The only way we'll know when we need COVID-19 boosters

Midway through America’s first mass-immunization campaign against the coronavirus, experts are already girding themselves for the next. The speedy rollout of wildly effective shots in countries such as the United States, where more than half the population has received at least one dose of a COVID-19 vaccine, has shown remarkable progress—finally, slowly, steadily beating the coronavirus back. But as people inch toward something tantalizingly resembling pre-pandemic life, a cloud hangs over our transcendent summer of change: the specter of vaccine failure.