British scientists at the University of Oxford examined national records of nearly 150,000 contacts that were traced from roughly 100,000 initial cases. The samples included people who were fully or partially vaccinated with either the Pfizer-BioNTech or the AstraZeneca vaccines, as well as people who were unvaccinated. The researchers then looked at how the vaccines affected the spread of the virus if a person had a breakthrough infection with either the alpha variant or the highly contagious delta variant.

Both vaccines reduced transmission, although they were more effective against the alpha variant compared to the delta variant. When infected with the delta variant, a given contact was 65 percent less likely to test positive if the person from whom the exposure occurred was fully vaccinated with two doses of the Pfizer vaccine. With AstraZeneca, a given contact was 36 percent less likely to test positive if the person from whom the exposure occurred was fully vaccinated.