While there was an increase in cases in older age groups — those aged 50 to 79 — in the first week of September, Kluge said, the biggest proportion of new cases is still among 25- to 49-year-olds.

In late August, Kluge said the gradual increase in Europe’s cases could be partly explained by “the relaxation of public health and social measures, where authorities have been easing some of the restrictions and people have been dropping their guard.”

He said he was “very concerned that more and more young people are counted among reported cases,” advising against large gatherings and parties.

In several countries, cases are rising particularly fast in densely populated cities, where people are returning to offices, schools and public places after measures eased following spring’s peak.