Effectiveness of bivalent mRNA booster vaccination against SARS-CoV-2 Omicron infection, the Netherlands, September to December 2022

Second booster doses of adenoviral- and mRNA-based COVID-19 vaccines increase protection against COVID-19 hospitalization: Final analysis from the REFORCO-Brazil real-world effectiveness study during Omicron

BACKGROUND

Booster doses of COVID-19 vaccines are required to maintain protection against SARS-CoV-2. However, real-world evidence from South America, needed to inform optimal vaccination strategies, is lacking. Herein, we present the final analysis of REFORCO-Brazil, a large-scale assessment of relative vaccine effectiveness (rVE) of second boosters (vs first boosters) against hospitalization with COVID-19.

METHODS

REFORCO-Brazil is a test-negative case-control study (NCT05697705) that utilized Brazilian national data on severe acute respiratory syndrome (SARS) surveillance and COVID-19 vaccination. Individuals hospitalized with SARS from January 1 to December 31, 2022, were classified as test-positive cases (via SARS-CoV-2 antigen/reverse transcription polymerase chain reaction [RT-PCR]) or test-negative case-controls (via RT-PCR) and matched by admission date, region, sex, preceding COVID-19 vaccinations, and age. We used conditional logistic regression combined with multiple covariate adjustments to estimate rVE for second boosters (versus first boosters received ≥4 months prior) overall, by type (AZD1222, Ad26.COV2·S, BNT162b2, and CoronaVac) and in vulnerable subgroups (elderly and immunocompromised/high-risk individuals).

RESULTS

Median (range) time between second booster and SARS hospitalization was 87.0 (8.0-307.0) and 79.0 (8.0-303.0) days among 5426 test-positive cases and 6131 test-negative controls, respectively. Overall rVE of any second booster against hospitalization was 18.7 % (95 % confidence interval [CI]: 10.5-26.1). The rVE of adenoviral- and mRNA-based vaccines was similar; 18.2 % (4.8-29.8) for AZD1222, 20.7 % (10.2-30.0) for Ad26.COV2·S, and 23.2 % (9.7-34.7) for BNT162b2. Similar levels of added protective benefit, or 'boosting', was observed in very elderly and immunocompromised/high-risk individuals. Additional protection was highest within 2 months post-dosing, decreasing thereafter. Exploratory analyses revealed increased protection against severe in-hospital outcomes, including mortality.

CONCLUSIONS

Our results support the use of monovalent adenoviral/mRNA-based vaccine maintain protection against COVID-19 hospitalization from Omicron subvariants. However, optimal timing of booster vaccinations will need to be carefully considered for future booster strategies, especially among vulnerable subgroups.

Effectiveness of mRNA COVID-19 Vaccines and Hybrid Immunity in Preventing SARS-CoV-2 Infection and Symptomatic COVID-19 Among Adults in the United States

BACKGROUND

Understanding protection against SARS-CoV-2 infection by vaccine and hybrid immunity is important for informing public health strategies as new variants emerge.

METHODS

We analyzed data from 3 cohort studies spanning 1 September 2022 to 31 July 2023 to estimate COVID-19 vaccine effectiveness (VE) against SARS-CoV-2 infection and symptomatic COVID-19 among adults with and without prior infection in the United States. Participants collected weekly nasal swabs irrespective of symptoms, participated in annual blood draws, and completed periodic surveys, which included vaccination status and infection history. Swabs were tested molecularly for SARS-CoV-2. VE was estimated by Cox proportional hazards models for the hazard ratios of infections, adjusting for covariates. VE was calculated considering prior infection and recency of vaccination.

RESULTS

Among 3344 adults, the adjusted VE of a bivalent vaccine against infection was 37.2% (95% CI, 12.3%-55.7%) within 7 to 59 days of vaccination and 21.1% (95% CI, -0.5% to 37.1%) within 60 to 179 days of vaccination when compared with participants who were unvaccinated or had received an original monovalent vaccine dose ≥180 days prior. Overall, the adjusted VE of a bivalent vaccine against infection, in conjunction with prior infection, was 62.2% (95% CI, 46.0%-74.5%) within 7 to 179 days of vaccination and 39.4% (95% CI, 12.5%-61.6%) at ≥180 days when compared with naive participants who were unvaccinated or had received a monovalent vaccine dose ≥180 days prior.

CONCLUSIONS

Adults with prior infection and recent vaccination had high protection against infection and symptomatic illness. Recent vaccination alone provided moderate protection.

Using SARS-CoV-2 nucleoprotein antibodies to detect (re)infection

We assessed the validity of serum total anti-nucleoprotein Immunoglobulin (N-antibodies) to identify SARS-CoV-2 (re)infections by estimating the persistence of N-antibody seropositivity and boosting following infection. From a prospective Dutch cohort study (VASCO), we included adult participants with ≥2 consecutive self-collected serum samples, 4-8 months apart, between May 2021-May 2023. Sample pairs were stratified by N-seropositivity of the first sample and by self-reported infection within the sampling interval. We calculated the proportions of participants with N-seroconversion and fold-increase (1.5, 2, 3, 4) of N-antibody concentration over time since infection and explored determinants. We included 67,632 sample pairs. Pairs with a seronegative first sample (70%) showed 89% N-seroconversion after reported infection and 11% when no infection was reported. In pairs with a seropositive first sample (30%), 82%-65% showed a 1.5- to 4-fold increase with a reported reinfection, and 19%-10% without a reported reinfection, respectively. After one year, 83% remained N-seropositive post-first infection and 93%-61% showed a 1.5-fold to 4-fold increase post-reinfection. Odds for seroconversion/fold increase were higher for symptomatic infections and Omicron infections. In the current era with limited antigen or PCR testing, N-serology can be validly used to detect SARS-CoV-2 (re)infections at least up to a year after infection, supporting the monitoring of COVID-19 burden and vaccine effectiveness.

