Discussion
ASYMPTOMATIC COVID-19, COMPLICATIONS
This Phase 3 study was conducted to assess heterologous booster effect of SII-NVX-CoV2373 in 372 adults who were primed with ChAdOx1 nCoV-19 or BBV152. The heterologous booster dose of SII-NVX-CoV2373 was non-inferior to homologous booster dose of either ChAdOx1 nCoV-19 or BBV152 in terms of anti-S IgG and nAbs. Also, the SII-NVX-CoV2373 booster showed significantly higher immune responses than BBV152 homologous booster in terms of anti-S IgG, nAbs and hACE2 receptor binding inhibition antibodies against prototype / ancestral strain as well as Omicron variants. All three vaccines were found safe and well tolerated.The homologous booster dose of COVID-19 vaccine in India started in January 2022 when the at-risk population (elderly, healthcare workers, etc.) were offered a free dose six months after the last doseGMTs/GMEUs of both the antibodies with SII-NVX-CoV2373 booster were at around 2 times higher than the ChAdOx1 nCoV-19 booster and > 5 times higher than the BBV152 booster. Even the seroresponse rates with SII-NVX-CoV2373 were numerically higher than those in the ChAdOx1 nCoV-19 and BBV152 groups. In the COV-BOOST study, NVX-CoV2373 was found to show higher immune response as compared to viral vector vaccines and inactivated vaccinesThere are no head to head efficacy studies but in general, primary immunization with NVX-CoV2373 has been found more protective against symptomatic COVID-19 than viral vector vaccines and inactivated vaccines. The vaccine gave around 90% efficacyThe real-world evidence from different COVID-19 vaccines (prototype strain) supports the completion of primary series and booster vaccinations where appropriate, especially to restore waning vaccine effectiveness against the more infectious Omicron variant and protect populations from severe outcomes, hospital admissions, and longer lasting post-COVID-19 complications, as well as mortalityNeutralizing antibody titers are an accepted measure of immunity against COVID-19 while there is a strong correlation for nAb titers with anti-S IgG and hACE-2 binding inhibition among vaccinated individualsIt is interesting to note that the baseline titers of both anti-S IgG and nAbs in the ChAdOx1 nCoV-19 prime group were almost two times as compared to the BBV152 prime group even though the intervals since the last vaccination were quite similar. This has been reported earlier that ChAdOx1 nCoV-19 gave higher, more durable antibody response than BBV152Effectiveness of ChAdOx1 nCoV-19 has also been higher than BBV152Our study had a few limitations. Our study was unable to assess the immunogenicity among the elderly population with sufficient sample size, as most of this population had already received a booster dose prior to the commencement of the study. This exclusionary factor limited the elderly cohort, constituting only 5% of our study participants. This subset of participants, especially those with comorbidities, might exhibit immunosenescence, thereby adding complexity to our observations. The study did not assess BA.2, BA.3, BA.4, and Delta-variants. We evaluated only one booster dose whereas in some countries second and third boosters have also been introduced particularly for older adults and immunocompromised individualsA study reported that the heterologous booster induced greater systemic reactogenicity than their homologous counterpartsIn conclusion, a heterologous booster of SII-NVX-CoV2373 in ChAdOx1 nCoV-19 and BBV152-primed adults demonstrated a non-inferior booster response, which was numerically higher than that achieved with a homologous booster of ChAdOx1 nCoV-19 and BBV152. The vaccine also exhibited an acceptable safety profile.