The World Health Organization (WHO) says older people and high-risk groups must protect themselves with a flu vaccination ahead of the winter months.

A rise in COVID-19 cases and hospital admissions is expected as temperatures drop and people begin to spend more time in indoor spaces, which could put the elderly at risk.

The WHO has also warned that Europe is entering a "risky moment" as schools reopen across the continent and large numbers of young people continue to socialize.

Hans Kluge, the WHO's regional director for Europe, told a news briefing on Thursday that throughout the summer, European countries have recorded higher numbers of COVID-19 infections among young people.

He said: "It may be that the younger people are not necessarily going to die from it, but it's a tornado with a long tail. It's a multi-organ disease, so the virus is really attacking the lungs, but also the heart and other organs.

"Younger people, particularly with the winter coming, will be in closer contact with the elder population."

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Health experts now fear that schools could become drivers of COVID-19 infections and Kluge says he intends to discuss this issue with the EU's member states at a meeting on 31 August.

"What we know is that we can't open societies without opening the schools first," he said. "So the key issue here is that it depends a lot on the level of transmission in the community. Basic measures have to be applied everywhere."

He added: "So far, we know that the school setting has not been a main contributor to the epidemic. There are also more and more publications that add to the body of evidence that children do play a role in the transmission, but that this is so far more linked with social gatherings."