Effect of mRNA vaccination and previous infections on SARS-CoV-2 transmission across four variants: adjusted analysis of 111’432 declared contacts

Abstract The immunity conferred by SARS-CoV-2 vaccines and infections reduces the transmission of the virus. But it is not clear how the effect of immunity is shared between a reduction of the contagiousness and an increased protection against infection. To answer this question, we used a register of >50’000 SARS-CoV-2 positive index cases and their >110’000 declared contacts to estimate the association between secondary attack rate and immunity status (natural infection, vaccine, or both). Analyses were stratified per four SARS-Cov-2 variants and adjusted for index cases and contacts socio-demographic characteristics and the propensity of the contacts to be tested. The reduction of the propagation conferred by immunity was mainly a protection of the contacts against infection rather than a diminution of the contagiousness of the index cases. The largest immunity effect was conferred for both by natural infection, especially when recent for the contacts. Although of smaller amplitude, the effect of vaccination for index, i.e. the reduction of contagiousness, was less affected by SarS-CoV-2 variant and time since vaccination than the effect for contacts, i.e. the increase of protection against infection. Indeed, vaccination offered protection against infection only if given less than 6 months before and only for the variants preceding omicron, while the vaccination kept moderately reducing the index infectivity during the omicron wave, even when given more than 6 months ago. Hybrid immunity (vaccination and infection) did not have increased effectiveness than recent infection. These findings support the idea that vaccination also protects others, and highlight the need for the implementation of non-personal interventions reducing Sars-CoV-2 propagation, such as ventilation or air filtration.