The question is were they living in in close association with vaccinated people who were (possibly asymptomatic) breakthrough cases. The Oxford Uni team at the Hospital of Tropical Diseases in Ho Chi Minh City where most staff were fully vaccinated with AZ reported that viral load in infected staff was highest from 3 day before symptoms developed to 3 days after.
During the study 700 hundred of 900 hospital staff were locked down when the first case was detected. About 70 developed Covid delta strain; 2 had had 1 jab; the rest were fully vaccinated. The researchers considered their findings to be similar to what is happening in Israel where the Pfizer jab is being used.
Genome testing showed that the staff had a single source of infection which did not match with the local community.