SARS-CoV-2 seroprevalence among healthcare workers in a highly vaccinated Japanese medical center from 2020-2023

Infection-induced SARS-CoV-2 seroprevalence has been studied worldwide. At Juntendo University Hospital (JUH) in Tokyo, Japan, we have consistently conducted serological studies using the blood residue of healthcare workers (HCWs) at annual health examinations since 2020. In this 2023 study (n = 3,594), N-specific seroprevalence (infection-induced) was examined while univariate and multivariate logistic regression analyses were performed to compute ORs of seroprevalence with respect to basic characteristics of participants. We found that the N-specific seroprevalence in 2023 was 54.1%-a jump from 17.7% in 2022, and 1.6% in 2021-with 37.9% as non-PCR-confirmed asymptomatic infection cases. Those younger than 50 (adjusted OR = 1.62; p < .001) and recipients with 4 doses or less of vaccine had a higher risk to be N-positive, ranging from 1.45 times higher for the participants with 4 doses (p < .001) to 4.31 times higher for the participants with 1 dose (p < .001), compared to those with 5 or more doses. Our findings indicate that robust vaccination programs may have helped alleviate symptoms but consequently caused asymptomatic spread in this hospital, especially among younger HCWs. Although having four doses or less was found to be associated with higher risk of infection, the optimal constitution and intervals for effective booster vaccines warrant further investigations.

Impact of Repeated Variant Exposures on Cellular and Humoral Immunogenicity Induced by SARS-CoV-2 Vaccines

BACKGROUND/OBJECTIVES

The emergence of the Omicron variant has complicated COVID-19 control and prompted vaccine updates. Recent studies have shown that a fourth dose significantly protects against infection and severe disease, though long-term immunity data remain limited. This study aimed to assess Anti-S-RBD antibodies and interferon-γ levels in healthcare workers 12 months after receiving bivalent Original/Omicron BA.4-5 fourth SARS-CoV-2 vaccine.

METHODS

In this prospective cohort study, 549 healthcare workers were categorized by the initial vaccination schedule, with 229 individuals having received the fourth SARS-CoV-2 vaccine dose. Blood samples were collected from all participants 12 months post-vaccination.

RESULTS

Significant differences in Anti-S-RBD antibody levels were observed between those receiving a fourth dose and those who did not, while no differences were seen in interferon-γ levels. After 12 months, there were no significant differences in humoral and cellular immunity response between volunteers primoinfected or reinfected across different periods by the Omicron variant. A multivariable analysis revealed an association between high antibody levels (>6000 U/mL) and interferon-γ levels (OR: 3.13; 95% CI: 1.3-7.7; p < 0.05). Regarding primary vaccine schedules, participants vaccinated with ChAdOx1 (a single or double dose) had notably lower antibody levels compared to those who received mRNA-based vaccines. Additionally, the study shows a higher frequency of multiple infections among those with a single-dose ChAdOx1 primary schedule (OR: 6.24; 95% CI: 1.25-31.15; p < 0.01).

CONCLUSIONS

Overall, mRNA-based vaccines exhibited stronger long-term immunogenicity. Repeated exposure to the Omicron variant seems to mitigate immune imprinting from the wild-type SARS-CoV-2. An association was observed between high antibody levels and a strong cellular response, although the correlation was not linear.

Effectiveness of Bivalent mRNA Vaccines in Preventing Symptomatic SARS-CoV-2 Infection-Increasing Community Access to Testing Program, United States, January-September 2023

BACKGROUND

On September 2, 2022, bivalent COVID-19 mRNA vaccines, were recommended to address reduced effectiveness of COVID-19 monovalent vaccines during SARS-CoV-2 Omicron variant predominance.

METHODS

Using national pharmacy-based SARS-CoV-2 testing program data from January 15 to September 11, 2023, this test-negative, case-control design study assessed bivalent COVID-19 vaccine effectiveness (VE) against symptomatic infection.

RESULTS

VE against symptomatic infection of a bivalent dose between 2 weeks and 1 month after bivalent vaccination ranged from 46% (95% CI: 38%-52%) for those aged ≥ 65 years to 61% (95% CI 41%-75%) for those aged 12-17 years.

CONCLUSION

Bivalent vaccines protected against symptomatic infection. However, effectiveness waned over time, emphasizing the need to stay up to date with COVID-19 vaccination.

Cohort profile: an observational population-based cohort study on COVID-19 vaccine effectiveness in the Netherlands - the VAccine Study COVID-19 (VASCO)

PURPOSE

VAccine Study COVID-19 (VASCO) is a cohort study with a 5-year follow-up that was initiated when COVID-19 vaccination was introduced in the Netherlands. The primary objective is to estimate real-world vaccine effectiveness (VE) of COVID-19 vaccines against SARS-CoV-2 infection in the Netherlands, overall and in four subpopulations defined by age and medical risk.

PARTICIPANTS

The cohort consists of 45 547 community-dwelling participants aged 18-85 years who were included irrespective of their COVID-19 vaccination status or intention to get vaccinated. A medical risk condition is present in 4289 (19.8%) of 21 679 individuals aged 18-59 years, and in 9135 (38.3%) of 23 821 individuals aged 60-85 years. After 1 year of follow-up, 5502 participants had dropped out of the study. At inclusion and several times after inclusion, participants are asked to take a self-collected fingerprick blood sample in which nucleoprotein and spike protein receptor binding domain-specific antibody concentrations are assessed. Participants are also asked to complete monthly digital questionnaires in the first year, and 3 monthly in years 2-5, including questions on sociodemographic factors, health status, COVID-19 vaccination, SARS-CoV-2-related symptoms and testing results, and behavioural responses to COVID-19 measures.

FINDINGS TO DATE

VASCO data have been used to describe VE against SARS-CoV-2 infection of primary vaccination, first and second booster and bivalent boosters, the impact of hybrid immunity on SARS-CoV-2 infection and VE against infectiousness. Furthermore, data were used to describe antibody response following vaccination and breakthrough infections and to investigate the relation between antibody response and reactogenicity.

FUTURE PLANS

VASCO will be able to contribute to policy decision-making regarding future COVID-19 vaccination. Furthermore, VASCO provides an infrastructure to conduct further studies and to respond to changes in vaccination campaigns and testing policy, and new virus variants.

TRIAL REGISTRATION NUMBER

NL9279.

Methodology of comparative studies on the relative effectiveness of COVID-19 vaccines: a systematic review

BACKGROUND

This study aimed to comprehensively outline the methodological approaches used in published research comparing the vaccine effectiveness (VE) of coronavirus disease 2019 (COVID-19) vaccines.

METHODS

A systematic search was conducted on June 13, 2024, to identify comparative studies evaluating the effectiveness of mRNA versus non-mRNA and monovalent versus bivalent COVID-19 vaccines. We screened titles, abstracts, and full texts, collecting data on publication year, country, sample size, study population composition, study design, VE estimates, outcomes, and covariates. Studies that reported relative VE (rVE) were analyzed separately from those that did not.

