Antibodies Responses to SARS-CoV-2 in a Large Cohort of Vaccinated Subjects and Seropositive Patients

Employment of a high throughput functional assay to define the critical factors that influence vaccine induced cross-variant neutralizing antibodies for SARS-CoV-2

The scale and duration of neutralizing antibody responses targeting SARS-CoV-2 viral variants represents a critically important serological parameter that predicts protective immunity for COVID-19. In this study, we describe the development and employment of a new functional assay that measures neutralizing antibodies for SARS-CoV-2 and present longitudinal data illustrating the impact of age, sex and comorbidities on the kinetics and strength of vaccine-induced antibody responses for key variants in an Asian volunteer cohort. We also present an accurate quantitation of serological responses for SARS-CoV-2 that exploits a unique set of in-house, recombinant human monoclonal antibodies targeting the viral Spike and nucleocapsid proteins and demonstrate a reduction in neutralizing antibody titres across all groups 6 months post-vaccination. We also observe a marked reduction in the serological binding activity and neutralizing responses targeting recently newly emerged Omicron variants including XBB 1.5 and highlight a significant increase in cross-protective neutralizing antibody responses following a third dose (boost) of vaccine. These data illustrate how key virological factors such as immune escape mutations combined with host demographic factors such as age and sex of the vaccinated individual influence the strength and duration of cross-protective serological immunity for COVID-19.

Use of Self-Collected Dried Blood Spots and a Multiplex Microsphere Immunoassay to Measure IgG Antibody Response to COVID-19 Vaccines

Serosurveys can determine the extent and spread of a pathogen in populations. However, collection of venous blood requires trained medical staff. Dried blood spots (DBS) are a suitable alternative because they can be self-collected and stored/shipped at room temperature. As COVID-19 vaccine deployment began in early 2021, we rapidly enrolled laboratory employees in a study to evaluate IgG antibody levels following vaccination. Participants received a DBS collection kit, self-collection instructions, and a brief questionnaire. Three DBS were collected by each of 168 participants pre- and/or postvaccination and tested with a multiplex microsphere immunoassay (MIA) that separately measures IgG antibodies to SARS-CoV-2 spike-S1 and nucleocapsid antigens. Most DBS (99.6%, 507/509) were suitable for testing. Participants with prior SARS-CoV-2 infection (n = 7) generated high S antibody levels after the first vaccine dose. Naïve individuals (n = 161) attained high S antibody levels after the second dose. Similar antibody levels were seen among those vaccinated with Moderna (n = 29) and Pfizer-BioNTech (n = 137). For those receiving either mRNA vaccine, local side effects were more common after the first vaccine dose, whereas systemic side effects were more common after the second dose. Individuals with the highest antibody levels in the week prior to the second vaccine dose experienced more side effects from the second dose. Our study demonstrated that combining self-collected DBS and a multiplex MIA is a convenient and effective way to assess antibody levels to vaccination and could easily be used for population serosurveys of SARS-CoV-2 or other emerging pathogens. IMPORTANCE Serosurveys are an essential tool for assessing immunity in a population (1, 2). However, common barriers to effective serosurveys, particularly during a pandemic, include high-costs, resources required to collect venous blood samples, lack of trained laboratory technicians, and time required to perform the assay. By utilizing self-collected dried blood spots (DBS) and our previously developed high-throughput microsphere immunoassay, we were able to significantly reduce many of these common challenges. Participants were asked to self-collect three DBS before and/or after they received their COVID-19 vaccines to measure antibody levels following vaccination. Participants successfully collected 507 DBS that were tested for IgG antibodies to the spike and nucleocapsid proteins of SARS-CoV-2. When used with self-collected DBS, our relatively low-cost assay significantly reduced common barriers to collecting serological data from a population and was able to effectively assess antibody response to vaccination.

Infection by SARS-CoV-2 with alternate frequencies of mRNA vaccine boosting

One of the most consequential unknowns of the COVID-19 pandemic is the frequency at which vaccine boosting provides sufficient protection from infection. We quantified the statistical likelihood of breakthrough infections over time following different boosting schedules with messenger RNA (mRNA)-1273 (Moderna) and BNT162b2 (Pfizer-BioNTech). We integrated anti-Spike IgG antibody optical densities with profiles of the waning of antibodies and corresponding probabilities of infection associated with coronavirus endemic transmission. Projecting antibody levels over time given boosting every 6 months, 1, 1.5, 2, or 3 years yielded respective probabilities of fending off infection over a 6-year span of >93%, 75%, 55%, 40%, and 24% (mRNA-1273) and >89%, 69%, 49%, 36%, and 23% (BNT162b2). Delaying the administration of updated boosters has bleak repercussions. It increases the probability of individual infection by SARS-CoV-2, and correspondingly, ongoing disease spread, prevalence, morbidity, hospitalization, and mortality. Instituting regular, population-wide booster vaccination updated to predominant variants has the potential to substantially forestall-and with global, widespread uptake, eliminate-COVID-19.

