Effects of SARS-CoV-2 Alpha, Beta, and Delta variants, age, vaccination, and prior infection on infectiousness of SARS-CoV-2 infections

Suelen H. Qassim, Mohammad R. Hasan, Patrick Tang, Hiam Chemaitelly, Houssein H. Ayoub, Hadi M. Yassine, Hebah A. Al-Khatib, Maria K. Smatti, Hanan F. Abdul-Rahim, Gheyath K. Nasrallah, Mohamed Ghaith Al-Kuwari, Abdullatif Al-Khal, Peter Coyle, Imtiaz Gillani, Anvar Hassan Kaleeckal, Riyazuddin Mohammad Shaik, Ali Nizar Latif, Einas Al-Kuwari, Andrew Jeremijenko, Adeel A. ButtRoberto Bertollini, Hamad Eid Al-Romaihi, Mohamed H. Al-Thani, Laith J. Abu-Raddad*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

5 Citations (Scopus)
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In 2021, Qatar experienced considerable incidence of SARS-CoV-2 infection that was dominated sequentially by the Alpha, Beta, and Delta variants. Using the cycle threshold (Ct) value of an RT-qPCR-positive test to proxy the inverse of infectiousness, we investigated infectiousness of SARS-CoV-2 infections by variant, age, sex, vaccination status, prior infection status, and reason for testing in a random sample of 18,355 RT-qPCR-genotyped infections. Regression analyses were conducted to estimate associations with the Ct value of RT-qPCR-positive tests. Compared to Beta infections, Alpha and Delta infections demonstrated 2.56 higher Ct cycles (95% CI: 2.35-2.78), and 4.92 fewer cycles (95% CI: 4.67- 5.16), respectively. The Ct value declined gradually with age and was especially high for children <10 years of age, signifying lower infectiousness in small children. Children <10 years of age had 2.18 higher Ct cycles (95% CI: 1.88-2.48) than those 10-19 years of age. Compared to unvaccinated individuals, the Ct value was higher among individuals who had received one or two vaccine doses, but the Ct value decreased gradually with time since the second-dose vaccination. Ct value was 2.07 cycles higher (95% CI: 1.42-2.72) for those with a prior infection than those without prior infection. The Ct value was lowest among individuals tested because of symptoms and was highest among individuals tested as a travel requirement. Delta was substantially more infectious than Beta. Prior immunity, whether due to vaccination or prior infection, is associated with lower infectiousness of breakthrough infections, but infectiousness increases gradually with time since the second-dose vaccination.

Original languageEnglish
Article number984784
Number of pages15
JournalFrontiers in Immunology
Publication statusPublished - 13 Sept 2022


  • Immunology
  • COVID-19
  • SARS-CoV-2 variant
  • vaccine
  • reinfection
  • breakthrough infection
  • immunity
  • epidemiology
  • PCR


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