RESULTS

We identified 25 articles comparing rVE between mRNA and non-mRNA COVID-19 vaccines, as well as between monovalent and bivalent formulations. Among the studies assessing VE by vaccine type, 126 did not provide rVE estimates. Comparative VE studies frequently employed retrospective cohort designs. Among the definitions of rVE used, the most common were hazard ratio and absolute VE, calculated as (1-odds ratio)×100. Studies were most frequently conducted in the United Kingdom and the United States, and the most common outcome was infection. Most targeted the general population and assessed the VE of mRNA vaccines using the AstraZeneca vaccine as a reference. A small proportion, 7.3% (n=11), did not adjust for any variables. Only 3 studies (2.0%) adjusted for all core confounding variables recommended by the World Health Organization.

CONCLUSION

Few comparative studies of COVID-19 vaccines have incorporated rVE methodologies. Reporting rVE and employing a consistent set of covariates can broaden our understanding of COVID-19 vaccines.

Safety and antibody responses of Omicron BA.4/5 bivalent booster vaccine among hybrid immunity with diverse vaccination histories: A cohort study

This cohort study, conducted between July and August 2023, evaluated the adverse events (AEs) and immune response to a bivalent mRNA-1273.222 (containing sequences of the original Wuhan-H1 strain and the Omicron BA.4/5 variant) booster vaccine in 122 participants. The study included individuals with diverse vaccination histories, and their responses were assessed based on anti-receptor binding domain (RBD) IgG levels and neutralizing antibodies against the wild-type, Omicron BA.5, and XBB.1.16 variants. Following administration of the BA.4/5 bivalent vaccine, AEs were generally mild to moderate and well-tolerated within a few days. There were no reports of vomiting and no serious AEs or death. The findings demonstrated robust immune responses, with significant increases in anti-RBD IgG levels, particularly in groups that had received 3 -6 doses before the booster dose. The BA.4/5 bivalent booster effectively induced neutralizing antibodies against the vaccine strains, providing robust neutralization, including the XBB.1.16 strain. The study also highlighted that individuals with hybrid immunity, especially those assumed infected with the BA.5 strain or who had been infected twice, showed higher levels of robust neutralizing activity against Omicron XBB.1.16. Overall, these results indicate that the BA.4/5 bivalent booster vaccines can induce potent and good antibody responses in emerging Omicron subvariants, supporting its efficacy as a booster in individuals with diverse vaccination histories.

Repeated COVID-19 mRNA vaccination results in IgG4 class switching and decreased NK cell activation by S1-specific antibodies in older adults

BACKGROUND

Previous research has shown that repeated COVID-19 mRNA vaccination leads to a marked increase of SARS-CoV-2 spike-specific serum antibodies of the IgG4 subclass, indicating far-reaching immunoglobulin class switching after booster immunization. Considering that repeated vaccination has been recommended especially for older adults, the aim of this study was to investigate IgG subclass responses in the ageing population and assess their relation with Fc-mediated antibody effector functionality.

RESULTS

Spike S1-specific IgG subclass concentrations (expressed in arbitrary units per mL), antibody-dependent NK cell activation, complement deposition and monocyte phagocytosis were quantified in serum from older adults (n = 38-50, 65-83 years) at one month post-second, -third and -fifth vaccination. Subclass distribution in serum was compared to that in younger adults (n = 64, 18-47 years) at one month post-second and -third vaccination. Compared to younger individuals, older adults showed increased levels of IgG2 and IgG4 at one month post-third vaccination (possibly related to factors other than age) and a further increase following a fifth dose. The capacity of specific serum antibodies to mediate NK cell activation and complement deposition relative to S1-specific total IgG concentrations decreased upon repeated vaccination. This decrease associated with an increased IgG4/IgG1 ratio.

CONCLUSIONS

In conclusion, these findings show that, like younger individuals, older adults produce antibodies with reduced functional capacity upon repeated COVID-19 mRNA vaccination. Additional research is needed to better understand the mechanisms underlying these responses and their potential implications for vaccine effectiveness. Such knowledge is vital for the future design of optimal vaccination strategies in the ageing population.

Immunogenicity and safety of beta variant COVID-19 vaccine AZD2816 and AZD1222 (ChAdOx1 nCoV-19) as primary-series vaccination for previously unvaccinated adults in Brazil, South Africa, Poland, and the UK: a randomised, partly double-blinded, phase 2/3 non-inferiority immunobridging study

BACKGROUND

AZD2816 is a variant-adapted COVID-19 vaccine that expresses the full-length SARS-CoV-2 beta variant spike protein but is otherwise similar to AZD1222 (ChAdOx1 nCoV-19). This study aimed to evaluate the safety and immunogenicity of AZD1222 or AZD2816 (or both) primary-series vaccination in a cohort of adult participants who were previously unvaccinated.

METHODS

In this phase 2/3, randomised, multinational, active-controlled, non-inferiority, immunobridging study, adult participants previously unvaccinated for COVID-19 were enrolled at 16 study sites in Brazil, South Africa, Poland, and the UK. Participants were stratified by age, sex, and comorbidity and randomly assigned 5:5:5:2 to receive a primary series of AZD1222 (AZD1222 group), AZD2816 (AZD2816 [4-week] group), or AZD1222-AZD2816 (AZD1222-AZD2816 group) at 4-week dosing intervals, or AZD2816 at a 12-week interval (AZD2816 [12-week] group) and evaluated for safety and immunogenicity through 180 days after dose 2. Primary outcomes were safety (rates of solicited adverse events occurring during 7 days and unsolicited adverse events occurring during 28 days after each dose) and immunogenicity (non-inferiority of pseudovirus neutralising antibody geometric mean titre [GMT], GMT ratio margin of 0·67, and seroresponse rate, rate difference margin of -10%, recorded 28 days after dose 2 with AZD2816 [4-week interval] against beta vs AZD1222 against ancestral SARS-CoV-2) in participants who were seronegative at baseline. This trial is registered with ClinicalTrials.gov, NCT04973449, and is completed.

