Zika, the mosquito-borne virus that triggered public health alarm bells last summer, has receded from the spotlight. But, experts say, expect the virus to pose a renewed threat this year.

How great of a threat? That’s where it gets tricky.

No locally acquired cases of the virus have been reported in the United States this year. But as public health agencies gear up for mosquito season, uncertainty remains around what resources states may need and whether they will receive adequate federal support. In addition, researchers still have questions about how the virus works and its long-term effects. These forces could complicate efforts to track outbreaks and provide accurate information about prevention and disease management.

“We still have much to learn. And much remains to be done,” Lyle Petersen, director of the Centers for Disease Control and Prevention’s Division of Vector-Borne Diseases, said last week at a House Energy and Commerce subcommittee hearing.

Zika, which can cause birth defects if contracted by pregnant women, is primarily transmitted by a mosquito more commonly found in southern areas of the country such as the Gulf Coast. Texas, Florida, Southern California and Louisiana were all deemed high-risk areas last year. It hit the Americas in 2015 and reached the United States via infected travelers.

Last year, 5,102 people in the United States were reported with the disease — most contracted the virus while traveling in South and Central America. Sixty-four babies in the U.S. have been affected since last June. In rare cases, people who had not traveled abroad contracted Zika through a local mosquito or through sexual contact. Puerto Rico, a U.S. territory, had 34,963 cases in 2016.

Again this year, determining if areas are experiencing active Zika transmission will require active surveillance and testing — both of mosquitoes and of people potentially exposed. Most people who contract the virus don’t show visible symptoms.