Since mid-December, flu viruses have been circulating in Europe at a higher-than-expected rate, the European Centre for Disease prevention and Control (ECDC) reported this month.

In December, the number of flu cases in European intensive care units (ICU) rose steadily to peak at 43 in the last week of the year, ECDC and World Health Organization data show. That is well below pre-pandemic levels but is a big increase on last year.

The return of the virus could be the start of an unusually long flu season that could stretch well into the summer, the ECDC's top expert on influenza Pasi Penttinen said.