Heterogeneous Longitudinal Antibody Responses to Covid-19 mRNA Vaccination.

BACKGROUND: Public health measures to stem the coronavirus disease 2019 (COVID-19) pandemic are challenged by social, economic, health status, and cultural disparities that facilitate disease transmission and amplify its severity. Prior pre-clinical biomedical technologic advances in nucleic acid-based vaccination enabled unprecedented speed of conceptualization, development, production, and widespread distribution of mRNA vaccines that target SARS-CoV-2's Spike (S) protein.
DESIGN: Twenty-five female and male volunteer fulltime employees at the Providence VA Medical Center participated in this study to examine longitudinal antibody responses to the Moderna mRNA-1273 vaccine. IgM-S and IgG-S were measured in serum using the Abbott IgM-S-Qualitative and IgG2-S-Quantitative chemiluminescent assays.
RESULTS: Peak IgM responses after Vaccine Dose #1 were delayed in 6 (24%) and absent in 7 (28%) participants. IgG2-S peak responses primarily occurred 40 to 44 days after Vaccine Dose #1, which was also 11 to 14 days after Vaccine Dose #2. However, subgroups exhibited Strong (n = 6; 24%), Normal (n = 13; 52%), or Weak (n = 6; 24%) peak level responses that differed significantly from each other (P < .005 or better). The post-peak IgG2-S levels declined progressively, and within 6 months reached the mean level measured 1 month after Vaccine Dose #1. Weak responders exhibited persistently low levels of IgG2-S. Variability in vaccine responsiveness was unrelated to age or gender.
CONCLUSION: Host responses to SARS-CoV-2-Spike mRNA vaccines vary in magnitude, duration and occurrence. This study raises concern about the lack of vaccine protection in as many as 8% of otherwise normal people, and the need for open dialog about future re-boosting requirements to ensure long-lasting immunity via mRNA vaccination versus natural infection.

Introduction

High rates of infectivity, morbidity and mortality from severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), combined with the lack of effective anti-viral therapeutics, drove the unprecedently rapid development of messenger RNA (mRNA)-based vaccines that quickly obtained emergency use authorization (EUA) from the U.S. Food and Drug Administration (FDA).[1,2] Previous work in experimental animal models provided ample data that nucleic acid-based vaccination could effectively generate host immune responses to expressed proteins.[3,4] The 2 major producers of SARS-CoV-2 mRNA vaccines, Moderna Inc. (Cambridge, MA) and Pfizer Inc. (Groton, CT), generated mRNA vaccines that express portions of the Spike (S) protein. Alternative vaccines produced by Astra-Zeneca Pharmaceuticals (Waltham, MA) and Janssen Biotech, Inc. (a Janssen Pharmaceutical Company of Johnson & Johnson, Horsham, PA) are recombinant Adenoviruses that encode the SARS-CoV-2 Spike protein. Since the Spike protein contains the SARS-CoV2 receptor binding domain, pre-formed antibodies (due to immunization) could potentially neutralize infection by blocking viral entry into target cells. In addition to structural modifications that stabilize the mRNA used for immunization, coupling the molecules with lipid protective coats enables entry into all cell types for intracellular synthesis of the spike protein. In addition, T cell activation could facilitate subsequent generation of neutralizing antibodies.

Although it has been projected that host immunity to SARS-CoV-2 mRNA-Spike vaccination would persist for 6 to 12 months, the time course and potential variability in host responses have not been adequately studied. This type of research is especially important in relation to widespread implementation of a novel disease prevention strategy to preserve public health and safety. The present study assessed longitudinal antibody responses to SARS-CoV-2 immunization in 25 female and male fully vaccinated employees at the Providence Veteran Administration Medical Center (PVAMC). The goals were to examine the time courses, peak levels, and uniformity of IgM and IgG responses.

Methods

Employees at the PVAMC volunteered to participate in a SARS-CoV-2 vaccine-associated antibody surveillance. All participants were immunized with two 100-µg doses of the Moderna COVID-19 vaccine (mRNA-1273) at the PVAMC. Vaccine Dose #1 was administered on Day 0, and Vaccine Dose #2 was given approximately 4 weeks later. Serum samples collected in 5 mL gold top tubes were obtained on the day of initial vaccination (baseline), and then sequentially at 2- to 4-week intervals up to 180 days later, including 2 weeks after the initial and second vaccine doses. The samples were coded and stored frozen for batch analysis using the chemiluminescent Abbott IgG-I-N, the IgM-S Qualitative, and the IgG II-S Quantitative assays. The IgG-I-N assay measures responses to the SARS-CoV-2 Nucleocapsid protein, whereas the IgM-S and IgG-II-S (IgG2-S) assays measure responses to the SARS-CoV-2 Spike protein. Since the vaccine immunogen only includes epitopes within the Spike protein, the IgG-I-N assay served as a negative control and index of recent natural infection. Negative results (data not shown) helped to ensure that observed responses were vaccine-driven rather than due to recent natural infection.

