Vexing and volatile variants of the coronavirus are adding fuel to the fire as Japan braces for its fourth and possibly largest wave of the pandemic.

Tokyo, Kyoto and Okinawa on Monday joined a growing list of prefectures that have imposed a fresh set of countermeasures to fend off a viral rebound inflamed by new strains of the coronavirus that are more contagious and harder to detect.

A significant increase in testing for COVID-19 — and genomic screening for its variants — would better reveal where and to what extent the mutant contagion is spreading, and allow public officials to respond accordingly, but experts question the country’s willingness and ability to do so.