FINDINGS

Between July 7 and Nov 12, 2021, 1449 participants were assigned to the AZD1222 group (n=413), the AZD2816 (4-week) group (n=415), the AZD1222-AZD2816 group (n=412), and the AZD2816 (12-week) group (n=209). Ten (2·6%) of 378 participants who were seronegative at baseline in the AZD1222 group, nine (2·4%) of 379 in the AZD2816 (4-week) group, eight (2·1%) of 380 in the AZD1222-AZD2816 group, and 11 (5·8%) of 191 in the AZD2816 (12-week) group had vaccine-related unsolicited adverse events. Serious adverse events were recorded in one (0·3%) participant in the AZD1222 group, one (0·3%) in the AZD2816 (4-week) group, two (0·5%) in the AZD1222-AZD2816 group, and none in the AZD2816 (12-week) group. Co-primary immunogenicity endpoints were met: neutralising antibody GMT (ratio 1·19 [95% CI 1·08-1·32]; lower bound greater than 0·67) and seroresponse rate (difference 1·7% [-3·1 to 6·5]; lower bound greater than -10%) at 28 days after dose 2 were non-inferior in the AZD2816 (4-week) group against beta versus in the AZD1222 group against ancestral SARS-CoV-2. Seroresponse rates were highest with AZD2816 against beta (12-week interval 94·3% [95% CI 89·4-97·3]; 4-week interval 85·7% [81·5-89·2]) and with AZD1222 (84·6% [80·3-88·2]) against ancestral SARS-CoV-2.

INTERPRETATION

Primary series of AZD1222 and AZD2816 were well tolerated, with no emergent safety concerns. Both vaccines elicited robust immunogenicity against beta and ancestral SARS-CoV-2 with greater responses demonstrated when testing against SARS-CoV-2 strains that matched those targeted by the respective vaccine. These findings demonstrate the continued importance of ancestral COVID-19 vaccines in protecting against severe COVID-19 and highlight the feasibility of using the ChAdOx1 platform to develop COVID-19 vaccines against future SARS-CoV-2 variants.

FUNDING

AstraZeneca.

A systematic review and meta-analysis on the effectiveness of bivalent mRNA booster vaccines against Omicron variants

BACKGROUND

A global shift to bivalent mRNA vaccines is ongoing to counterbalance the diminishing effectiveness of the original monovalent vaccines due to the evolution of SARS-CoV-2 variants, yet substantial variation in the bivalent vaccine effectiveness (VE) exists across studies and a complete picture is lacking.

METHODS

We searched papers evaluating absolute or relative effectiveness of SARS-CoV-2 BA.1 type or BA.4/5 type bivalent mRNA vaccines on eight publication databases published from September 1st, 2022, to November 8th, 2023. Pooled VE against Omicron-associated infection and severe events (hospitalization and/or death) was estimated in reference to unvaccinated, ≥2 original monovalent doses, and ≥ 3 original monovalent doses.

RESULTS

From 630 citations identified, 28 studies were included, involving 55,393,303 individuals. Bivalent boosters demonstrated higher effectiveness against symptomatic or any infection for all ages combined, with an absolute VE of 53.5 % (95 % CI: -22.2-82.3 %) when compared to unvaccinated and relative VE of 30.8 % (95 % CI: 22.5-38.2 %) and 28.4 % (95 % CI: 10.2-42.9 %) when compared to ≥ 2 and ≥ 3 original monovalent doses, respectively. The corresponding VE estimates for adults ≥ 60 years old were 22.5 % (95 % CI: 16.8-39.8 %), 31.4 % (95 % CI: 27.7-35.0 %), and 30.6 % (95 % CI: -13.2-57.5 %). Pooled bivalent VE estimates against severe events were higher, 72.9 % (95 % CI: 60.5-82.4 %), 57.6 % (95 % CI: 42.4-68.8 %), and 62.1 % (95 % CI: 54.6-68.3 %) for all ages, and 72.0 % (95 % CI: 51.4-83.9 %), 63.4 % (95 % CI: 41.0-77.3 %), and 60.7 % (95 % CI: 52.4-67.6 %) for adults ≥ 60 years old, compared to unvaccinated, ≥2 original monovalent doses, and ≥ 3 original monovalent doses, respectively.

CONCLUSIONS

The bivalent boosters demonstrated superior protection against severe outcomes than the original monovalent boosters across age groups, highlighting the critical need for improving vaccine coverage, especially among the vulnerable older subpopulation.

Effectiveness of a bivalent mRNA vaccine dose against symptomatic SARS-CoV-2 infection among U.S. Healthcare personnel, September 2022-May 2023

BACKGROUND

Bivalent mRNA vaccines were recommended since September 2022. However, coverage with a recent vaccine dose has been limited, and there are few robust estimates of bivalent VE against symptomatic SARS-CoV-2 infection (COVID-19). We estimated VE of a bivalent mRNA vaccine dose against COVID-19 among eligible U.S. healthcare personnel who had previously received monovalent mRNA vaccine doses.

METHODS

We conducted a case-control study in 22 U.S. states, and enrolled healthcare personnel with COVID-19 (case-participants) or without COVID-19 (control-participants) during September 2022-May 2023. Participants were considered eligible for a bivalent mRNA dose if they had received 2-4 monovalent (ancestral-strain) mRNA vaccine doses, and were ≥67 days after the most recent vaccine dose. We estimated VE of a bivalent mRNA dose using conditional logistic regression, accounting for matching by region and four-week calendar period. We adjusted estimates for age group, sex, race and ethnicity, educational level, underlying health conditions, community COVID-19 exposure, prior SARS-CoV-2 infection, and days since the last monovalent mRNA dose.

RESULTS

Among 3,647 healthcare personnel, 1,528 were included as case-participants and 2,119 as control-participants. Participants received their last monovalent mRNA dose a median of 404 days previously; 1,234 (33.8%) also received a bivalent mRNA dose a median of 93 days previously. Overall, VE of a bivalent dose was 34.1% (95% CI, 22.6%-43.9%) against COVID-19 and was similar by product, days since last monovalent dose, number of prior doses, age group, and presence of underlying health conditions. However, VE declined from 54.8% (95% CI, 40.7%-65.6%) after 7-59 days to 21.6% (95% CI 5.6%-34.9%) after ≥60 days.

CONCLUSIONS

Bivalent mRNA COVID-19 vaccines initially conferred approximately 55% protection against COVID-19 among U.S. healthcare personnel. However, protection waned after two months. These findings indicate moderate initial protection against symptomatic SARS-CoV-2 infection by remaining up-to-date with COVID-19 vaccines.

