WHO officials said this month the proportion of younger people among those infected had risen globally, putting at risk vulnerable sectors of the population worldwide, including the elderly and sick people in densely populated areas with weak health services.

“The epidemic is changing,” WHO Western Pacific regional director, Takeshi Kasai, told a virtual briefing. “People in their 20s, 30s and 40s are increasingly driving the spread. Many are unaware they are infected.”

“This increases the risk of spillovers to the more vulnerable,” he added.

A surge in new cases has prompted some countries to re-impose curbs as companies race to find a vaccine for a virus that has battered economies, killed more than 770,000 people and infected nearly 22 million, according to a Reuters tally.