A World Health Organization (WHO) special COVID-19 envoy predicts that Europe could see a third wave of the pandemic in early 2021 if governments repeated failures that enabled a second wave of infections.

"They missed building up the necessary infrastructure during the summer months, after they brought the first wave under the control," Reuters quotes the WHO's David Nabarro.

"Now we have the second wave. If they don't build the necessary infrastructure, we'll have a third wave early next year," said Nabarro.

His warning came as Europe remains the second-hardest hit region by the pandemic, having reported more than 15.54 million cases with over 353,000 deaths. Only Asia has recorded more cases.

European countries had started seeing a decline in their caseloads, but infections began rising again in October, forcing some governments to reimpose restrictions that had been lifted.

Nabarro singled out Switzerland's move to allow skiing - with masks required in gondolas - as other Alpine nations like Austria have shuttered resorts. Nabarro said Switzerland could reach a "very high level of sicknesses and deaths."

"Once the infection rates sink, and they will sink, then we can be as free as we want," Reuters quotes Nabarro. "But right now? Should ski resorts open? Under what conditions?"

The WHO COVID-19 envoy said countries ought not to relax restrictions until case numbers decline and remain low.