Multivalent and Sequential Heterologous Spike Protein Vaccinations Effectively Induce Protective Humoral Immunity against SARS-CoV-2 Variants

The emergence of new SARS-CoV-2 variants continues to cause challenging problems for the effective control of COVID-19. In this study, we tested the hypothesis of whether a strategy of multivalent and sequential heterologous spike protein vaccinations would induce a broader range and higher levels of neutralizing antibodies against SARS-CoV-2 variants and more effective protection than homologous spike protein vaccination in a mouse model. We determined spike-specific IgG, receptor-binding inhibition titers, and protective efficacy in the groups of mice that were vaccinated with multivalent recombinant spike proteins (Wuhan, Delta, Omicron), sequentially with heterologous spike protein variants, or with homologous spike proteins. Trivalent (Wuhan + Delta + Omicron) and sequential heterologous spike protein vaccinations were more effective in inducing serum inhibition activities of receptor binding to spike variants and virus neutralizing antibody titers than homologous spike protein vaccination. The higher efficacy of protection was observed in mice with trivalent and sequential heterologous spike protein vaccination after a challenge with a mouse-adapted SARS-CoV-2 MA10 strain compared to homologous spike protein vaccination. This study provides evidence that a strategy of multivalent and sequential heterologous variant spike vaccination might provide more effective protection against emerging SARS-CoV-2 variants than homologous spike vaccination and significantly alleviate severe inflammation due to COVID-19.

Bivalent mRNA COVID vaccines elicit predominantly cross-reactive CD4+ T cell clonotypes

Bivalent COVID vaccines containing mRNA for ancestral and Omicron BA.5 spike proteins do not induce stronger T cell responses to Omicron BA.5 spike proteins than monovalent vaccines that contain only ancestral spike mRNA. The reasons for this finding have not been elucidated. Here, we show that healthy donors (HDs) and people living with HIV (PLWH) on antiretroviral therapy mostly target T cell epitopes that are not affected by BA.5 mutations. We use the functional expansion of specific T cells (FEST) assay to determine the percentage of CD4+ T cells that cross-recognize both spike proteins and those that are monoreactive for each protein. We show a predominance of cross-reactive CD4+ T cells; less than 10% percent of spike-specific CD4+ T cell receptors were BA.5 monoreactive in most HDs and PLWH. Our data suggest that the current bivalent vaccines do not induce robust BA.5-monoreactive T cell responses.

Effectiveness of Omicron XBB.1.5 vaccine against infection with SARS-CoV-2 Omicron XBB and JN.1 variants, prospective cohort study, the Netherlands, October 2023 to January 2024

We estimated vaccine effectiveness (VE) of SARS-CoV-2 Omicron XBB.1.5 vaccination against self-reported infection between 9 October 2023 and 9 January 2024 in 23,895 XBB.1.5 vaccine-eligible adults who had previously received at least one booster. VE was 41% (95% CI: 23-55) in 18-59-year-olds and 50% (95% CI: 44-56) in 60-85-year-olds. Sequencing data suggest lower protection against the BA.2.86 (including JN.1) variant from recent prior infection (OR = 2.8; 95% CI:1.2-6.5) and, not statistically significant, from XBB.1.5 vaccination (OR = 1.5; 95% CI:0.8-2.6).

Relative effectiveness of bivalent COVID-19 vaccine: a systematic review and meta-analysis

Objective

The rapid development of COVID-19 bivalent vaccines (BVs) has encompassed both the original virus strains and the variant strain. However, the effectiveness of BVs is largely unknown. Therefore, we conducted a systematic review and meta-analysis of the effectiveness of BVs.

Methods

Literature research was conducted through PubMed, Cochrane Library, Embase, and Web of Science up until November 4, 2023. Both randomized control trials and observational studies were considered for inclusion. Pooled estimates were calculated using a random effects model. The Newcastle-Ottawa Scale (NOS) was used to assess the risk of bias in cohort and case-control studies.

Results

A total of 1,174 articles were reviewed and 22 eligible studies were included. All included studies were observational (15 cohort studies, 7 case-control studies). The total number of participants was 39,673,160, and the number of people vaccinated with BVs as an intervention group was 11,585,182. Two mRNA BVs were mainly involved, including the ancestral strain and the BA.1 or BA.4-5 variants. Meta-analysis results showed, compared with the monovalent vaccines (MVs), the relative effectiveness (rVE) of the BVs in COVID-19-associated infections/symptomatic infections, illnesses, hospitalizations, and deaths was 30.90% [95% confidence interval (CI), 8.43-53.37], 39.83% (95% CI, 27.34-52.32), 59.70% (95% CI, 44.08-75.32), and 72.23% (95% CI, 62.08-82.38), respectively. For those aged 50 years and older, BVs provided an additional 49.69% (95% CI, 41.44-57.94) effective protection compared with MVs. During the dominance period of the omicron XBB variant strain, BVs provided an additional 47.63% (95% CI, 27.45-67.82) effective protection compared with MVs.

Conclusion

Our findings show that the rVE of BVs in preventing COVID-19-associated infections, symptomatic infections, illnesses, hospitalizations, and deaths is higher compared to MVs. Particularly for people over 50 years of age and during the Omicron variant XBB dominance phase, BVs provided superior protection. Therefore, BVs may have a broader application in the prevention and control of coronaviruses variant.

Bivalent mRNA vaccine effectiveness against COVID-19 among older adults in Japan: a test-negative study from the VENUS study

BACKGROUND

Bivalent COVID-19 vaccines have been implemented worldwide since the booster vaccination campaigns of autumn of 2022, but little is known about their effectiveness. Thus, this study holistically evaluated the effectiveness of bivalent vaccines against infection in older adults in Japan.

METHODS

We adopted the test-negative design using COVID-19 test data of individuals, aged ≥ 65 years, residing in three municipalities in Japan, who underwent tests in medical institutions between October 1 and December 30, 2022. Logistic regression analyses were conducted to estimate the odds of testing positive according to vaccination status. Vaccine effectiveness was defined as (1 - odds ratio) × 100%.

RESULTS

A total of 3,908 positive and 16,090 negative results were included in the analyses. Receiving a bivalent dose in addition to ≥ 2 monovalent doses was 33.6% (95% confidence interval [CI]: 20.8, 44.3%) more effective than receiving no vaccination, and 18.2% (95% CI: 9.4, 26.0%) more effective than receiving ≥ 2 monovalent doses but not receiving a bivalent vaccination. In addition, the effectiveness peaked at 14-20 days after administration and then gradually declined over time. Furthermore, a bivalent booster dose provided 18.6% (95% CI: 9.9, 26.5%) additional protection among those vaccinated with ≥ 2 monovalent doses, in the absence of a previous infection history. However, we did not find sufficient evidence of effectiveness of bivalent vaccines among previously infected older adults.

