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

Laith J Abu-Raddad, Hiam Chemaitelly, Houssein H Ayoub, Patrick Tang, Peter Coyle, Mohammad R Hasan, Hadi M Yassine, Fatiha M Benslimane, Hebah A Al-Khatib, Zaina Al-Kanaani, Einas Al-Kuwari, Andrew Jeremijenko, Anvar Hassan Kaleeckal, Ali Nizar Latif, Riyazuddin Mohammad Shaik, Hanan F Abdul-Rahim, Gheyath K Nasrallah, Mohamed Ghaith Al-Kuwari, Adeel A Butt, Hamad Eid Al-RomaihiAbdullatif Al-Khal, Mohametabd H Al-Thani, Roberto Bertollini

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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.
Original languageEnglish
Article number532
JournalNature Communications
Publication statusPublished - 27 Jan 2022


  • Humans
  • Vaccination
  • Viral Load
  • Adolescent
  • Adult
  • Aged
  • Middle Aged
  • Child
  • Qatar
  • Female
  • Male
  • Young Adult
  • COVID-19
  • SARS-CoV-2
  • COVID-19 Vaccines
  • BNT162 Vaccine
  • 2019-nCoV Vaccine mRNA-1273


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