Methodology to estimate natural- and vaccine-induced antibodies to SARS-CoV-2 in a large geographic region

Accurate estimates of natural and/or vaccine-induced antibodies to SARS-CoV-2 are difficult to obtain. Although model-based estimates of seroprevalence have been proposed, they require inputting unknown parameters including viral reproduction number, longevity of immune response, and other dynamic factors. In contrast to a model-based approach, the current study presents a data-driven detailed statistical procedure for estimating total seroprevalence (defined as antibodies from natural infection or from full vaccination) in a region using prospectively collected serological data and state-level vaccination data. Specifically, we conducted a longitudinal statewide serological survey with 88,605 participants 5 years or older with 3 prospective blood draws beginning September 30, 2020. Along with state vaccination data, as of October 31, 2021, the estimated percentage of those 5 years or older with naturally occurring antibodies to SARS-CoV-2 in Texas is 35.0% (95% CI = (33.1%, 36.9%)). This is 3× higher than, state-confirmed COVID-19 cases (11.83%) for all ages. The percentage with naturally occurring or vaccine-induced antibodies (total seroprevalence) is 77.42%. This methodology is integral to pandemic preparedness as accurate estimates of seroprevalence can inform policy-making decisions relevant to SARS-CoV-2.

Impaired immunity and high attack rates caused by SARS‐CoV‐2 variants among vaccinated long‐term care facility residents

Introduction

Long‐term care facilities (LTCF) residents are at high risk for severe coronavirus disease 2019 (COVID‐19), and therefore, COVID‐19 vaccinations were prioritized for residents and personnel in Finland at the beginning of 2021.

Methods

We investigated COVID‐19 outbreaks in two LTCFs, where residents were once or twice vaccinated. After the outbreaks we measured immunoglobulin G (IgG) antibodies to severe acute respiratory syndrome coronavirus 2 spike glycoprotein, neutralizing antibody (NAb) titers, and cell‐mediated immunity markers from residents and healthcare workers (HCWs).

Results

In LTFC‐1, the outbreak was caused by an Alpha variant (B.1.1.7) and the attack rate (AR) among once vaccinated residents was 23%. In LTCF‐2 the outbreak was caused by a Beta variant (B.1.351). Its AR was 47% although all residents had received their second dose 1 month before the outbreak. We observed that vaccination had induced lower IgG concentrations, NAb titers and cell‐mediated immune responses in residents compared to HCWs. Only 1/8 residents had NAb to the Beta variant after two vaccine doses.

Conclusions

The vaccinated elderly remain susceptible to breakthrough infections caused by Alpha and Beta variants. The weaker vaccine response in the elderly needs to be addressed in vaccination protocols, while new variants capable of evading vaccine‐induced immunity continue to emerge.

The durability of natural infection and vaccine-induced immunity against future infection by SARS-CoV-2

The durability of natural and vaccine-mediated immunity to future SARS-CoV-2 infection and the necessity of booster vaccination are crucial knowledge for pandemic response, yet these factors remain poorly understood. Here, we use comparative evolutionary approaches to estimate the durability of immunity and the likelihood of future infections over time following vaccination by messenger RNA (mRNA) and viral vector COVID-19 vaccines. These findings provide quantitative evidence supporting booster vaccination as a crucial approach toward the curtailment of breakthrough infections and reinfections.