The antibody measurements were performed on an Abbott ci8200 Chemistry Analyzer (Chicago, IL) by PVAMC Medical Laboratory Scientists. The assay results were reported in arbitrary units (AU) as designated by the manufacturer. The manufacturer’s positive cut-off values and suggested levels of immune protection based on the IgG2-S assay are listed in Table 1. The data were graphed and analyzed using Prism Graphpad 9.11 (San Diego, CA). Box plots reflect the group mean (horizontal), 95% confidence interval limits (upper and lower boundaries of the box plots), and range (stems). Inter-group statistical comparisons were made by 1-way ANOVA with the post-hoc Tukey or linear trend tests. This study was approved by the Human Subjects Institutional Review Board (IRB) at the PVAMC. The requirement for written informed consent was waived since the procedure carried less than minimal risk and the participants were self-enrolled.

Table 1.

Threshold antibody response levels.

Assay	Cut-off (AU/mL)	Interpretation
IgG1-N Qualitative	<1.4	Negative
	⩾1.4	Positive
IgM-S Qualitative	<1.0	Negative
	⩾1.0	Positive
IgG2-S Quantitative	<50	Negative
	50-839	Low positive
	840-3999	Moderate positive
	⩾4000	High positive (immune)

Threshold positive cut-off values for IgG-1-N Qualitative and IgM-S-Qualitative, and categorical levels of antibody responsiveness based on IgG2-S Quantitative titers. The values listed were provided by the manufacturer (Abbott, Inc, Chicago, IL).

Results

Participants

The 25 participants included 18 females and 7 males between 24 and 68 years, and with a mean age (±SD) of 44.7 ± 15.1 years (Table 2). The higher proportion of women compared with men reflects the PVAMC’s employee demographics. The mean interval between Vaccine Dose #1 and Dose #2 was 29 ± 1.9 days (Range 27-35 days). The participants’ serum IgG1-N assays were negative at the time of Vaccine Dose #1 administration (data not shown), minimizing the likelihood that subsequent immune responses to SARS-CoV-2 Spike protein resulted from recent natural infection rather than vaccination.

Table 2.

Research participants.

Index	Number (%) or average ± SD (range)
Participants	25
Age	44.7 ± 15.1 (24-68)
Female	18 (72)
Male	7 (28)
Interval: Dose #1-Dose #2	29 ± 1.9 (27-35)

Characteristics of the study population.

Overview of antibody responses

The mean IgM-S antibody levels increased sharply and peaked from baseline approximately 2 weeks after Vaccine Dose #1, and then declined progressively (Figure 1A). IgG2-S antibody responses were initially detected in 24 of 25 participants 14.4 days after Vaccine Dose #1, and slightly higher mean levels were detected 29.1 days after Vaccine Dose #1, coinciding with the administration of Vaccine Dose #2 (Figure 1B). The mean peak IgG2-S responses occurred 44.2 days after Vaccine Dose #1 and 15.1 days after Vaccine Dose #2. Similar observations were reported in a double-blind phase 2 placebo-control study in which pronounced neutralizing antibody responses were generated approximately 2 weeks after Vaccine Dose #2, or 43 days from the start of immunization.

Figure 1.

Time course of (A) IgM-S and (B) IgG2-S responses to the Moderna mRNA-1273 vaccine in 25 volunteer participants. The left arrows correspond to the time of Vaccine Dose #1 (100 µg, i.m.) and the right arrows to Vaccine Dose #2 (100 µg, i.m.). Serum IgM-S and IgG2-S were measured at the plotted time points using the Abbott IgM-Spike Qualitative and IgG II-Spike Quantitative chemiluminescent assays. Analyses were performed on an Abbott Architect ci8200 Integrated Analyzer. Antibody levels are represented in arbitrary units (AU) based on standard curves. Data reflect mean (±SD) corresponding to (A) IgM-S or (B) IgG2-S levels (Y-axis) and the number of days after administrating Vaccine Dose #1 (See Table 1). Note the earlier IgM-S versus IgG2-S peak, and the progressive tapering of IgG2-S over a 6-month interval.

