The indirect effect of mRNA-based Covid-19 vaccination on unvaccinated household members

ABSTRACT

Mass vaccination is effective in reducing SARS-CoV-2 infections among vaccinated individuals. However, it remains unclear how effectively Covid-19 vaccines prevent people from spreading the virus to their close contacts in real-world circumstances. Here, using nationwide administrative datasets on SARS-CoV-2 infections, vaccination records, demographics, and unique household IDs, we provide evidence about the direct and indirect effectiveness of Covid-19 vaccines in reducing infections among vaccinated individuals and their unvaccinated household members. Our estimates for adults imply indirect effectiveness of 8.7% (95% CI -28.9 to 35.4) two weeks and 42.9% (95% CI 22.3 to 58.1) 10 weeks after the first dose. We find that the indirect effect of Covid-19 vaccines is smaller and less precise for unvaccinated children aged between 3 to 18 years than for adults. These results provide household-level evidence that vaccines do not only protect vaccinated individuals but provide indirect protection to unvaccinated individuals in a real-world setting.