On Friday, the World Health Organization announced that the Zika virus is no longer a global health emergency. Now it’s a chronic threat.

The organization formally ended its “Public Health Emergency of International Concern,” which it initiated in February 2016 as the mosquito-borne virus was blazing through South and Central America. Since then, the virus has spread through nearly the entire Western Hemisphere. Researchers collected enough data to confirm that the virus causes devastating birth defects, most notably microcephaly, as well as other neurological disorders. Though Zika is still on the move, the WHO said now is the time to switch from crisis mode to long-term planning.

“We are not downgrading the importance of Zika,” Peter Salama, executive director of the WHO’s emergencies program, told The New York Times. “We are sending the message that Zika is here to stay, and the WHO response is here to stay.”

Health officials expect that the virus will become endemic and circulate seasonally in some countries. As such, the WHO will treat Zika like other perennial, mosquito-borne viruses, such as malaria, yellow fever, and dengue. “A robust longer-term technical mechanism [is] now required to manage the global response,” the organization said. And research on the virus “should be escalated into a sustained program of work with dedicated resources to address the long-term nature of the disease and its associated consequences.”

However, some experts were nervous about the end of the emergency period. Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases, expressed concern that the shift is coming just before the start of summer—and mosquito season—in the Southern Hemisphere.

“Are we going to see a resurgence in Brazil, Colombia, and elsewhere?” he asked. “If they pull back on the emergency, they’d better be able to reinstate it. Why not wait a couple of months to see what happens?”

Dr. Fauci and other researchers around the globe are hastily working on vaccines against the virus. Dozens of candidates are in the works. The National Institutes of Health have already started human clinical trials for their vaccine candidates, including one last week.