Infectious viral load (VL) expelled as droplets and aerosols by infected individuals
partly determines transmission of severe acute respiratory syndrome coronavirus 2
(SARS-CoV-2). RNA VL measured by qRT-PCR is only a weak proxy for infectiousness.
Studies on the kinetics of infectious VL are important to understand the mechanisms
behind the different transmissibility of SARS-CoV-2 variants and the effect of vaccination
on transmission, which allows guidance of public health measures. In this study, we
quantified infectious VL in individuals infected with SARS-CoV-2 during the first
five symptomatic days by in vitro culturability assay in unvaccinated or vaccinated
individuals infected with pre-variant of concern (pre-VOC) SARS-CoV-2, Delta or Omicron
BA.1. Unvaccinated individuals infected with pre-VOC SARS-CoV-2 had lower infectious
VL than Delta-infected unvaccinated individuals. Full vaccination (defined as >2 weeks
after receipt of the second dose during the primary vaccination series) significantly
reduced infectious VL for Delta breakthrough cases compared to unvaccinated individuals.
For Omicron BA.1 breakthrough cases, reduced infectious VL was observed only in boosted
but not in fully vaccinated individuals compared to unvaccinated individuals. In addition,
infectious VL was lower in fully vaccinated Omicron BA.1-infected individuals compared
to fully vaccinated Delta-infected individuals, suggesting that mechanisms other than
increased infectious VL contribute to the high infectiousness of SARS-CoV-2 Omicron
BA.1. Our findings indicate that vaccines may lower transmission risk and, therefore,
have a public health benefit beyond the individual protection from severe disease.