Taiwan won plaudits for its successful containment of Covid-19 last year, which made its recent virus resurgence all the more surprising. Data show it was unusually deadly as well.

While the country has seen fewer than 800 Covid-19 deaths, 500 of them occurred in June alone, amid its biggest virus wave to date. The pathogen got through the stringent border curbs that had kept local infections at bay for most of 2020, seeding an outbreak that tore through the then-largely unvaccinated elderly population...