Bivalent COVID-19 vaccines boost the capacity of pre-existing SARS-CoV-2-specific memory B cells to cross-recognize Omicron subvariants (preprint)

Bivalent COVID-19 vaccines comprising ancestral Wuhan-Hu-1 (WH1) and the Omicron BA.1 or BA.5 subvariant elicit enhanced serum antibody responses to emerging Omicron subvariants. We characterized the memory B-cell (Bmem) response following a fourth dose with a BA.1 or BA.5 bivalent vaccine, and compared the immunogenicity with a WH1 monovalent fourth dose. Healthcare workers previously immunized with mRNA or adenoviral vector monovalent vaccines were sampled before and one-month after a monovalent, BA.1 or BA.5 bivalent fourth dose COVID-19 vaccine. RBD-specific Bmem were quantified with an in-depth spectral flow cytometry panel including recombinant RBD proteins of the WH1, BA.1, BA.5, BQ.1.1, and XBB.1.5 variants. All recipients had slightly increased WH1 RBD-specific Bmem numbers. Recognition of Omicron subvariants was not enhanced following monovalent vaccination, while both bivalent vaccines significantly increased WH1 RBD-specific Bmem cross-recognition of all Omicron subvariants tested by flow cytometry. Thus, Omicron-based bivalent vaccines can improve recognition of descendent Omicron subvariants by pre-existing, WH1-specific Bmem, beyond that of a conventional, monovalent vaccine. This provides new insights into the capacity of variant-based mRNA booster vaccines to improve immune memory against emerging SARS-CoV-2 variants.

Distinct COVID-19 vaccine combinations result in divergent immune responses (preprint)

Waning antibody responses after COVID-19 vaccination combined with the emergence of the SARS-CoV-2 Omicron lineage led to reduced vaccine effectiveness. As a countermeasure, bivalent mRNA-based booster vaccines encoding the ancestral spike protein in combination with that of Omicron BA.1 or BA.5 were introduced. Since then, BA.2-descendent lineages have become dominant, such as XBB.1.5 or BA.2.86. Here, we assessed how different COVID-19 priming regimens affect the immunogenicity of the recently used bivalent booster vaccinations and breakthrough infections. BA.1 and BA.5 bivalent vaccines boosted neutralizing antibodies and T-cells up to 3 months after boost; however, cross-neutralization of XBB.1.5 was poor. Interestingly, different combinations of prime-boost regimens induced divergent responses: participants primed with Ad26.COV2.S developed lower binding antibody levels after bivalent boost while neutralization and T-cell responses were similar to mRNA-based primed participants. In contrast, the breadth of neutralization was higher in mRNA-primed and bivalent BA.5 boosted participants. Combined, we highlight important "lessons learned" from the employed COVID-19 vaccination strategies. Our data further support the use of monovalent vaccines based on circulating strains when vaccinating risk groups, as recently recommended by the WHO. We emphasize the importance of the continuous assessment of immune responses targeting circulating variants to guide future COVID-19 vaccination policies.

Evaluation of bivalent Omicron BA.1 booster vaccination after different priming regimens in healthcare workers (SWITCH ON): a randomized controlled trial (preprint)

Background Bivalent mRNA-based COVID-19 vaccines encoding the ancestral and Omicron spike protein were developed as a countermeasure against antigenically distinct SARS-CoV-2 variants. We compared the (variant-specific) immunogenicity and reactogenicity of mRNA-based bivalent Omicron BA.1 vaccines in individuals who were primed with adenovirus- or mRNA-based vaccines. Methods In this open-label, multicenter, randomized, controlled trial, healthcare workers primed with Ad26.COV2.S or mRNA-based vaccines were boosted with mRNA-1273.214 or BNT162b2 OMI BA.1. The primary endpoint was the fold change in S1-specific IgG antibodies pre- and 28 days after booster vaccination. Secondary outcomes were fast response, (antibody levels on day 7), reactogenicity, neutralization of circulating variants and (cross-reactive) SARS-CoV-2-specific T-cell responses. Findings No effect of different priming regimens was observed on bivalent vaccination boosted S1-specific IgG antibodies. The largest increase in S1-specific IgG antibodies occurred between day 0 and 7 after bivalent booster. Neutralizing antibodies targeting the variants in the bivalent vaccine (ancestral SARS-CoV-2 and Omicron BA.1) were boosted. In addition, neutralizing antibodies against the circulating Omicron BA.5 variant increased after BA.1 bivalent booster. T-cell responses were boosted and retained reactivity with variants from the Omicron sub-lineage. Interpretation Bivalent booster vaccination with mRNA-1273.214 or BNT162b2 OMI BA.1 resulted in a rapid recall of humoral and cellular immune responses independent of the initial priming regimen. Although no preferential boosting of variant-specific responses was observed, the induced antibodies and T-cells cross-reacted with Omicron BA.1 and BA.5. It remains crucial to monitor immunity at the population level, and simultaneously antigenic drift at the virus level, to determine the necessity (and timing) of COVID-19 booster vaccinations.