The durability of vaccine-mediated immunity to SARS-CoV-2, the durations to breakthrough infection, and the optimal timings of booster vaccination are crucial knowledge for pandemic response. Here, we applied comparative evolutionary analyses to estimate the durability of immunity and the likelihood of breakthrough infections over time following vaccination by BNT162b2 (Pfizer-BioNTech), mRNA-1273 (Moderna), ChAdOx1 (Oxford-AstraZeneca), and Ad26.COV2.S (Johnson & Johnson/Janssen). We evaluated anti-Spike (S) immunoglobulin G (IgG) antibody levels elicited by each vaccine relative to natural infection. We estimated typical trajectories of waning and corresponding infection probabilities, providing the distribution of times to breakthrough infection for each vaccine under endemic conditions. Peak antibody levels elicited by messenger RNA (mRNA) vaccines mRNA-1273 and BNT1262b2 exceeded that of natural infection and are expected to typically yield more durable protection against breakthrough infections (median 29.6 mo; 5 to 95% quantiles 10.9 mo to 7.9 y) than natural infection (median 21.5 mo; 5 to 95% quantiles 3.5 mo to 7.1 y). Relative to mRNA-1273 and BNT1262b2, viral vector vaccines ChAdOx1 and Ad26.COV2.S exhibit similar peak anti-S IgG antibody responses to that from natural infection and are projected to yield lower, shorter-term protection against breakthrough infection (median 22.4 mo and 5 to 95% quantiles 4.3 mo to 7.2 y; and median 20.5 mo and 5 to 95% quantiles 2.6 mo to 7.0 y; respectively). These results leverage the tools from evolutionary biology to provide a quantitative basis for otherwise unknown parameters that are fundamental to public health policy decision-making.

Neutralizing Antibodies Response against SARS-CoV-2 Variants of Concern Elicited by Prior Infection or mRNA BNT162b2 Vaccination

In order to determine the humoral protective response against SARS-CoV-2, the vaccine-induced and naturally induced neutralizing antibodies (NtAbs) responses against SARS-CoV-2 variants circulating in Italy through in vitro live virus neutralization assay were evaluated. A total of 39 SARS-CoV-2 recovered subjects (COVID-19+) and 63 subjects with a two-dose cycle of the BNT16262 vaccine were enrolled. A single serum sample was tested for COVID-19+ at 35–52 days post-positive swab, while vaccinees blood samples were taken at one (V1) and at three months (V3) after administration of the second vaccine dose. Significantly higher NtAb titers were found against B.1 and Alpha in both COVID-19+ and vaccinees, while lower NtAb titers were detected against Delta, Gamma, and Omicron variants. A comparison between groups showed that NtAb titers were significantly higher in both V1 and V3 than in COVID-19+, except against the Omicron variant where no significant difference was found. COVID-19+ showed lower neutralizing titers against all viral variants when compared to the vaccinees. Two-dose vaccination induced a sustained antibody response against each analyzed variant, except for Omicron. The evolution process of SARS-CoV-2, through variants originating from an accumulation of mutations, can erode the neutralizing effectiveness of natural and vaccine-elicited immunity. Therefore, a need for new vaccines should be evaluated to contain the ongoing pandemic.

Differing kinetics of anti-spike protein IgGs and neutralizing antibodies against SARS-CoV-2 after Comirnaty (BNT162b2) immunization

AIMS

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection has had a serious worldwide impact on human health. On December 2020, an immunization campaign with a COVID-19 mRNA vaccine (Comirnaty-BNT162b2 Pfizer-BioNTech) was started in Italy, first targeting healthcare workers (HCWs). This study aims to investigate the antibodies that are response against SARS-CoV-2 vaccine.

METHODS AND RESULTS

The kinetics and the persistence of both anti-S1/S2 IgGs and neutralizing antibodies (Nt-Abs) were investigated in 76 HCWs through a 4-month follow-up with multiple testing points starting at the first dose. Temporal analysis of SARS-CoV-2 Abs titre kinetics showed three different stages, with an initial slow growth in the anti-S1/S2 IgGs and Nt-Abs titres, corresponding to the first 4 weeks after the first dose of vaccine, followed by a second stage with peaks in titres, around 35 days after the first dose, and by a third stage (38 to 90-120 days after the first dose) showing a steady decrease in anti-S1/S2 IgGs while Nt-Abs are maintained at stable levels. Moreover, the levels of specific Nt-Abs to SARS-CoV-2 Spike protein are correlated to the anti-S1/S2 IgG titre (R-squared = 0.47; p < 0.001).

CONCLUSIONS

The levels of specific Nt-Abs to SARS-CoV-2 Spike protein are correlated to the anti-S1/S2 IgG titre, although Nt-Abs could maintain a more stable titre over the time despite declining IgG Abs titre.

