PARIS (AP) — A top world health official Monday warned that countries are essentially driving blind in reopening their economies without setting up strong contact tracing to beat back flare-ups of the coronavirus.

The warning came as France and Belgium emerged from lockdowns, the Netherlands sent children back to school, and a number of U.S. states continued to lift their business restrictions.

Fears of infection spikes in places that have loosened up have been borne out in recent days in Germany, where new clusters were linked to three slaughterhouses; in Wuhan, the Chinese city where the crisis started; and in South Korea, where one nightclub customer was linked to 85 new cases.

Authorities have warned that the scourge could come back with a vengeance without widespread testing and tracing of infected people’s contacts.

Health officials in the U.S. will be watching closely in the coming days for any resurgence of the virus two weeks after states began gradually reopening, and efforts to assemble contact-tracing teams are underway there and in Europe.

The World Health Organization’s emergencies chief, Dr. Michael Ryan, said that robust contact tracing measures adopted by Germany and South Korea provide hope that those countries can detect and stop virus clusters before they get out of control.

But he said the same is not true of other countries exiting their lockdowns, declining to name specific countries.

“Shutting your eyes and trying to drive through this blind is about as silly an equation as I’ve seen,” Ryan said. “And I’m really concerned that certain countries are setting themselves up for some seriously blind driving over the next few months.”

Other countries are far behind Germany. Britain abandoned an initial contact- tracing effort in mid-March when the virus’s rapid spread made it impossible. Now it is recruiting 18,000 people to do the legwork of tracking contacts.

Britain and other countries are also developing contact-tracing cellphone apps that can show whether someone has crossed paths with an infected person.

In the hardest-hit corner of the U.S., contact tracers in New York began online training Monday, and Gov. Andrew Cuomo said some upstate areas, including the Finger Lakes, could ease their restrictions after Friday.

Contract tracing across the rest of the U.S. is a patchwork of approaches and readiness levels.