As European countries prepare to reopen to tourists, two leading German doctors on Sunday warned that the mass movement of holidaymakers could prompt a second wave of the coronavirus pandemic.

Max Geraedts, a doctor and health researcher at the University of Marburg, told the dpa news agency that large gatherings could lead to "another sharp increase" in infections.

"If you walk alone on the beach, no matter where in the world, then, of course it doesn't matter," he said. "But when you go to a bar where there are lots of people, that can have unpleasant results."

Geraedts said that tourist destinations could be responsible for a second phase of exponential growth in cases which are then brought back to visitors' home countries.

Read more: Will EU's summer travel rebound from coronavirus pandemic?

He also warned that medical care abroad may not be as good as it is in Germany, especially if a traveler falls seriously ill with COVID-19.

"In the places where there is poorer health care, it is more dangerous — particularly for people who are already at risk," Geraedts added.

He said holidaymakers also risked endangering the local population: "If foreigners in need of treatment have to be factored in [by health systems abroad], that is, of course bad," he said. "Ethically it is at least questionable if you have few resources for those already there and then have to account for travelers too."

Not enough immunity

Another senior medical expert, Hajo Zeep, from the Leibniz Institute for Prevention Research and Epidemiology in Bremen warned that there is still not enough immunity in the population to prevent a second wave.

"Considering that a very small portion of the population is likely to be immune so far, it is clear that the virus could spread quickly if the conditions are right," Zeep told dpa.

Studies in Sweden and Italy last week showed that only around 7% of their respective populations have antibodies for COVID-19, a sign that herd immunity to the disease in Europe could still be a long way off.

Zeep, however, took comfort that infection rates are decreasing not only in Germany but in many Mediterranean travel destinations too.

Read more: EU tourist hot spots urge Germans to keep vacation plans

Restrictions eased

Germany has begun to relax some lockdown measures earlier this month, which allowed bars, restaurants, hair salons and retail stores to reopen their doors.

The nation's tourism industry has received a boost as hotels and vacation homes have been told they can welcome guests once more — under certain conditions and depending on state rules.

German Foreign Minister Heiko Maas and his counterparts in other European Union states have said that travel restrictions across the bloc could be lifted by mid-June.

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