited T cell memory data are available for several other vaccines, including Coronavac, Sinopharm, and Sputnik. Memory CD4 T cells elicited by mRNA vaccines recognize diverse spike epitopes. 50 , 51 , 77 Recognition of variants by memory CD4 T cells is maintained in mRNA, Ad26.COV2.S, and NVX‐CoV2373 vaccinees. 77 , 81 , 96 , 97

Human B Mem cells can be exceptionally long‐lived, with smallpox vaccine B Mem lasting &gt;50 years, 158 and B Mem cells generated from infections during the 1918 pandemic lasting at least 90 years. 159 B Mem cells are re‐activated upon an infection and are the source of classic anamnestic antibody responses. B Mem cells serve two purposes. The first is a cellular source for the anamnestic antibody response. B Mem cells can plausibly reactive and generate an anamnestic antibody response within 3–5 days. 160 The second important value of B Mem cells is to serve as a library of predictions by the immune system of possible future viral variants. 2 , 161 The COVID‐19 pandemic has dramatically demonstrated the importance of B Mem cell diversity in the recognition of a pathogen and variants, also highlighting the brilliance of the immune system at predicting viral mutations, embedding those predictions in the B Mem cell repertoire. B Mem cells likely play a role in protective immunity against SARS‐CoV‐2 infection by both of the mechanisms above, and protection by B Mem cells is reviewed in the accompanying article [Goldblatt et al.]. 24 4.1 B cell memory to SARS‐CoV ‐2 infection Detectable B Mem cells develop within two weeks of symptom onset after SARS‐CoV‐2 infection. 3 , 4 Strikingly, B Mem cell frequencies continuously increase over the course of 3–6 months post‐infection. 3 , 4 , 162 Spike‐, RBD‐, and nucleocapsid‐specific B Mem cells all exhibit this increase, in a cohort of 160 individuals. 3 SARS‐CoV‐2‐specific B Mem cell frequencies stabilize approximately 4 months post‐infection 3 and are maintained for at least 15 months 162 , 163 (Figure 2 ). These spike‐ and RBD‐binding B Mem cell frequency increases are associated with substantial somatic hypermutation (SHM) for 6 months, 162 , 164 , 165 continuing for at least 12 months. 162 The B Mem cell antibody mutations accumulated over 6–12 months demonstrated increased affinity maturation and increased neutralization potency, particularly against variants. 162 , 165 These patterns are all indicative of long‐lasting germinal centers after SARS‐CoV‐2 infection; an exception, however, is fatal COVID‐19, in which profound disruption of germinal centers can be observed in autopsies. 166 , 167 The high quality of the B Mem cells after SARS‐CoV‐2 infection is also evidenced by the anamnestic nAb responses to variants after a subsequent vaccination or infection, as discussed in the “Antibody durability” sections below. While IgM + B Mem cells initially comprise approximately 1/3rd of SARS‐CoV‐2‐specific B Mem cells, IgM + cells decline rapidly and are mostly undetectable after 5 months. 3 , 4 IgA + B Mem cells are uncommon, comprising only approximately 5% of spike or RBD‐specific B Mem cells on average, 3 , 4 , 165 but the IgA + B Mem cells are stably maintained over 8 months post‐infection, in contrast to the IgM + B Mem cells. 3 B Mem cells can have diverse phenotypes. After SARS‐CoV‐2 infection, activated SARS‐CoV‐2‐specific B Mem cell frequencies are initially high, but decline over the course of 7 months, with a reciprocal increase in resting B Mem cells. 164 COVID‐19 severity does impact the magnitude of the B Mem cell response. Patients with hospitalization‐level COVID‐19 develop higher RBD‐specific B Mem cell frequencies compared to individuals with mild COVID‐19, 3 , 164 similar to what is observed for antibody titers. 168 Asymptomatic cases develop similar Spike‐specific B Mem cell frequencies compared to symptomatic but non‐hospitalized COVID‐19 cases [Crotty manuscript in prep]. The detailed study of SARS‐CoV‐2‐specific B Mem cells in response to infection, over periods of 6–12 months, in multiple large independent cohorts, with a range of disease severities, and intensive BCR sequencing, amounts to the most detailed understanding of the development of B cell memory to any acute infection. In a small data set from two YFV vaccine (a live viral vaccine) recipients, increases in B Mem cell frequencies were observed for 6 months, increases in affinity maturation were observed for over 6 months, and declining frequencies of IgM + or activated B Mem cells were observed over 6+ months. 169 All of those features are commonalities shared with B Mem cell responses to SARS‐CoV‐2 infection. Ebola infection B Mem cell responses also have some commonalities, though the severity of Ebola disease and longevity of high viral loads may alter that response. 170 Overall, the B Mem cell response to SARS‐CoV‐2 infection is quite impressive, with substantial RBD‐ and spike‐specific B Mem cell generation; and with only exposure to a single viral strain, the B Mem cell compartment develops over several months to contain B Mem cells with high neutralization potency and B Mem cells capable of recognizing and neutralizing a range of variants. Tissue‐resident B Mem cells (B RM ) can exist in some cases. Pathogen‐specific tissue B RM have been observed in lungs of mouse models, 171

