“For now, the vaccines remain effective, but there is a trend toward less effectiveness,” said Dr. Céline Gounder, an infectious disease physician and epidemiologist at Bellevue Hospital in New York. Vaccine makers say they are poised to develop booster shots that would tackle especially troublesome variants, but such a fix would be of little help to poorer nations already struggling to obtain the existing vaccines. Experts say the best way to head off the emergence of dangerous variants is to tamp down new infections and immunize most of humanity as quickly as possible. Dr. Michael Diamond, a viral immunologist at Washington University in St. Louis, said that the longer the coronavirus circulates, the more time it has to mutate, which could eventually threaten vaccinated people; the only way to break the cycle is to ensure countries like India get enough vaccines.