Moreover, questions about transmission remain unsettled. Vaccinated people with asymptomatic or breakthrough infections may still be able to pass the virus on to others, but it is not yet clear how often that happens.
Until that science is more definitive, or until vaccination rates rise, it is best to err on the side of safety and regular testing, many experts said. At the Olympics, for instance, frequent testing could help protect the broader Japanese population, which has relatively low vaccination rates, as well as the support staff, who may be older and at higher risk.
“It’s those folks I’m most worried about, really,” said Dr. Lisa Brosseau, a research consultant at University of Minnesota’s Center for Infection Disease Research and Policy.
Not only can they contract the virus, adding strain on the Japanese health care system, but they can also become sources of transmission: “Everybody’s at risk, and everybody could potentially be infected,” she said.
According to the Tokyo 2020 press office, all Olympics staff and volunteers have been offered the opportunity to be vaccinated, though officials did not provide data on how many had received the shots.
Instead of testing less frequently, officials could rethink how they respond to positive tests, Dr. Binney said. For instance, if someone who is vaccinated and asymptomatic tests positive, he or she should still be isolated — but perhaps close contacts could simply be monitored, rather than being placed into quarantine.
“You’re trying to balance the disruptive nature of what you do when somebody vaccinated tests positive against any gains at slowing or stopping the spread of the virus,” Dr. Binney said.