The top U.S. infectious disease specialist called the coronavirus pandemic his “worst nightmare” and warned that the deadly outbreak is far from over.

In just four months, COVID-19 has devastated countries around the world, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Tuesday in an online address to the Biotechnology Innovation Organization, an industry group.

Societies, businesses and economies have all suffered as people have sequestered themselves from others to prevent the spread of the virus and inundation of health systems with severely ill patients. Now that some nations are emerging from lockdowns, there’s still a risk that the virus will also return, Fauci said.

The infection won’t “burn itself out with mere public health measures,” he said. “We’re going to need a vaccine for the entire world, billions and billions of doses.”

More than 100 vaccines are in development against the coronavirus, according to the World Health Organization. While some have been predicted for delivery as early as this year, they must first show safety and effectiveness in human tests that are already underway.