MIAMI — The waning of Zika outbreaks in the Caribbean and South America has helped slow the spread of the mosquito-borne virus in Florida this year, according to health officials. Herd immunity, when enough people in an area are infected with a virus and develop resistance to it, likely has contributed to Zika's decline outside the continental United States, Dr. Henry Walke, the U.S. Centers for Disease Control and Prevention's incident manager for Zika response, said in a Miami Herald report . "People that were infected before can't be infected again. That's our understanding," Walke said. "So you don't have as much of the virus circulating. That's true not only in Puerto Rico but throughout the Caribbean and throughout South America." However, experts warn that herd immunity elsewhere won't stop the virus from re-emerging in this country. That has happened in Florida with other mosquito-borne viruses in recent years. "As we've seen with chikungunya and dengue, it is not unlikely that we will experience small outbreaks of Zika in the future," Florida Department of Health spokeswoman Mara Gambineri said in a written statement. Also, herd immunity eventually wears off, said Derek Cummings, an epidemiologist with the University of Florida's Emerging Pathogens Institute. "As years go by, more people are born into a population and they haven't encountered Zika, and so they're going to introduce susceptibility into the population," Cummings said. "Some number of years from now, those susceptible populations will rise to where you'd have sufficient numbers and then maybe we'd be doing this all over again." By the end of 2016, state health officials had confirmed 1,456 Zika infections in Florida, including 285 cases spread by mosquitoes in Miami and Miami Beach. The infections caused the CDC to issue an unprecedented domestic travel advisory warning pregnant women to avoid Miami-Dade County because the virus can cause severe birth defects. There is no vaccine or treatment for the virus, which can also spread through sexual contact. Florida health officials have reported 384 pregnant women who tested positive for Zika since January 2016, with nine delivering children with Zika-related birth defects. So far this year, Florida health officials have reported a total of 135 Zika cases, but none have been linked to mosquitoes in the state. The CDC lifted its travel warning for Miami-Dade County in June. Experts warn residents and visitors not feel complacent about contracting the virus. "The threat is still there," Walke said. "It hasn't gone away. It will not go away any time soon." Frequent rainfall, which benefits breeding mosquitoes, and the fact that many people infected with Zika don't feel any symptoms — such as fever, joint pain, red eyes or a rash — can fuel an outbreak in spite of increased mosquito-control efforts, said Bill Petrie, Miami-Dade's new mosquito control director. "So it could be circulating and we don't know it," he said.