Circulating spike and RBD IgG + B Mem cell frequencies increase substantially in hybrid immunity, 80 , 162 , 181 but become similar to 2‐dose mRNA vaccination after 6 months 80 (Figure 2 ). In hybrid immunity, the RBD‐binding B Mem cells have substantially more SHM and affinity maturation than after vaccination alone. 80 , 162 , 181 Functionally, this is observed most clearly with the significantly higher potency and variant breadth of nAbs from B Mem cells in people with hybrid immunity compared to vaccination alone or infection alone. 162 , 181 The robustness and quality of these responses are likely driven by memory T FH cells and B Mem cells, and can occur after infection + vax or vax + infection (“breakthrough”), discussed in the ”Antibody durability” section below.

Two doses of an COVID‐19 mRNA vaccine are incredibly successful at eliciting high titers of nAbs. However, the biggest shortcoming of the mRNA COVID‐19 vaccines has been that the nAb titers decline continuously over a period of months. Vaccination with the mRNA‐1273 mRNA vaccine generates peak RBD IgG and SARS‐CoV‐2 nAb titers twofold higher than the BNT162b2 mRNA vaccine, on average 92 , 198 ; as a result, Ab durability analyses are confounded in studies that mix vaccinees receiving the two vaccines. In one large study of 2600 recipients of the 2‐dose BNT162b2 vaccine in Israel, RBD IgG continuously declined over 7 months from peak after the 2nd dose, with a 16‐fold reduction in RBD IgG from peak 199 (Figure 2 ). In that study, a vaccinated cohort and a previous‐infected cohort were directly compared; the RBD IgG titers declined extensively in the vaccinated individuals but were largely stable in the previously infected individuals. 199 NAb titers and RBD IgG similarly declined fivefold and 10‐fold to the mRNA‐1273 vaccine from peak over 6 months after the 2nd dose, ending with low but detectable levels of nAbs in 100% of subjects. 200 , 201 , 202 Ninefold to 10‐fold nAb declines were also observed for a low dose (25 μg) of the mRNA‐1273 mRNA COVID vaccine (instead of 100 μg), 76 comparable to the BNT162b2 dose (30 μg), indicating that the durability of the Ab responses to 2‐dose mRNA vaccines is consistent, and the kinetics are not determined by the vaccine dose, though the absolute magnitude of the Ab response is higher with a higher vaccine dose. Long‐lived plasma cells specific for SARS‐CoV‐2 spike are detectable in a majority of individuals 6 months after 2‐dose mRNA vaccination 179 ; however, given that nAb and RBD IgG titers continue to decline for at least 8 months after 2‐dose RNA vaccination, these long‐lived BPCs apparently represent low frequencies, or do not have the durability observed for B PC s generated to other antigen exposures. Due to the precipitous drop in nAb titers over 6–8 months after two doses, and the emergence of VOCs Delta and Omicron, 3‐dose mRNA vaccine regimens have been implemented as the norm in many countries (i.e., 2‐dose regimen plus a “booster” at approximately 6 months) (Figure 3 ). A critical question about 3‐dose mRNA regimens is whether they induce more durable Ab responses than 2‐dose regimens. Given that the 2‐dose mRNA vaccines were immunogenic and elicited substantial memory CD8 T cells, memory CD4 T cells, B Mem cells, and at least a few long‐lived PCs, it was reasonable to predict that 3‐dose mRNA vaccine regimens would induce substantially more durable Abs than the 2‐dose regimen. Results from previously infected individuals cleared demonstrated that the human immune system is capable of making durable Ab responses to SARS‐CoV‐2, and hybrid immunity also demonstrated that the human immune system is clearly capable of making high nAb titers to SARS‐CoV‐2. Additionally, many vaccines are three dose regimens, with durable Abs only being developed after the third dose. Teleologically, one can consider that this is because the immune system performs a cost‐benefit analysis of durable memory to each antigen exposure. Durable Ab responses for 10 years or more have a high caloric resource cost commitment, whereas durable B Mem cells or T cells have significantly lower caloric costs. As such, it frequently takes multiple antigen exposures to trigger significant durable Ab responses. Nevertheless, it was unclear if mRNA vaccines were capable of triggering durable Ab responses. Acute Ab responses to a 3rd dose of mRNA vaccine were strong, with peak nAb titers above that of 2‐dose immunization. 203 , 204 Two studies found much more durable nAb titers at 4 or 6 months after a 3rd dose of mRNA vaccine compared to 2‐doses. 202 , 205 NAb titers against Ac SARS‐CoV‐2 only declined 1.6‐fold for the BNT162b2 vaccine at 4 months and 2.3‐fold for the mRNA‐1273 vaccine at 6 months after a 3rd dose. 202 , 205 Those findings indicate robust long‐lived Ab production after 3 doses (Figure 2 ). However, not all results agree. In the study of mRNA‐1273 vaccinees, there was a substantial discordance between the durability of nAbs against Ac SARS‐CoV‐2 or Omicron, with nAbs against Ac SARS‐CoV‐2 only declining 2.3‐fold after 6 months, but nAbs against Omicron declining 6.3‐fold. 202 In a third study, of an Israeli population receiving the BNT162b2 vaccine, Ac SARS‐CoV‐2 nAbs declined 5.5‐fold over approximately 4 months after 3 doses. 206 Thus, conclusions about durability of Abs after 3‐dose mRNA vaccination remain uncertain. Adenoviral vector COVID‐19 vaccines ChAdOx1 (2‐dose) and Ad26.COV2.S (1‐dose) initially elicit substantially lower Ab responses than mRNA vaccines. Spike or RBD IgG titers after 1‐dose Ad26.COV2.S are approximately 70‐fold to 355‐fold lower than 2‐dose mRNA vaccines. 79 , 82 , 207 NAb titers are approximately 10‐ to 70‐fold lower 30 to 60 days after Ad26.COV2.S co

