WASHINGTON — As Europe and the U.S. loosen their lockdowns against the coronavirus, health experts are expressing growing dread over what they say is an all-but-certain second wave of deaths and infections that could force governments to clamp back down.

“We’re risking a backslide that will be intolerable,” said Dr. Ian Lipkin of Columbia University’s Center for Infection and Immunity.

Around the world, German authorities began drawing up plans in case of a resurgence of the virus. Experts in Italy urged intensified efforts to identify new victims and trace their contacts. And France, which hasn’t yet eased its lockdown, has already worked up a “reconfinement plan” in the event of a new wave.

“There will be a second wave, but the problem is to which extent. Is it a small wave or a big wave? It’s too early to say,” said Olivier Schwartz, head of the virus unit at France’s Pasteur Institute.

In the U.S., with about half of the states easing their shutdowns to get their economies restarted and cellphone data showing that people are becoming restless and increasingly leaving home, public health authorities are worried.

Many states have not put in place the robust testing that experts believe is necessary to detect and contain new outbreaks. And many governors have pressed ahead before their states met one of the key benchmarks in the Trump administration’s guidelines for reopening — a 14-day downward trajectory in new illnesses and infections.