CONCLUSIONS

Bivalent vaccines are effective against COVID-19 infections among older adults without a history of infection.

Effect of second booster vaccinations and prior infection against SARS-CoV-2 in the UK SIREN healthcare worker cohort

Background

The protection of fourth dose mRNA vaccination against SARS-CoV-2 is relevant to current global policy decisions regarding ongoing booster roll-out. We aimed to estimate the effect of fourth dose vaccination, prior infection, and duration of PCR positivity in a highly-vaccinated and largely prior-COVID-19 infected cohort of UK healthcare workers.

Methods

Participants underwent fortnightly PCR and regular antibody testing for SARS-CoV-2 and completed symptoms questionnaires. A multi-state model was used to estimate vaccine effectiveness (VE) against infection from a fourth dose compared to a waned third dose, with protection from prior infection and duration of PCR positivity jointly estimated.

Findings

1298 infections were detected among 9560 individuals under active follow-up between September 2022 and March 2023. Compared to a waned third dose, fourth dose VE was 13.1% (95% CI 0.9 to 23.8) overall; 24.0% (95% CI 8.5 to 36.8) in the first 2 months post-vaccination, reducing to 10.3% (95% CI -11.4 to 27.8) and 1.7% (95% CI -17.0 to 17.4) at 2-4 and 4-6 months, respectively. Relative to an infection >2 years ago and controlling for vaccination, 63.6% (95% CI 46.9 to 75.0) and 29.1% (95% CI 3.8 to 43.1) greater protection against infection was estimated for an infection within the past 0-6, and 6-12 months, respectively. A fourth dose was associated with greater protection against asymptomatic infection than symptomatic infection, whilst prior infection independently provided more protection against symptomatic infection, particularly if the infection had occurred within the previous 6 months. Duration of PCR positivity was significantly lower for asymptomatic compared to symptomatic infection.

Interpretation

Despite rapid waning of protection, vaccine boosters remain an important tool in responding to the dynamic COVID-19 landscape; boosting population immunity in advance of periods of anticipated pressure, such as surging infection rates or emerging variants of concern.

Funding

UK Health Security Agency, Medical Research Council, NIHR HPRU Oxford, Bristol, and others.

Design and analysis heterogeneity in observational studies of COVID-19 booster effectiveness: A review and case study

We investigated the design and analysis of observational booster vaccine effectiveness (VE) studies by performing a scoping review of booster VE literature with a focus on study design and analytic choices. We then applied 20 different approaches, including those found in the literature, to a single dataset from Michigan Medicine. We identified 80 studies in our review, including over 150 million observations in total. We found that while protection against infection is variable and dependent on several factors including the study population and time period, both monovalent boosters and particularly the bivalent booster offer strong protection against severe COVID-19. In addition, VE analyses with a severe disease outcome (hospitalization, intensive care unit admission, or death) appear to be more robust to design and analytic choices than an infection endpoint. In terms of design choices, we found that test-negative designs and their variants may offer advantages in statistical efficiency compared to cohort designs.

Effectiveness of bivalent mRNA vaccines against medically attended symptomatic SARS-CoV-2 infection and COVID-19-related hospital admission among SARS-CoV-2-naive and previously infected individuals: a retrospective cohort study

BACKGROUND

Emergence of the SARS-CoV-2 omicron (B.1.1.529) variant with high immune evasion has led to the development and roll-out of bivalent mRNA vaccines targeting original and omicron strains. However, real-world observational data on effectiveness of bivalent vaccines are scarce. We aimed to assess the relative effectiveness of a fourth vaccine dose with the BA.1-adapted or BA.4/BA.5-adapted bivalent vaccines against medically attended symptomatic SARS-CoV-2 infection and COVID-19-related hospital admission among SARS-CoV-2-naive and previously infected individuals in Singapore.

METHODS

We conducted a retrospective cohort study among Singapore residents aged 18 years and older who had received three monovalent mRNA vaccine doses and were eligible for a fourth dose. Data were collected from official databases on COVID-19 cases and vaccinations maintained by the Singapore Ministry of Health. We analysed the incidence of medically attended symptomatic SARS-CoV-2 infection and COVID-19-related hospital admission between Oct 14, 2022, and Jan 31, 2023, by previous infection status and type of fourth vaccine dose received. Inverse probability-weighted Cox regressions were used to estimate hazard ratios (HRs).

FINDINGS

2 749 819 individuals were included in the analysis. For the SARS-CoV-2-naive group, a fourth monovalent vaccine dose did not confer additional protection over three monovalent doses against symptomatic infection (HR 1·09 [95% CI 1·07-1·11]), whereas the bivalent vaccine did provide additional protection (0·18 [0·17-0·19]). Among individuals with previous infection, the HR was 0·87 (95% CI 0·84-0·91) and 0·14 (0·13-0·15) with receipt of the fourth monovalent and bivalent doses, respectively. Against COVID-19-related hospital admission, the bivalent vaccine (HR 0·12 [95% CI 0·08-0·18] in SARS-CoV-2-naive participants and 0·04 [0·01-0·15] in previously infected participants) conferred greater benefit compared with the fourth monovalent dose (0·84 [0·77-0·91] in SARS-CoV-2-naive participants and 0·85 [0·69-1·04] in previously infected participants).

INTERPRETATION

A fourth dose with the bivalent vaccine was substantially more effective against medically attended symptomatic SARS-CoV-2 infection and COVID-19-related hospital admission than four monovalent doses among both SARS-CoV-2-naive and previously infected individuals. Boosters with the bivalent vaccine might be preferred in this omicron-predominant pandemic, regardless of previous infection history.

FUNDING

None.

The Effectiveness of Bivalent COVID-19 Vaccination: A Preliminary Report

Vaccination has been a game-changer in the long battle against COVID-19. However, waning vaccine-induced immunity and the immune evasion of emerging variants create challenges. The rapid-fire development of bivalent vaccines (BVs), comprising ancestral strains and a new variant, was authorized to prevent COVID-19, but the effectiveness of the updated vaccines remains largely unclear. Electronic databases were searched to investigate the immunogenicity and reactogenicity of BVs in humans. As of March 2023, 20 trials were identified. Compared with monovalent vaccination, the induced immunogenicity against ancestral strains was similar. The BVs demonstrated approximately 33-50% higher immunogenicity values against additional variant strains. An observational cohort study showed the additional clinical effectiveness of the BVs. The adverse events were similar. In conclusion, our systematic review found that the BVs had equal immunogenicity against ancestral strains without safety concerns. Approximately 33-50% increased additional antibody titers and clinical effectiveness against additional variant strains were observed in subjects with a BV vaccine with moderate heterogeneity, especially for BA.1-containing BVs.