Studies of SARS‐CoV‐2 memory are the first time that large datasets have been collected of multiple antigen‐specific memory cell compartments over a period of 6+ months after an acute infection. This provides key opportunities to understand relationships between different aspects of immune memory. It was observed that each compartment of immune memory after infection exhibit distinct kinetics over time, and different quantitative relationships to the other compartments of immune memory. 3 Some of the relationships changed dramatically over time. 3 Perhaps most importantly from a practical perspective, serum RBD IgG titers were not quantitatively predictive of the other components of immune memory, notably memory T cells. 3 Nevertheless, other relationships were observed. 3 T FH cells, B Mem cells, and circulating Ab titers are functionally associated. 104 However, cT FH cell frequencies after infection were not quantitatively predictive of germinal centers, 153 or nAb titers, 110 suggesting more complex relationships between circulating T cell memory, germinal centers, and nAbs. For mRNA vaccines, relationships between nAbs and memory CD4 T cells are clear, and early T FH cell responses do correlate with subsequent nAb titers. 79 However, at any given memory timepoint, no clear association is observed between serum Ab titers and memory CD4 T cell and memory CD8 T cell frequencies. 79 CD4 T cells provide help for CD8 T cell differentiation and memory CD8 T cells in multiple contexts. 224 Nevertheless, memory CD4 T cell frequencies and memory CD8 T cell frequencies do not show a strong relationship in mRNA‐vaccinated individuals. 79 Overall, interrelationships between immune memory compartments exist, but much remains to be learned.

Many papers show substantial immunological and epidemiological evidence that hybrid immunity is the most robust immunity against COVID‐19. 2 , 9 This includes vax+vax+inf, 20 , 212 , 227 inf+vax, and inf+vax+vax. These individuals have the best neutralizing Ab breadth—able to recognize every single known variant, include Omicron, and even able to recognize another species of virus (SARS) 20 , 157 , 228 —and they also have substantially better local immunity in the nose and mouth, 62 , 212 which is not generated by intramuscular vaccination. They also have more durable Ab responses, based on the available data. 3 , 4 , 100 , 186 , 200 , 229 , 230 However, many governments have booster vaccination requirements within 90 days of an infection. For people who had breakthrough infections with Delta or Omicron after being double vaccinated, this is most likely to be far sooner than needed, and may be counterproductive. It is plausible that such individuals may have such good immune memory that they do not need a booster for years. The quality of the Ab response needs time to develop. The immune system has done an amazing job making Ab responses and memory B cells against SARS‐CoV‐2 that are educated guesses about potential future variants. 2 , 20 , 162 , 165 , 215 , 229 , 231 , 232 That is important for immunity against this virus, but takes time in germinal centers, 233 and it is likely disrupted by a new immunization. Hence, immunizations too close together are shortsighted and result in poorer quality immunity. We also know that memory B cell frequencies increase for almost 6 months after infection, 3 , 215 , 229 or after vaccination. 162 , 229 We know that germinal centers can persist for more than 6 months after SARS‐CoV‐2 infection. 62 We know that germinal centers can persist and be productive for more than 6 months after two doses of COVID‐19 mRNA vaccines. 180 , 234 , 235 In addition, we know that the quality of neutralizing Abs can improve over 3–6 months, 86 , 162 , 173 , 215 , 236 reflective of outcomes of these long processes of developing higher quality immune memory. Boosters too close together may disrupt those processes of generating broader protection against future variants.

SC has consulted for GSK, JP Morgan, Citi, Morgan Stanley, Avalia NZ, Nutcracker Therapeutics, University of California, California State Universities, United Airlines, and Roche. A.S. is a consultant for Gritstone Bio, Flow Pharma, ImmunoScape, Moderna, AstraZeneca, Avalia, Fortress, Repertoire, Gilead, Gerson Lehrman Group, RiverVest, MedaCorp, and Guggenheim.