The mean peak IgG2-S responses coincided with IgM-S’s decline to 1.32 ± 1.22 AU/mL, which was still above the positive cut-off value of 1.0 AU. Over a 5-month interval, the mean IgG2-S progressively declined to a relatively low level, comparable to that measured at the time of Vaccine Dose #2. Note that mean IgG2-S level of 3440 ± 708 measured approximately 6 months after the initial immunization was below the suggested threshold for full immunity (Figure 1B, Table 1). Of further note is that the host immune responses to SARS-CoV-2 mRNA Spike vaccine exhibited the classical sequence of IgM followed by IgG (Figure 1).

IgM-S antibody response profiles

Graphical representation of the serum IgM-S responses to intramuscular administration of Vaccine Dose #1 revealed individual variability in the time course and peak level responses (Figure 2A). However, participants’ antibody responses clustered into 4 subgroups based on the interval from Vaccine Dose #1 to peak IgM-S as follows: Normal Responders (n = 11; 44%), Late Responders (n = 3; 12%), Latest Responders (n = 3; 12%), and Non-Responders (n = 8; 32%) (Figure 2). Non-responders were defined as participants whose IgM-S levels never reached the manufacturer’s positive threshold value (⩾1.0 AU/mL). The Normal Responders exhibited peak antibody responses approximately 2 weeks after Vaccine Dose #1 (Figure 2B). Among the Late (Figure 2C) and Latest (Figure 2D) responders, peak IgM-S levels were respectively detected 4 and 6 weeks after Vaccine Dose #1. In addition, both the overall and peak responses in the Latest Responder group were muted relative to those of Normal and Late Responders (Figure 2).

Figure 2.

Time course of individual IgM-S responses to Moderna mRNA-1273 among: (A) all participants, (B) normal responders, (C) late responders, and (D) latest responders. IgM-S levels (AU) are reflected on the Y-axis, and the number of days after Vaccine Dose #1 is represented on the X-axis. Data from non-responders are not shown. Day 0 (left arrow) corresponds to the day in which immunization was initiated. The right arrow corresponds to Vaccine Dose #2. Note the rightward shifts in IgM-S peaks from normal to the late and then the latest responders.

The mean peak IgM-S responses were significantly delayed in the Late and Latest relative to Normal Responder groups (Figure 3A). However, the difference in mean peak response time between the Late and Latest Responder groups only reached a statistical trend (P = .06). The mean peak IgM-S level was significantly higher in the Late compared with the Normal and Latest Responder groups (P < .05 or better; Figure 3B). Inter-group differences with respect to the IgM-S response times and peak levels were not linked to age (Figure 3C). Chi-square test demonstrated similar female/male ratios across all groups (χ2 = 0.54, NS; Supplemental Table 1).

Figure 3.

IgM-S sub-group responder comparisons of the (A) timing of peak responses, (B) magnitude of responses, and (C) participants’ mean age. (A) Peak responses were significantly delayed in the late and latest responder groups relative to normal responders. (B) Late responders had significantly higher peak-levels of IgM-S relative to all other groups. (C) The mean ages did not differ significantly among the groups, including non-responders (none).

IgG2-Spike antibody response profiles

The IgG2-S antibody time course graphs revealed peak responses approximately 60 days after Vaccine Dose #1 (1 month after Vaccine Dose #2), and progressive declines in antibody levels over the subsequent 150 days (Figure 4). Mean IgG2-Spike antibody levels significantly changed over time (F = 8.861; P < .0001), and post-hoc Tukey tests revealed elevated responses relative to the 30-day time point at the 60- (P = .0001) and 90- (P = .002) day intervals from Vaccine Dose #1 (Figure 4). One-way ANOVA with post-hoc linear trend analysis for declining antibody titers was statistically significant (F(1,133) = 19.95; P < .0001; R2 = .1125).

Figure 4.

Overall time-dependent trend in IgG-S responses. The mean levels of IgG2-S were calculated for measurements obtained at 30-day intervals after i.m. administration of Vaccine Dose #1. Day 30 was also the date of Vaccine Dose #2. Note that the highest protection levels were observed 60 and 90 days after vaccinations were initiated and 30 to 60 days after the second dose. One-way ANOVA with post hoc Tukey tests demonstrated significant differences from the 30-day time point. Furthermore, post hoc linear analysis was significant for progressive declines in mean IgG2-S over time, within 6 months of initial immunization (P < .0001).

