Relative infectiousness of SARS-CoV-2 vaccine breakthrough infections, reinfections, and primary infections

SARS-CoV-2 breakthrough infections in vaccinated individuals and reinfections in those with prior infection are a public health concern. Here, the authors show that viral loads of vaccine breakthrough infections and reinfections are generally lower than primary infections of unvaccinated individuals, potentially indicating lower infectiousness. SARS-CoV-2 breakthrough infections in vaccinated individuals and in those who had a prior infection have been observed globally, but the transmission potential of these infections is unknown. The RT-qPCR cycle threshold (Ct) value is inversely correlated with viral load and culturable virus. Here, we investigate differences in RT-qPCR Ct values across Qatar's national cohorts of primary infections, reinfections, BNT162b2 (Pfizer-BioNTech) breakthrough infections, and mRNA-1273 (Moderna) breakthrough infections. Our matched-cohort analyses of the randomly diagnosed infections show higher mean Ct value in all cohorts of breakthrough infections compared to the cohort of primary infections in unvaccinated individuals. The Ct value is 1.3 (95% CI: 0.9-1.8) cycles higher for BNT162b2 breakthrough infections, 3.2 (95% CI: 1.9-4.5) cycles higher for mRNA-1273 breakthrough infections, and 4.0 (95% CI: 3.5-4.5) cycles higher for reinfections in unvaccinated individuals. Since Ct value correlates inversely with SARS-CoV-2 infectiousness, these differences imply that vaccine breakthrough infections and reinfections are less infectious than primary infections in unvaccinated individuals. Public health benefits of vaccination may have been underestimated, as COVID-19 vaccines not only protect against acquisition of infection, but also appear to protect against transmission of infection.

Automated summary

The viral loads of SARS-CoV-2 breakthrough infections and reinfections are generally lower than primary infections in unvaccinated individuals, indicating potentially lower infectiousness. COVID-19 vaccines not only protect against acquisition of infection but also appear to protect against transmission.