Viral loads of Delta-variant SARS-CoV-2 breakthrough infections after vaccination and booster with BNT162b2

Nat Med. 2021 Dec;27(12):2108-2110. doi: 10.1038/s41591-021-01575-4. Epub 2021 Nov 2.


The effectiveness of the coronavirus disease 2019 (COVID-19) BNT162b2 vaccine in preventing disease and reducing viral loads of breakthrough infections (BTIs) has been decreasing, concomitantly with the rise of the Delta variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, it is unclear whether the observed decreased effectiveness of the vaccine in reducing viral loads is inherent to the Delta variant or is dependent on time from immunization. By analyzing viral loads of over 16,000 infections during the current, Delta-variant-dominated pandemic wave in Israel, we found that BTIs in recently fully vaccinated individuals have lower viral loads than infections in unvaccinated individuals. However, this effect starts to decline 2 months after vaccination and ultimately vanishes 6 months or longer after vaccination. Notably, we found that the effect of BNT162b2 on reducing BTI viral loads is restored after a booster dose. These results suggest that BNT162b2 might decrease the infectiousness of BTIs even with the Delta variant, and that, although this protective effect declines with time, it can be restored, at least temporarily, with a third, booster, vaccine dose.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antibodies, Neutralizing / blood
  • Antibodies, Viral / blood
  • BNT162 Vaccine / administration & dosage
  • BNT162 Vaccine / immunology*
  • COVID-19 / epidemiology
  • COVID-19 / immunology
  • COVID-19 / prevention & control*
  • COVID-19 Vaccines
  • Humans
  • Immunization, Secondary*
  • Israel / epidemiology
  • SARS-CoV-2 / immunology*
  • SARS-CoV-2 / isolation & purification
  • Time Factors
  • Vaccination / methods
  • Viral Load*


  • Antibodies, Neutralizing
  • Antibodies, Viral
  • COVID-19 Vaccines
  • BNT162 Vaccine

Supplementary concepts

  • COVID-19 breakthrough infections
  • SARS-CoV-2 variants