Effectiveness of the Original COVID-19 Vaccine against COVID-19 Exacerbations during the Omicron Wave: A Population-based Study in Okayama, Japan

Introduction

In Japan, approximately 97 million individuals have received their primary two doses of coronavirus disease 2019 (COVID-19) vaccine at the end of 2022. In this study, we aim to examine the effectiveness of the primary vaccines and compare its efficacy to booster vaccine shots in terms of preventing COVID-19 exacerbations during the Omicron-predominant period in Japan.

Methods

For this analysis, we have collected all the confirmed COVID-19-positive cases from different medical institutions in Okayama City and have also utilized the information from the public Vaccination Record System. Taking the number of vaccinations into consideration, we then conducted a population-based study to assess the effectiveness of the two primary vaccine doses in preventing COVID-19 exacerbations during the Omicron waves. Our primary and secondary outcomes were COVID-19 exacerbations with respiratory failure (i.e., oxygen saturation on room air ≤ 93%, requiring supplemental oxygen), intensive care unit admission and/or mechanical ventilator requirement, or death, in accordance with the Japanese COVID-19 guidelines, and pneumonia during the course of COVID-19 infection, respectively.

Results

In total, 95,329 COVID-19-positive individuals, aged 5 years and above, were included in this analysis (study period from January 1 to September 10, 2022). As per our findings, the effectiveness of the primary two doses against COVID-19 exacerbations compared with those who had never been vaccinated was 55.5% (95% confidential interval [CI]: 32.6-71.7), whereas it was higher after the third dose (76.9%; 95% CI: 66.7-84.0) and the fourth dose (75.7%; 95% CI: 58.8-85.7). Effectiveness was sustained for ≥ 5 months after the third vaccination, and preventive effectiveness was observed in individuals aged ≥ 65 years.

Conclusions

As per the results of this study, we can conclude that the efficacy of the primary two doses of SARS-CoV-2 vaccine can be further strengthened in terms of preventing COVID-19 exacerbations by administering third and fourth booster vaccine shots. The additional bivalent vaccine is anticipated to further increase its efficacy against the Omicron strain, suggesting that individuals who have not received their booster shots yet should consider getting them to prevent COVID-19 exacerbations.

COVID-19 vaccine booster doses provide increased protection against COVID-19 hospitalization compared with previously vaccinated individuals: Interim findings from the REFORCO-Brazil real-world effectiveness study during Delta and Omicron

BACKGROUND

Although COVID-19 booster vaccination is widely recommended, there is limited long-term, population-level, real-world evidence on the magnitude of improved protection against severe COVID-19 conferred by boosting with monovalent COVID-19 vaccines developed against ancestral SARS-CoV-2, especially in low- or middle-income countries. We present interim results from the first large-scale assessment of the relative vaccine effectiveness (rVE) of first and second booster doses against severe COVID-19 in a low-/middle-income country.

METHODS

REFORCO-Brazil is an ongoing, test-negative case-control study (NCT05697705) utilizing Brazil national severe acute respiratory syndrome (SARS) surveillance and vaccination data. In SARS hospitalizations from August 1, 2021 to July 31, 2022, we matched test-positive (via SARS-CoV-2 antigen/reverse transcription polymerase chain reaction [RT-PCR]) cases and test-negative case-controls (via RT-PCR) based on admission date, preceding vaccinations, and age. We evaluated the rVEs of four monovalent COVID-19 vaccines (AZD1222, Ad26.COV2.S, CoronaVac, and BNT162b2) as second boosters compared with any first boosters received ≥4 months previously, and as first boosters compared with primary-series vaccinations completed ≥4 months previously.

RESULTS

The overall rVE of second boosters, from 5668 (2238 test-positive) evaluated hospitalizations, was 24.7 % (95 % confidence interval [CI]: 12.6-35.1); the overall rVE of first boosters, from 30,272 (12,063 test-positive) hospitalizations, was 46.8 % (95 % CI: 43.3-50.0). The rVEs of AZD1222 and BNT162b2 were similar: 29.4 % (95 % CI: 8.6-45.5) and 25.5 % (95 % CI: 4.2-42.2), respectively, for second boosters; and 42.5 % (95 % CI: 28.0-54.0) and 50.8 % (95 % CI: 47.5-54.0), respectively, for first boosters. In general, rVEs were higher in elderly (≥80 years) and immunocompromised/high-risk individuals.

CONCLUSIONS

Our results support the use of AZD1222 and other adenoviral/mRNA vaccine boosters to maintain protection against COVID-19 hospitalization from Omicron subvariants, including in elderly and immunocompromised individuals at increased risk of accelerated waning or severe outcomes.

Bivalent mRNA-1273.214 vaccine effectiveness against SARS-CoV-2 omicron XBB* infections

Effectiveness of the 50-μg mRNA-1273.214 bivalent vaccine against SARS-CoV-2 infection was modest at 25% in a matched, retrospective, cohort study in Qatar comparing infection incidence in the bivalent cohort to that in the national no-recent-vaccination resident cohort. XBB* immune evasion, immune imprinting effects, or both, may explain findings.

Comparative effectiveness of bivalent BA.4-5 and BA.1 mRNA booster vaccines among adults aged ≥50 years in Nordic countries: nationwide cohort study

OBJECTIVE

To estimate the effectiveness of the bivalent mRNA booster vaccines containing the original SARS-CoV-2 and omicron BA.4-5 or BA.1 subvariants as the fourth dose against severe covid-19.

DESIGN

Nationwide cohort analyses, using target trial emulation.

SETTING

Denmark, Finland, Norway, and Sweden, from 1 July 2022 to 10 April 2023.

PARTICIPANTS

People aged ≥50 years who had received at least three doses of covid-19 vaccine (that is, a primary course and a first booster).

MAIN OUTCOME MEASURES

The Kaplan-Meier estimator was used to compare the risk of hospital admission and death related to covid-19 in people who received a bivalent Comirnaty (Pfizer-BioNTech) or Spikevax (Moderna) BA.4-5 or BA.1 mRNA booster vaccine as a fourth dose (second booster) with three dose (first booster) vaccinated people and between four dose vaccinated people.