SIGNIFICANCE AND IMPACT

This study highlights the kinetics and the persistence of Nt-Abs in HCWs vaccinated with Comirnaty (BNT162b2) Pfizer-BioNTech, and compared the Nt-Abs levels with anti-SARS-CoV-2 S1/S2 IgGs titres during a 4-month follow-up starting at the first dose of vaccine.

Antibody responses and correlates of protection in the general population after two doses of the ChAdOx1 or BNT162b2 vaccines

Antibody responses are an important part of immunity after Coronavirus Disease 2019 (COVID-19) vaccination. However, antibody trajectories and the associated duration of protection after a second vaccine dose remain unclear. In this study, we investigated anti-spike IgG antibody responses and correlates of protection after second doses of ChAdOx1 or BNT162b2 vaccines for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the United Kingdom general population. In 222,493 individuals, we found significant boosting of anti-spike IgG by the second doses of both vaccines in all ages and using different dosing intervals, including the 3-week interval for BNT162b2. After second vaccination, BNT162b2 generated higher peak levels than ChAdOX1. Older individuals and males had lower peak levels with BNT162b2 but not ChAdOx1, whereas declines were similar across ages and sexes with ChAdOX1 or BNT162b2. Prior infection significantly increased antibody peak level and half-life with both vaccines. Anti-spike IgG levels were associated with protection from infection after vaccination and, to an even greater degree, after prior infection. At least 67% protection against infection was estimated to last for 2-3 months after two ChAdOx1 doses, for 5-8 months after two BNT162b2 doses in those without prior infection and for 1-2 years for those unvaccinated after natural infection. A third booster dose might be needed, prioritized to ChAdOx1 recipients and those more clinically vulnerable.

Neutralizing antibodies to SARS-CoV-2 Omicron variant after third mRNA vaccination in health care workers and elderly subjects

The emergence of SARS-CoV-2 Omicron variant (B.1.1.529) with major spike protein mutations has raised concern over potential neutralization escape and breakthrough infections among vaccinated and previously SARS-CoV-2-infected subjects. We measured cross-protective antibodies against variants in health care workers (HCW, n = 20) and nursing home residents (n = 9) from samples collected at 1-2 months, following the booster (3rd) dose. We also assessed the antibody responses in subjects infected before the Omicron era (n = 38) with subsequent administration of a single mRNA vaccine dose. Following booster vaccination, HCWs had high IgG antibody concentrations to the spike protein and neutralizing antibodies (NAb) were detectable against all variants. IgG concentrations among the elderly remained lower, and some lacked NAbs against the Beta and Omicron variants. NAb titers were significantly reduced against Delta, Beta, and Omicron compared to WT virus regardless of age. Vaccination induced high IgG concentrations and variable titers of cross-reactive NAbs in previously infected subjects, whereas NAb titers against Omicron were barely detectable 1 month postinfection. High IgG concentrations with cross-protective neutralizing activity were detected after three Coronavirus Disease 2019 (COVID-19) vaccine doses in HCWs. However, lower NAb titers seen in the frail elderly suggest inadequate protection against Omicron breakthrough infections, yet protection against severe COVID-19 is expected.

Early and Longitudinal Humoral Response to the SARS-CoV-2 mRNA BNT162b2 Vaccine in Healthcare Workers: Significance of BMI, Adipose Tissue and Muscle Mass on Long-Lasting Post-Vaccinal Immunity

BACKGROUND

This study aimed to investigate the early and longitudinal humoral response in Healthcare Workers (HCWs) after two doses of the BNT162b2 vaccine and to assess the association between metabolic and anthropometric parameters and the humoral response after vaccination.

METHODS

The study included 243 fully vaccinated HCWs: 25.50% previously infected with SARS-CoV-2 (with prior history of COVID-19-PH) and 74.40%-uninfected, seronegative before the first vaccination (with no prior history of COVID-19-NPH). IgG antibodies were measured, and sera were collected: prior to the vaccination, 21 days after the first dose, and 14 days and 8 months after the second dose.

RESULTS

21 days after the first dose, 90.95% of individuals were seropositive; 14 days after the second dose, persistent immunity was observed in 99.18% HCWs, 8 months after complete vaccination-in 61.73%. Statistical analysis revealed that HCWs with PH had a greater chance of maintaining a humoral response beyond eight months after vaccination. Increased muscle mass, decreased fat mass, and younger age may positively affect long-term immunity. Smokers have a reduced chance of developing immunity compared to non-smokers.

CONCLUSIONS

Fully vaccinated HCWs with PH are more likely to be seropositive than fully inoculated volunteers with NPH.

