It comes as the UK government was warned NHS managers are "concerned" about the prospect of a second wave of Covid-19.
Dr Hans Kluge, Europe regional director for WHO says increasing infections among young people could be driving recent spikes in Covid-19 across Europe.
He told BBC Radio 4's Today programme that authorities needed to communicate better with younger members of society to avoid further outbreaks.
"An increasing number of countries are experiencing localised outbreaks and a resurge in cases. What we do know, is that it's a consequence of change in human behaviour," he said.
"We're receiving reports from several health authorities of a higher proportion of new infections among young people. So for me, the call is loud enough to rethink how to better involve young people."
Dr Kluge said that as a father of two daughters he understood that young people "do not want to miss the summer".
But added: "They have a responsibility towards themselves, their parents, grandparents and their communities and we do know, now, how to adopt good healthy behaviours so let's take advantage of the knowledge."
The warning comes as NHS bosses in the UK voiced fears about a potential second wave of the virus.
Niall Dickson, chief executive of the NHS Confederation, told the All-Party Parliamentary Group on Coronavirus (APPG), that the levels of concern on the issue were "very high".
He said: "I would say in relation to the second spike issue or something coming, the levels of concern among our members – the people who are leading NHS trusts, who are leading in primary care and all levels in the systems – is very high".
Mr Dickson continued: "I mean, of course, there’s real concern about winter and the compounding factors there, but also about an earlier spike.
"We have already mentioned exhausted staff (and) we are already trying to rebuild other services."
He said non-Covid productivity in NHS trusts was currently at about 60%.
The British Medical Association (BMA) has added its voice to the chorus of warnings about a second wave.
In its submission paper for the APPG, the BMA said the government needs to provide a detailed plan on how it will provide adequate PPE in the event of a second wave.
The union criticised Downing Street for publicising "arbitrary figures" of "billions of items of PPE" earlier in the crisis, adding that single gloves were classed as one item - when medics need to wear a pair at once.
Dr Chaand Nagpaul, chairman of the council of the BMA, warned the NHS is entering its "busiest" time of year and is already facing a "huge backlog of care".
He said the system needs to "coexist" by treating Covid-19 patients, as well as non-Covid-19 patients and those who suffer winter flu. But he said that a second spike should not be seen as an "inevitability".
Dr Nagpaul called for more concise public messaging over measures people can take to stop the spread of the virus.
He said: "If you look at the figures in the moment – the last ONS weekly figures from last Friday – the infection rate has increased. We’re now seeing about 2,700 new cases a day compared to 2,500 the week before.
"I think now is the time, we must be much more robust and rigorous around how we mitigate better."
One expert said that it is "too early" to say whether some European countries were experiencing a second wave of coronavirus.
Dr Michael Head, senior research fellow in global health at the University of Southampton, said: "It is a little early to say definitively whether some European countries, such as Spain or Germany, are experiencing the start of a second wave, or simply seeing spikes in their caseloads.
"The long-term decline to zero cases of Covid-19 will always see bumps in their graphs within the downward trend.
"It is up to all of these countries to ensure that these are only 'spikes' and not a 'second wave'," he said.
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