There is always the chance, though small, the authors write, that the virus could evolve resistance to a vaccine, what researchers call “viral escape.” They urge monitoring of vaccine effects and viral response, just in case.

“Nothing that we’re saying is suggesting that we slow down development of vaccines,” Dr. Kennedy said. An effective vaccine is of utmost importance, he said, “But let’s make sure that it stays efficacious.”

Vaccine makers could use the results of nasal swabs taken from volunteers during trials to look for any genetic changes in the virus. Test results need not stop or slow down vaccine rollout, but if recipients of the vaccine had changes in the virus that those who received the placebo did not, that would indicate “the potential for resistance to evolve,” something researchers ought to keep monitoring.