In New York City, health officials said Friday that of 1,160 people hospitalized with COVID-19 symptoms, one in four were between ages 18 and 49. That squares with what appears to be happening nationwide: Across the United States, about 38% of coronavirus patients sick enough to be hospitalized were ages 20 to 54, the CDC reported last week.

Young people outside the US have also been severely affected. In Spain, about 18% of hospitalized patients are under 50, according to the latest data. And in South Korea, more than half of confirmed cases are under 50, with the ages 20-29 being the largest age group. (The government doesn’t break down how many of those cases required hospitalization.) Anecdotal data out of the UK, where demographic data on patient ages has yet to be released by the government, also suggests that young people have needed intensive care in hospitals…

Some experts and health agencies have speculated that certain lifestyle factors may be playing a role in these cases of younger patients. Vapers and smokers may be especially susceptible, the National Institute on Drug Abuse suggested on its website this month.

But it’s too early to know to what extent those factors are contributing to the outbreak, or if they are unique to the US.