Waning Humoral Response after COVID-19 mRNA Vaccination in Maintenance Dialysis Patients and Recovery after a Complementary Third Dose

The aim of this study was to analyze the waning of anti-spike (S) antibodies after mRNA vaccination against COVID-19 in maintenance dialysis patients, and to assess the safety and effectiveness of the complementary third dose. This was a prospective, longitudinal study in which we analyzed the kinetics of antibodies up to six months after a two-dose vaccination (first protocol) in infection-naïve dialysis patients (IN-Ds), previously infected dialysis patients (PI-Ds) and subjects without chronic kidney disease (the controls), as well as their humoral response to the third dose of the same mRNA vaccine (second protocol). The respective reduction in antibody titer after 3 and 6 months by 82.9% and 93.03% in IN-Ds (n = 109), 73.4% and 93.36% in PI-Ds (n = 32) and 75.5% and 88.8% in the controls (n = 20) was demonstrated. Consequently, a protective antibody titer above 141 BAU/mL was found in only 47.7% and 23.8% of IN-Ds after 3 and 6 months, respectively. After the third vaccine dose, a significant increase in antibody titer was observed in all groups, with increases by a factor of ×51.6 in IN-Ds, ×30.1 in the controls and ×8.4 in PI-Ds. The median antibody titer after the third dose differed significantly between groups, and was the highest in PI-Ds: PI-Ds, 9090 (3300−15,000) BAU/mL; the controls, 6945 (2130−11,800); IN-Ds, 3715 (1470−7325) (p < 0.001). In conclusion, we observed similar degrees of antibody waning in all patients. After 3 months, over half of the infection-naïve dialysis patients had a very low antibody titer, and almost twenty percent of them had no antibodies at all. The humoral response to the third dose was very good, raising their titer of antibodies to a higher level than those in the general population who have received the primary two-dose scheme. The results support the administration of a complementary third dose of the mRNA vaccine for dialysis patients as soon as possible.

Decline of Anti-SARS-CoV-2 IgG Antibody Levels 6 Months after Complete BNT162b2 Vaccination in Healthcare Workers to Levels Observed Following the First Vaccine Dose

Research on post-vaccination antibody dynamics has become pivotal in estimating COVID-19 vaccine efficacy. We studied anti-SARS-CoV-2 Spike RBD IgG levels in 587 healthcare workers (2038 sera) who completed BNT162b2 vaccination. Average antibody titer 3 weeks after the first dose in COVID-19-naïve participants (median 873.5 AU/mL) was 18-fold higher than the test threshold, with a significant increase 1 month (median 9927.2 AU/mL) and an exponential decrease 3 (median 2976.7 AU/mL) and 6 (median 966.0 AU/mL) months after complete vaccination. Participants with a history of COVID-19 prior to vaccination showed significantly higher antibody levels, particularly after the first dose (median 14,280.2 AU/mL), with a slight decline 1 month (median 12,700.0 AU/mL) and an exponential decline in antibody titers 3 (median 4831.0 AU/mL) and 6 (median 1465.2 AU/mL) months after vaccination. Antibody levels of COVID-19-naïve subjects after the first dose were moderately correlated with age (r = −0.4). Multivariate analysis showed a strong independent correlation between IgG levels 6 months after vaccination and both IgG titers after the first dose and 1 month after vaccination (R² = 0.709). Regardless of pre-vaccination COVID-19 history, IgG levels 6 months after vaccination were comparable to antibody levels reached by COVID-19-naïve participants after the first vaccine dose.

Physicians' Experience with COVID-19 Vaccination: A Survey Study

BACKGROUND

COVID-19 vaccine hesitancy remains high in the general population and is the main determinant of low vaccination rates and of the fourth pandemic wave severity in Romania. Additional information is needed to raise awareness over vaccine efficiency and the safety profile.

OBJECTIVE

To assess self-reported experience related to COVID-19 vaccination in Romanian physicians.

DESIGN, SETTING, AND PARTICIPANTS

A cross-sectional, self-administered questionnaire-based survey, distributed online in the period 24 March to 24 May 2021. The survey included 30 cascade questions with skip logic filters. All physicians included filled in the questionnaire voluntarily and anonymously. Not all respondents filled in all questions. Main outcome and measure: Primary outcomes addressed were related to the COVID-19 vaccine safety profile.

