Note: I will be on vacation for a few days.  Q&A will pause until I return on June 29th.  In the meantime, feel free to send your questions.

Question: Have you been looking into breakthrough cases of vaccinated individuals? I’ve been in lots of discussions about this topic, especially with colleagues in Africa.

Answer: None of the COVID vaccines are 100% effective, which means that breakthrough cases– cases occurring 14+ days after full vaccination– are possible.  Given the high rates of vaccine effectiveness, however, breakthrough cases are relatively rare, occurring in only a small percentage of fully vaccinated people.  Additionally, when breakthrough cases do occur, they are generally milder than cases among non-vaccinated individuals.  That said, the frequency of breakthrough cases varies based on a number of factors including individual-level susceptibility, underlying risk in the community, type of variant(s) circulating, and type of vaccine administered.  

For example, individuals who are vaccinated but immunocompromised may be at increased risk of breakthrough infection. Communities with high levels of transmission and individuals working in settings with increased risk of exposure are at increased risk of breakthrough infection– the higher the virus attack rate, the more breakthrough cases we’re likely to see.  And while Pfizer and Astra-Zeneca have been shown to remain effective against the Delta variant, their effectiveness is slightly reduced, which means that in places with the Delta variant, the risk of breakthrough infections is also higher.  Finally, not all vaccines are equal in terms of effectiveness, including effectiveness against variants of concern.  For example, Astra-Zeneca is less effective than Pfizer, and recent experiences in a number of countries that administered Sinovac and Sinopharm vaccines indicate that vaccine effectiveness may be lower for these vaccines and breakthrough infections may be more common.