RESULTS

A total of 1 634 199 people receiving bivalent BA.4-5 fourth dose booster and 1 042 124 receiving bivalent BA.1 fourth dose booster across the four Nordic countries were included. Receipt of a bivalent BA.4-5 booster as a fourth dose was associated with a comparative vaccine effectiveness against admission to hospital with covid-19 of 67.8% (95% confidence interval 63.1% to 72.5%) and a risk difference of -91.9 (95% confidence interval -152.4 to -31.4) per 100 000 people at three months of follow-up compared with having received three doses of vaccine (289 v 893 events). The corresponding comparative vaccine effectiveness and risk difference for bivalent BA.1 boosters (332 v 977 events) were 65.8% (59.1% to 72.4%) and -112.9 (-179.6 to -46.2) per 100 000, respectively. Comparative vaccine effectiveness and risk difference against covid-19 related death were 69.8% (52.8% to 86.8%) and -34.1 (-40.1 to -28.2) per 100 000 for bivalent BA.4-5 booster (93 v 325 events) and 70.0% (50.3% to 89.7%) and -38.7 (-65.4 to -12.0) per 100 000 for BA.1 booster (86 v 286) as a fourth dose. Comparing bivalent BA.4-5 and BA.1 boosters as a fourth dose directly resulted in a three month comparative vaccine effectiveness and corresponding risk difference of -14.9% (-62.3% to 32.4%) and 10.0 (-14.4 to 34.4) per 100 000 people for admission to hospital with covid-19 (802 v 932 unweighted events) and -40.7% (-123.4% to 42.1%) and 8.1 (-3.3 to 19.4) per 100 000 for covid-19 related death (229 v 243 unweighted events). The comparative vaccine effectiveness did not differ across sex and age (</≥70 years) and seemed to be sustained up to six months from the day of vaccination with modest waning.

CONCLUSION

Vaccination with bivalent BA.4-5 or BA.1 mRNA booster vaccines as a fourth dose was associated with reduced rates of covid-19 related hospital admission and death among adults aged ≥50 years. The protection afforded by the bivalent BA.4-5 and BA.1 boosters did not differ significantly when directly compared, and any potential difference would most likely be very small in absolute numbers.

Design and Analysis Heterogeneity in Observational Studies of COVID-19 Booster Effectiveness: A Review and Case Study

Background

Observational vaccine effectiveness (VE) studies based on real-world data are a crucial supplement to initial randomized clinical trials of Coronavirus Disease 2019 (COVID-19) vaccines. However, there exists substantial heterogeneity in study designs and statistical methods for estimating VE. The impact of such heterogeneity on VE estimates is not clear.

Methods

We conducted a two-step literature review of booster VE: a literature search for first or second monovalent boosters on January 1, 2023, and a rapid search for bivalent boosters on March 28, 2023. For each study identified, study design, methods, and VE estimates for infection, hospitalization, and/or death were extracted and summarized via forest plots. We then applied methods identified in the literature to a single dataset from Michigan Medicine (MM), providing a comparison of the impact of different statistical methodologies on the same dataset.

Results

We identified 53 studies estimating VE of the first booster, 16 for the second booster. Of these studies, 2 were case-control, 17 were test-negative, and 50 were cohort studies. Together, they included nearly 130 million people worldwide. VE for all outcomes was very high (around 90%) in earlier studies (i.e., in 2021), but became attenuated and more heterogeneous over time (around 40%-50% for infection, 60%-90% for hospitalization, and 50%-90% for death). VE compared to the previous dose was lower for the second booster (10-30% for infection, 30-60% against hospitalization, and 50-90% against death). We also identified 11 bivalent booster studies including over 20 million people. Early studies of the bivalent booster showed increased effectiveness compared to the monovalent booster (VE around 50-80% for hospitalization and death).Our primary analysis with MM data using a cohort design included 186,495 individuals overall (including 153,811 boosted and 32,684 with only a primary series vaccination), and a secondary test-negative design included 65,992 individuals tested for SARS-CoV-2. When different statistical designs and methods were applied to MM data, VE estimates for hospitalization and death were robust to analytic choices, with test-negative designs leading to narrower confidence intervals. Adjusting either for the propensity of getting boosted or directly adjusting for covariates reduced the heterogeneity across VE estimates for the infection outcome.

Conclusion

While the advantage of the second monovalent booster is not obvious from the literature review, the first monovalent booster and the bivalent booster appear to offer strong protection against severe COVID-19. Based on both the literature view and data analysis, VE analyses with a severe disease outcome (hospitalization, ICU admission, or death) appear to be more robust to design and analytic choices than an infection endpoint. Test-negative designs can extend to severe disease outcomes and may offer advantages in statistical efficiency when used properly.

Antibody Response to the BA.5 Bivalent Vaccine Shot: a Two-Year Follow-Up Study following Initial COVID-19 mRNA Vaccination

Although many studies have been conducted on the increase in spike antibody levels after vaccination, there is insufficient prospective and longitudinal information on the BA.5-adapted bivalent vaccine up to the fifth vaccination. In this study, we conducted a follow-up study of spike antibody levels and infection history in 46 health care workers who received up to 5 vaccinations. Monovalent vaccines were administered for the first to fourth vaccinations, and a bivalent vaccine was administered for the fifth vaccination. 11 serum samples were collected from each participant, and antibody levels were measured in a total of 506 serum samples. During the observation period, 43 of the 46 health care workers had no infection history, and 3 had a history of infection. Spike antibody levels peaked at 1 week after the second booster vaccination and gradually declined until the 27th week after the second vaccination. After 2 weeks following the fifth BA.5-adapted bivalent vaccine, the spike antibody levels significantly increased (median: 23,756 [IQR: 16,450 to 37,326]), compared to those measured before vaccination (median: 9,354 [IQR: 5,904 to 15,784]) (paired Wilcoxon signed-rank test, P = 5.7 × 10-14). These changes in antibody kinetics were observed regardless of age or sex. These results suggest that booster vaccination increased the spike antibody levels. Regular vaccination is effective in maintaining long-term antibody levels. IMPORTANCE A COVID-19 bivalent mRNA vaccine was developed and administered to health care workers. The COVID-19 mRNA vaccine induces a robust antibody response. However, little is known about the antibody response to vaccines in serially collected blood samples from the same individuals. Here, we provide two-year follow-up data on the humoral immune response to COVID-19 mRNA vaccines in health care workers who received up to five vaccinations, including the BA.5-adapted bivalent vaccine. The results suggest that regular vaccination is effective in maintaining long-term antibody levels and have implications for vaccine efficacy and booster dose strategies in health care settings.