Abstract
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.
Highlights
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
Leveraging the national, federated databases that have captured all SARS-CoV-2 vaccinations and polymerase chain reaction (PCR) testing in Qatar since the start of the epidemic (“Methods”), we investigated the effect of vaccination on infectiousness by comparing SARS-CoV-2 real-time reverse transcription-PCR (RT-qPCR) cycle threshold (Ct) values of individuals infected and fully vaccinated with the values of those infected and unvaccinated
The real-time (quantitative) reverse transcription-PCR (RT-qPCR) Ct value is a measure of the inverse of viral load and correlates strongly with culturable virus;[17] it can be used as a proxy of SARS-CoV-2 infectiousness[17–21]
Summary
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. 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. 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. Leveraging the national, federated databases that have captured all SARS-CoV-2 vaccinations and polymerase chain reaction (PCR) testing in Qatar since the start of the epidemic (“Methods”), we investigated the effect of vaccination on infectiousness by comparing SARS-CoV-2 real-time (quantitative) reverse transcription-PCR (RT-qPCR) cycle threshold (Ct) values of individuals infected and fully vaccinated with the values of those infected and unvaccinated. The RT-qPCR Ct value is a measure of the inverse of viral load and correlates strongly with culturable virus;[17] it can be used as a proxy of SARS-CoV-2 infectiousness[17–21]. The comparisons were implemented utilizing: (i) the national cohort of all 384,452 RT-qPCR-confirmed primary infections since epidemic onset (February 28, 2020) until the end of the study (July 11, 2021; Fig. 1a); (ii) the national cohort of all 1695 RT-qPCR-confirmed reinfections during the same period (Fig. 1b); (iii) the national cohort of all 898,648 individuals vaccinated with the BNT162b21 (Pfizer-BioNTech) vaccine since the first recorded vaccination in Qatar on December 16, 2020 until the end of the study (July 11, 2021; Fig. 1c); and (iv) the national cohort of all 468,872 vaccinated individuals with the mRNA-12732 (Moderna) vaccine during the same period (Fig. 1d)
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