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 immunisation 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...