Immunogenicity of an additional mRNA-1273 SARS-CoV-2 vaccination in people living with HIV with hyporesponse after primary vaccination (preprint)

Background The COVIH study is a prospective SARS-CoV-2 vaccination study in people living with HIV (PLWH). Of the 1154 PLWH enrolled, 14% showed a reduced or absent antibody response after a primary vaccination regimen. As the response to an additional vaccination in PLWH with hyporesponse is unknown, we evaluated whether an additional vaccination boosts immune responses in these hyporesponders. Methods Consenting hyporesponders received an additional 100 g of mRNA-1273. Hyporesponse was defined as [≤]300 spike(S)-specific binding antibody units [BAU]/mL. The primary endpoint was the increase in antibodies 28 days after the additional vaccination. Secondary endpoints were the correlation between patient characteristics and antibody response, levels of neutralizing antibodies, S-specific T-cell and B-cell responses, and reactogenicity. Results Of the 75 PLWH enrolled, five were excluded as their antibody level had increased to >300 BAU/mL at baseline, two for a SARS-CoV-2 infection before the primary endpoint evaluation and two were lost to follow-up. Of the 66 remaining participants, 40 previously received ChAdOx1-S, 22 BNT162b2, and four Ad26.COV2.S. The median age was 63[IQR:60-66], 86% were male, pre-vaccination and nadir CD4+ T-cell counts were 650/L[IQR:423-941] and 230/L[IQR:145-345] and 96% had HIV-RNA <50 copies/mL. The mean antibody level before the additional vaccination was 35 BAU/mL (SEM 5.4) and 45/66 (68%) were antibody negative. After the additional mRNA-1273 vaccination, antibodies were >300 BAU/mL in 64/66 (97%) with a mean increase of 4282 BAU/mL (95%CI:3241-5323). No patient characteristics correlated with the magnitude of the antibody response, nor did the primary vaccination regimen. The additional vaccination significantly increased the proportion of participants with detectable ancestral S-specific B-cells (p=0.016) and CD4+ T-cells (p=0.037). Conclusion An additional mRNA-1273 vaccination induced a robust serological response in 97% of the PLWH with a hyporesponse after a primary vaccination regimen. This response was observed regardless of the primary vaccination regimen or patient characteristics.

Ad26.COV2.S priming provides a solid immunological base for mRNA-based COVID-19 booster vaccination (preprint)

A large proportion of the global population received a single dose of the Ad26.COV2.S coronavirus disease-2019 (COVID-19) vaccine as priming vaccination, which was shown to provide protection against moderate to severe COVID-19. However, the emergence of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) variants that harbor immune-evasive mutations in the spike protein led to the recommendation of booster vaccinations after Ad26.COV2.S priming. Recent studies showed that heterologous booster vaccination with an mRNA-based vaccine following Ad26.COV2.S priming leads to high antibody levels. However, how heterologous booster vaccination affects other functional aspects of the immune response remains unknown. Here, we performed immunological profiling on samples obtained from Ad26.COV2.S-vaccinated individuals before and after a homologous (Ad26.COV2.S) or heterologous (mRNA-1273 or BNT162b2) booster vaccination. Both homologous and heterologous booster vaccination increased antibodies with multiple functionalities towards ancestral SARS-CoV-2, the Delta and Omicron BA.1 variants. Especially, mRNA-based booster vaccination induced high levels of neutralizing antibodies and antibodies with various Fc-mediated effector functions such as antibody-dependent cellular cytotoxicity and phagocytosis. In contrast, T cell responses were similar in magnitude following homologous or heterologous booster vaccination, and retained functionality towards Delta and Omicron BA.1. However, only heterologous booster vaccination with an mRNA-based vaccine led to the expansion of SARS-CoV-2-specific T cell clones, without an increase in the breadth of the T cell repertoire as assessed by T cell receptor sequencing. In conclusion, we show that Ad26.COV2.S priming vaccination provides a solid immunological base for heterologous boosting with an mRNA-based COVID-19 vaccine, increasing humoral and cellular responses targeting newly emerging variants of concern.

Divergent SARS CoV-2 Omicron-specific T- and B-cell responses in COVID-19 vaccine recipients (preprint)

The severe acute respiratory distress syndrome coronavirus-2 (SARS-CoV-2) Omicron variant (B.1.1.529) is spreading rapidly, even in vaccinated individuals, raising concerns about immune escape. Here, we studied neutralizing antibodies and T-cell responses to SARS-CoV-2 D614G (wildtype, WT), and the B.1.351 (Beta), B.1.617.2 (Delta), and B.1.1.529 (Omicron) variants of concern (VOC) in a cohort of 60 health care workers (HCW) after immunization with ChAdOx-1 S, Ad26.COV2.S, mRNA-1273 or BNT162b2. High binding antibody levels against WT SARS-CoV-2 spike (S) were detected 28 days after vaccination with both mRNA vaccines (mRNA-1273 or BNT162b2), which significantly decreased after 6 months. In contrast, antibody levels were lower after Ad26.COV2.S vaccination but did not wane. Neutralization assays with authentic virus showed consistent cross-neutralization of the Beta and Delta variants in study participants, but Omicron-specific responses were significantly lower or absent (up to a 34-fold decrease compared to D614G). Notably, BNT162b2 booster vaccination after either two mRNA-1273 immunizations or Ad26.COV.2 priming partially restored neutralization of the Omicron variant, but responses were still up to-17-fold decreased compared to D614G. CD4+ T-cell responses were detected up to 6 months after all vaccination regimens; S-specific T-cell responses were highest after mRNA-1273 vaccination. No significant differences were detected between D614G- and variant-specific T-cell responses, including Omicron, indicating minimal escape at the T-cell level. This study shows that vaccinated individuals retain T-cell immunity to the SARS-CoV-2 Omicron variant, potentially balancing the lack of neutralizing antibodies in preventing or limiting severe COVID-19. Booster vaccinations may be needed to further restore Omicron cross-neutralization by antibodies.