A more granular appreciation of the variability and complexity of responses was revealed with overlaid individual time-course graphs (Figure 5A). In addition, subgroup analysis identified 3 main types of responders: Strong (Figure 5B), Normal (Figure 5C), and Weak (Figure 5D). The Strong responders (n = 6; 25%) exhibited peak serum IgG2-S levels above 50 000 AU/mL followed by gradual or precipitous declines. The Normal responders included most participants (n = 13; 54%). Their peak IgG2-S levels ranged between 16 977 and 43 400 AU/mL and gradually tapered toward baseline over a 120-day period. The Weak responders (n = 6; 24%) included 4 participants who had muted peak responses and 2 who had no detectable responses.

Figure 5.

Time course of individual participant IgG2-S responses to the Moderna mRNA-1273 vaccine. The levels of IgG2-S (AU, arbitrary units) are reflected on the Y-axis and the days after Vaccine Dose #1 (0 time point) are represented on the X-axis for (A) all participants, (B) strong responders, (C) normal responders, and (D) weak responders. Note the higher IgG2-S peak levels in strong versus normal, and among strong and normal relative to weak responders.

Further analysis of the sub-groups demonstrated significantly higher mean levels of IgG2-S in Strong responders compared with Normal (P = .0003) and Weak (P = .0007) responders, and significantly higher mean levels of IgG2-S among Normal compared with Weak (P = .003) Responders (Figure 6A). In addition, the Strong responders had significantly higher mean nadir IgG2-S levels than Normal (P = .004) and Weak (P = .0008) responders (Figure 6B). Significant differences in the peak IgG2-S levels were not associated with differences in the lag period between vaccination and peak response (Figure 6C), participant’s age (Figure 6D), or gender (Supplemental Table 2; χ2 = 0.9025; P > .05, NS).

Figure 6.

Subgroup differences in mean (±SD): (A) IgG2-S peak responses, (B) IgG2-S nadir responses, (C) number of days between Vaccine Dose #1 and peak responses, and (D) ages. The (A) peak and (B) nadir IgG2-S levels were significantly higher in strong compared with normal or weak responders. There were no significant inter-group differences in the (C) interval from vaccine administration to peak IgG2-S or (D) age.

Analytics of Late IgM and Weak IgG2 Spike antibody responders

As mentioned earlier, age and sex were not significantly associated with delayed or absent IgM-S responses, or Strong versus Weak IgG2-S responses. Although no demographic factors appeared to account for Weak IgG2-S responses, 5 of the 6 participants (85.7%) with Strong IgG2-S responses were known to have had a COVID-19 infection in the past or else they had been exposed to family members or close friends who tested positive and had symptomatic COVID-19 infections. Note that none of those participants had detectable IgG-N responses and therefore were very unlikely to have had a recent primary infection close to the time of vaccination.

Discussion

In the wake of increasing incident, morbidity and mortality rates caused by COVID-19 and its complications in highly vulnerable populations, beyond quarantine, masking, and hand hygiene measures, the need for additional targeted approaches to stem the crisis became an international emergency. To date, the pandemic has resulted in over 219 million infections and 4.55 million deaths (2.08% mortality) worldwide (https://www.worldometers.info/coronavirus/), and 38 million cases with 630 000 deaths (1.66% mortality) in the United States (https://www.worldometers.info/coronavirus/). Escalating rates of infection and mortality led to EUA of a large range of diagnostics ranging from high-complexity to home-based tests (https://www.fda.gov/emergency-preparedness-and-response/mcm-legal-regulatory-and-policy-framework/emergency-use-authorization). Expectations that rapid diagnostics would effectively reduce the spread of disease have been thwarted by high rates of asymptomatic infection and under-diagnosis.[7-9] Therefore, the best option going forward was to develop vaccines to protect susceptible and highly vulnerable hosts.

