| Summary: | Vaccination against infectious disease affords direct protection from vaccine-induced immunity and additional indirect protection for unvaccinated persons. A systematic review was conducted to estimate the indirect effect of COVID-19 vaccination. From PubMed and Embase, 31 studies were included describing the impact of original wild-type COVID-19 vaccines on disease transmission or viral load. Overall, study results showed the effectiveness of COVID-19 vaccination against SARS-CoV-2 transmission (range 16–95%), regardless of vaccine type or number of doses. The effect was apparent, but less pronounced against omicron (range 24–95% for pre-omicron variants versus 16–31% for omicron). Results from viral load studies were supportive, showing SARS-CoV-2 infections in vaccinated individuals had higher Ct values, suggesting lower viral load, compared to infections among the unvaccinated. Based on these findings, well-timed vaccination programs may help reduce SARS-CoV-2 transmission—even in the omicron era. Whether better-matched vaccines can improve effectiveness against transmission in the omicron era needs further study.
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