RESULTS

407/467 (87.15%) of the respondents' physicians were fully vaccinated, mostly with the Pfizer-BioNTech (Comirnaty)-BNT162b2 vaccine, with the peak of immunization in January 2021, with almost four-fifths of the study participants. Regarding COVID-19, almost 20% physicians had the infection and one declared COVID-19 re-infection. A number of 48/420 (11.42%) and 47/419 (11.22%) of the vaccinated physicians did not report any side effects after the first or second vaccine dose. However, most of the side effects reported were minor. Only 50/360 (13.88%) physicians reported the vaccine side effects on the dedicated online national platform. Approximately 40% respondents checked the anti-spike SARS-CoV2 antibodies' titer after complete vaccination, of which two cases reported indeterminate levels. Lower anti-spike SARS-CoV2 antibodies' titer of 100-1000 times the laboratory limit was more frequent in naive physicians (36.95% versus 14.28%, p = 0.012), moderate titers were similar, while very high levels, more than 10,000 times laboratory limit, were more frequent in physicians with previous COVID-19 infection (2.17% versus 42.85%, p < 0.001).

CONCLUSIONS AND RELEVANCE

In this cross-sectional survey study on the COVID-19 vaccination among Romanian physicians, we describe a safety vaccination profile among Romanian physicians.

Serological Response to SARS-CoV-2 Messenger RNA Vaccine: Real-World Evidence from Italian Adult Population

BACKGROUND

This study aims to investigate the extent of the BNT162b2 mRNA vaccine-induced antibodies against SARS-CoV-2 in a large cohort of Italian subjects belonging to the early vaccinated cohort in Italy.

METHODS

A prospective study was conducted between December 2020 and May 2021. Three blood samples were collected for each participant: one at the time of the first vaccine dose (T0), one at the time of the second vaccine dose, (T1) and the third 30 days after this last dose (T2).

RESULTS

We enrolled 2591 fully vaccinated subjects; 16.5% were frail subjects, and 9.8% were over 80 years old. Overall, 98.1% of subjects were seropositive when tested at T2, and 76.3% developed an anti-S IgG titer ≥4160 AU/mL, which is adequate to develop viral neutralizing antibodies. Seronegative subjects at T1 were more likely to remain seronegative at T2 or to develop a low-intermediate anti-S IgG titer (51-4159 AU/mL).

CONCLUSIONS

In summary, vaccination leads to detectable anti-S IgG titer in nearly all vaccine recipients. Stratification of the seroconversion level could be useful to promptly identify high-risk groups who may not develop a viral neutralizing response, even in the presence of seroconversion, and therefore may remain at higher risk of infection, despite vaccination.

Antibody Persistence 6 Months Post-Vaccination with BNT162b2 among Health Care Workers

BACKGROUND

We present immunogenicity data 6 months after the first dose of BNT162b2 in correlation with age, gender, BMI, comorbidities and previous SARS-CoV-2 infection.

METHODS

An immunogenicity evaluation was carried out among health care workers (HCW) vaccinated at the Istituti Fisioterapici Ospitalieri (IFO). All HCW were asked to be vaccine by the national vaccine campaign at the beginning of 2021. Serum samples were collected on day 1 just prior to the first dose of the vaccine and on day 21 just prior to the second vaccination dose. Thereafter sera samples were collected 28, 49, 84 and 168 days after the first dose of BNT162b2. Quantitative measurement of IgG antibodies against S1/S2 antigens of SARS-CoV-2 was performed with a commercial chemiluminescent immunoassay.

RESULTS

Two hundred seventy-four HWCs were analyzed, 175 women (63.9%) and 99 men (36.1%). The maximum antibody geometric mean concentration (AbGMC) was reached at T2 (299.89 AU/mL; 95% CI: 263.53-339.52) with a significant increase compared to baseline (p < 0.0001). Thereafter, a progressive decrease was observed. At T5, a median decrease of 59.6% in COVID-19 negative, and of 67.8% in COVID-19 positive individuals were identified with respect to the highest antibody response. At T1, age and previous COVID-19 were associated with differences in antibody response, while at T2 and T3 differences in immune response were associated with age, gender and previous COVID-19. At T4 and T5, only COVID-19 positive participants demonstrated a greater antibody response, whereas no other variables seemed to influence antibody levels.

CONCLUSIONS

Overall our study clearly shows antibody persistence at 6 months, albeit with a certain decline. Thus, the use of this vaccine in addressing the COVID-19 pandemic is supported by our results that in turn open debate about the need for further boosts.