The strategy employed was to generate synthetic mRNA and use lipid nanoparticle technology to drive target cells throughout the body to produce SARS-CoV-2 Spike protein sequences (https://www.statnews.com/2020/11/10/the-story-of-mrna-how-a-once-dismissed-idea-became-a-leading-technology-in-the-covid-vaccine-race/) and drive a host response to generate neutralizing antibodies and memory T cells.[11,12] Ease of production and re-design in the future make mRNA vaccines attractive for both the immediate need and future vaccine development. Karikó et al, a Hungarian-born scientist, in essence gave birth to the concept, laboring indefatigably for years to demonstrate the importance of mRNA as a disease-fighting therapeutic. Subsequent collaboration with Drew Weissman, led to critical engineering of Kariko’s synthetic mRNA to a more stable hybrid molecule for widespread and indiscriminate cell entry,[4,14,15] a critical attribute of the COVID-19 mRNA vaccine. It is noteworthy that until August 2021, mRNA vaccines have never been FDA-cleared for humans, but their success and the lessons learned from this vast global experiment will pave the way for future therapies and vaccines.

This study examined longitudinal host antibody responses to the Moderna SARS-CoV-2 mRNA-Spike vaccine (mRNA-1273) in healthy adult volunteer employees of the PVAMC. One of the main findings was that responses to the vaccine varied in time course, magnitude, and duration. Regarding IgM-S, peak responses to Vaccine Dose #1 were significantly delayed in 25% and absent in 29% of participants. However, the magnitude of response was not compromised, and instead, the peak IgM-S levels were significantly higher in the Late compared with Normal and Latest responders. Together, these findings suggest that a high percentage of people, the initial vaccine dose may provide little or no early protection from COVID-19, or their immune responses will be inadequate, rendering them fully vulnerable to infection prior to Vaccine Dose #2.

IgG2-S responses were more uniform relative to the timing of Vaccine Doses #1 and #2, but they differed in magnitude among the sub-groups defined as Strong, Normal, or Weak Responders. Weak Responders had no distinctive age or sex demographics, and their IgM-S responses were not more frequently delayed or absent relative to Normal or Strong Responders. These results contrast with data from another study that reported reduced lower antibody titers to the SARS-CoV-2 BNT162b1 (Pfizer-BioNTech, Groton, CT) or mRNA-1273 (Moderna Inc., Cambridge, MA) in older age people. Subsequent studies showed that the combined effects of age and multiple co-morbid chronic disease states, or immunocompromised status in general[18,19] were linked to Weak antibody responses to COVID-19 mRNA vaccines. Additional work is needed to clarify factors that contribute to Weak vaccine responsiveness since nearly 21% of otherwise healthy PVAMC employee participants in our study had markedly muted or absent IgG2-S responses. The main concern is that across the population, a significant percentage of fully vaccinated people may never have mounted protective antibody responses, yet they are perceived to be “safe.” Regardless of the forward strategy, the fact that Weak Responders exist among fully vaccinated people argues in favor of continued surveillance, particularly as Delta variants of SARS-CoV-2 continue to spread.

Although no private health information was collected, 6 of the 7 IgG2-S Strong Responders reported having had COVID-19 or were in close contact with relatives who had documented COVID-19 within the previous 6 months. Their robust responses suggest that the vaccinations served as boosters. In alignment with this concept are previous reports of stronger responses to the SARS-CoV-2 BNT162b1 mRNA vaccine (Pfizer-BioNTech, Groton, CT) in people who had prior COVID-19.[19,20] Superior protection following natural infection was further demonstrated by stronger Spike protein-specific IgG neutralizing antibody responses to the BNT162b2 mRNA vaccine in people with previous COVID-19 infections than in naïve vaccinees. These results have been confirmed in several independent human studies[22,23] and suggest that recovered survivors of natural infection should be granted the same social/political status of COVID-19 immunity as people who have been fully vaccinated. Additional noteworthy points are that robust immune responses to either SARS-CoV-2 natural infection or the vaccine can be associated with more severe clinical symptoms. Since infection- or vaccine-induced clinical symptoms are partly mediated by cytokine activation, severity of host responses may reflect immune activation and likelihood of future immune protection.

In another report, 8 weeks after the second vaccine dose, recipient volunteers showed high levels of anti-SARS-CoV-2 spike and receptor-binding-domain binding IgM and IgG titers, plasma neutralizing activity, and specific memory B cells equivalent to those who had recovered from natural infection. However, activity against SARS-CoV-2 variants that encode E484K-, N501Y-, or K417N/E484K/N501-mutant S was reduced. Similarly, the monoclonal antibody activity elicited by the vaccines for neutralization was reduced or abolished by the K417N, E484K, or N501Y mutation. These findings suggest that monoclonal antibodies should be tested against newly arising variants, and that mRNA vaccines may need to be updated to avoid a potential loss of clinical efficacy.

