For epidemiologists, the past year and a half has been a voyage of discovery. Recently their journey aboard SARS-CoV-2 took an unexpected turn toward Seychelles, a palm-fringed archipelago in the Indian Ocean with fewer than 100,000 inhabitants. A country that few could pinpoint on a map suddenly became internet-famous as the most vaccinated nation on Earth, with 64% of the population having received the requisite two shots. Yet to the surprise of virologists—and the dismay of the government, which had been counting on the immunization drive to reopen the tourism-dependent economy—the infection count has been ticking up. As of May 13 a third of active cases—about 900 in all—were among residents who’d been fully vaccinated.
Vaccine skeptics pronounced themselves vindicated, while international health experts have been scrambling to answer a host of questions without the benefit of robust data. Did one or both of the vaccines used in Seychelles fail? Has herd immunity not been reached? Is the nation grappling with a more infectious variant capable of evading the defenses that certain types of vaccines provide?