Inactivated SARS-CoV-2 vaccine (Vero cell) (Beijing Bio-Institute)

COVID-19 vaccine booster doses counteract waning immunity and vaccine escape by emerging variants. We evaluated long-term immunogenicity and safety of fractional and standard BNT162b2 vaccine booster doses in Mongolia.

In this randomised, controlled, non-inferiority trial, adults primed with two doses of ChAdOx1-S, BBIBP-CorV, or Gam-COVID-Vac were randomised (1:1) to receive a 15 μg (fractional dose) or 30 μg (standard dose) BNT162b2 booster. Geometric mean ratios (GMR) of IgG and surrogate virus neutralising test (sVNT) levels (Wuhan-Hu-1 and Omicron BA.1) were compared over 12 months. SARS-CoV-2 infections, and adverse and serious adverse events (SAEs), were documented.

gov Identifier: NCT05265065.

Of 601 participants randomised between May 27th and September 30th, 2022, 2 (0.3 %) were lost to follow-up and 19 (3.2 %) withdrew by 12 months. IgG levels declined from 28 days to six months, stabilising thereafter. At 12 months, IgG levels were lower in the fractional compared with the standard arm for ChAdOx1-S primed participants (GMR 0.78 [95 % CI 0.63-0.96], p = 0.017). At six and 12 months, the median sVNT inhibition percentages were comparable by study arm and priming strata. Documented SARS-CoV-2 infections occurred in 25 participants (fractional dose arm n = 12; standard dose arm n = 13). From 28 days, 228 undocumented infections (≥ 1.2-fold IgG increase) occurred (fractional arm n = 112; standard arm n = 116). SAEs (n = 41) were balanced between arms, with no severe vaccine-related AEs or SAEs reported.

15 μg and 30 μg BNT162b2 boosters demonstrated comparable immunogenicity and favourable safety. 15 μg BNT162b2 booster doses may improve vaccine acceptability due to lower reactogenicity.

To assess the dynamics of humoral immune responses to inactivated SARS-CoV-2 vaccines across populations with and without prior COVID-19 infection, a longitudinal cohort study was conducted. A total of 38 COVID-19-recovered individuals and 165 naïve participants (without prior COVID-19 infection) were enrolled, all of whom completed a two-dose vaccination regimen. Levels of anti-spike (anti-S) and anti-nucleocapsid (anti-N) antibodies were analyzed at baseline and post-vaccination time points, including 6 weeks post-first dose, and 1 month and 6 months post-second dose. Among naïve participants, the seropositivity rate for anti-S antibodies increased to 96.23% at 1 month after the second dose with anti-S titers peaking at a median of 54.59 U/mL (p < 0.0001). Conversely, COVID-19-recovered participants exhibited significantly elevated anti-S levels after the first dose (median titer: 637.70 U/mL, p < 0.0001), with no notable changes following the second dose. Anti-S levels in both groups declined by 6 months post-second dose. The dynamic pattern of anti-N antibodies was comparable to that of anti-S, albeit with weaker vaccine-induced responses. Notably, levels of both anti-S and anti-N antibodies decreased with advancing age (p < 0.001). Males demonstrated lower anti-N antibody levels compared with females (p = 0.038), while the presence of underlying diseases was associated with higher anti-S antibody levels (p = 0.030). In conclusion, two doses effectively augmented antibody levels in naïve individuals, whereas a single dose may suffice to confer immune protection in COVID-19-recovered individuals. Antibody levels wane over time, necessitating further investigations into the durability of vaccine-mediated immune protection, evidence-based recommendations for preventive vaccination, and the formulation of immunization strategies tailored to distinct populations.