Another concern revealed by our longitudinal monitoring of antibody responses is that IgG2-S titers waned within a period of 4 to 6 months. Data from Strong Responders to mRNA vaccines suggest that immunologic memory already existed due to prior natural infections, and that very low “pre-booster” IgG-S titers were not predictive of subsequent responses to SARS-CoV-2 vaccination. There is still insufficient data about the benefits of re-boosting with SARS-CoV-2 mRNA or other COVID-19 vaccines in people who have not had prior natural infection or exposure. This point is particularly relevant to public health measures that will be needed to respond to the rise in Delta variants given concerns about the neutralizing efficacy of antibodies generated to alpha variants of SARS-CoV-2. Whether booster immunizations with Spike-mRNA (the same or different sequences), a SARS-CoV-2 mRNA cocktail, or peptide/protein-based molecules would effectively circumvent lackluster immune responses to variants remains to be determined.

Initial concerns that SARS-CoV-2 adenovirus or mRNA vaccines would not prevent COVID-19 spread through respiratory droplets or aerosols due to persistence of the virus in nasal swabs and potential for transmission were thwarted by subsequent systematic experiments.[27-30] Following non-human primate immunization with Moderna mRNA-1273 which encodes the prefusion-stabilized SARS-CoV-2 spike protein encapsulated in a lipid nanoparticle, both circulating and mucosal antibody responses with anti-S IgG binding and neutralizing activity to upper respiratory infections were detected, as well as type 1 helper (Th1) based CD4 T cell responses.[16,31] Correspondingly, in a rodent model, immune responses to Moderna’s mRNA-1273 included both antibody and T-cell activation with strong CD4 cytokine responses.

One critical factor governing long-term immunity is the development of a robust T cell response. In 1 human study, the investigators showed that individuals who had a prior SARS-CoV-2 infection exhibited enhanced T cell immunity, antibody secreting memory B cell responses to Spike, and neutralizing antibodies that were effective against the B.1.1.7 and B.1.3.5.1 variants, whereas COVID-19 naïve participants who received a single vaccine dose showed reduced B and T cell responses to the same variants and that mutations further abrogated the host immune response. In contrast, in a non-randomized open-label phase I/II trial in healthy adults, 18 to 55 years of age, 2 doses of BNT162b1 elicited robust CD4(+) and CD8(+) T cell responses and strong antibody responses, with receptor binding domain IgG-binding titers above those measured in convalescent serum.

Study Limitations

Despite the many strengths of this study, there were weaknesses that could potentially limit the generalizability of the results. For example, the sample size was relatively small and with the moderately broad age distribution, we were unable to confidently assess the impact of age on host antibody response. The sex ratio was skewed such that females were recruited into the study more than twice the rate of males due to inherent sex ratio biases among our hospital employees. A third limitation is that we elected not to collect health information. Although all the employees were full-time and active with no overt disabilities or impairments that prohibited them from participating in the study, the opportunity to evaluate links to weak or absent antibody responses did not exist. It also would have been of interest to assess T cell responses. This approach was considered but deemed logistically impractical. Going forward, future studies should address these limitations in larger scale evaluations of host immune responses to the highly anticipated booster program (https://www.cdc.gov/coronavirus/2019-ncov/vaccines/booster-shot.html).

Conclusions

In conclusion, this study demonstrated heterogeneity in time course, magnitude, and duration of IgM and IgG antibody responses to the Moderna-1273 mRNA vaccine in healthy employees of the PVDMC. Vaccine responsiveness was not age- or sex-dependent. Major concerns centered on the absence of antibody responses in approximately 21% of participants, and sharp declines in serum IgG titers to Spike protein after 4 to 6 weeks, reaching levels considered non-protective. Additional studies are needed to determine how widespread Weak or absent vaccine responsiveness is in the general population, and devise methods of ensuring vaccine effectiveness. It is possible that “breakthrough” COVID-19 cases actually occur in people who never responded to the vaccines. Monitoring loss of host antibody and possibly T cell responses over time could be important for assessing vulnerability to evolving COVID-19 variants.

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Supplemental material, sj-docx-1-pat-10.1177_2632010X211049255 for Heterogeneous Longitudinal Antibody Responses to Covid-19 mRNA Vaccination by Suzanne M de la Monte, Christine Long, Nicole Szczepanski, Christopher Griffin, Amanda Fitzgerald and Kimberle Chapin in Clinical Pathology