Effectiveness of vaccination against symptomatic and asymptomatic SARS-CoV-2 infection: a systematic review and meta-analysis

Meggiolaro, A.; Sane Schepisi, M.; Nikolaidis, G.; Mipatrini, D.; Siddu, A.; Rezza, G.

OBJECTIVE: To assess the effectiveness of SARS-CoV-2 vaccines in terms of prevention of disease and transmission. The evaluation was narrowed to two mRNA vaccines and two modified adenovirus vectored vaccines. METHODS: A frequentist random effects meta-analysis was carried out after data extraction. Risk of bias of the included studies was assessed using New-Castle-Ottawa Scale. The overall risk of SARS-CoV-2 infection confirmed by real time Polymerase Chain Reaction (PCR) was estimated in partially and fully vaccinated individuals. The effect size was expressed as Relative Risk (RR) and RRR (RR reduction) of SARS-CoV-2 infection after vaccination. Potential sources of heterogeneity were investigated through between-study heterogeneity analysis and subgroup meta-analysis. RESULTS: The systematic review identified 27 studies eligible for the quantitative synthesis. Partially vaccinated individuals presented a RRR=73% (95%CI=59%-83%) for any positive SARS-CoV-2 PCR (RR=0.27) and a RRR=79% (95%CI=30%-93%) for symptomatic SARS-CoV-2 PCR (RR=0.21). Fully vaccinated individuals showed a RRR=94% (95%CI=88%-98%) for any SARS-CoV-2 positive PCR (RR=0.06) compared to unvaccinated. According to the subgroup meta-analysis, full BNT162b2 vaccination protocol achieved a RRR=84%-94% against any SARS-CoV-2 positive PCR and a RRR=68%-84% against symptomatic positive PCR. The RR for any SARS-CoV-2 positive PCR remained higher within elderly groups aged [≥]69 years (RR=0.12-0.15) compared to younger individuals (RR=0.05-0.12). The RR against B.1.351 infection approached 0.40 for any positive PCR and 0.36 for symptomatic SARS-COV-2 while the RR of any B.1.1.7 infection was 0.14. CONCLUSION: The current licensed vaccines may be transmission blocking, especially after full vaccination protocol. Given the substantial heterogeneity, results should be interpreted with caution. Subgroups meta-analyses suggested that the risk of any SARS-CoV-2 infection may be higher for non-B.1.1.7 variants and individuals aged [≥]69 years. Further data and longer follow-up are required to investigate additional sources of heterogeneity and the effectiveness of SARS-CoV-2 vaccination within population subgroups.