The World Health Organization's emergencies director warned Wednesday that vaccines would not arrive in time to defeat the second wave of the Covid-19 pandemic.

The WHO's Michael Ryan said vaccines should not be seen as a "unicorn" magic solution -- and countries battling a resurgence of the virus would once again have to "climb this mountain" without them.

"I think it's at least four to six months before we have significant levels of vaccination going on anywhere," he said, during a public question and answer session live on social media.

Despite recent promising announcements from final-phase candidate vaccine trials, "We're not there with vaccines yet," said Ryan.

"Many countries are going through this wave, and they're going to go through this wave, and continue through this wave, without vaccines.

"We need to understand and internalise that, and realise: we have got to climb this mountain this time, without vaccines."

Pfizer said Wednesday that a completed study of its experimental vaccine showed it was 95 per cent effective, while fellow US firm Moderna said this week that its own candidate was 94.5 per cent effective. Russia claims its candidate is more than 90 per cent effective.

Ryan warned against slackening off individual vigilance against the virus in the mistaken belief that vaccines would now solve the problem instead.

"Some people think a vaccine will be, in a sense, the solution: the unicorn we've all been chasing. It's not," the Irishman said.

"If we add vaccines and forget the other things, Covid does not go to zero."

The number of new Covid-19 cases in Europe declined last week for the first time in over three months, but deaths in the region continued to climb, WHO data showed Wednesday.

While at least 55.6 million cases worldwide have been registered since the outbreak emerged in China last December, the novel coronavirus has killed more than 1.3 million people around the globe, according to a tally from official sources compiled by AFP.

Ryan voiced concerns about how the pandemic has left many bereaved grandchildren who have been unable to go through the normal grieving process due to the restrictions imposed to combat the virus.

"A lot of kids in this have lost grandparents," he said. "There's quite a bit of trauma out there amongst kids.

"The grieving process for kids -- it's tough because you don't have all the natural things: the funeral situation, you don't get to say goodbye, you don't have the gatherings, so it's grieving interrupted.

"I am concerned if the child's need to grieve the loss of a grandparent has been interrupted because that has a lifelong impact."