Just under 400 people will be infected with the Zika virus by mosquitoes in Florida by mid-September, and about 80 of them will develop symptoms, according to projections developed by an international team of scientists from the University of Florida and half a dozen other research institutions.
The study’s authors caution that the numbers, issued Tuesday by UF, are preliminary and subject to considerable uncertainty, but they say the projections provide reassurance that the current outbreak will be likely small and limited to Florida and other Southern states whose warm climates make them susceptible to mosquito-borne transmission. The outbreak is almost certain to peter out on its own by November or December as dipping temperatures put a damper on mosquito populations and an effective end to transmission, the projections show.
“I don’t think large-scale epidemics are possible in the U.S. because of window screening and air conditioning,” said Ira Longini, a professor of biostatistics at UF and one of the lead authors of the study. “We’re talking about small clusters here and there.”
The projections also put the number of babies in Florida that will be born from mothers who are infected by the virus during the first trimester — the period at which research suggests they’re most vulnerable to Zika-related birth defects — at eight. Infection does not necessarily mean that babies will be born with smaller than normal brains or any of the other birth defects, ranging from eye problems to joint problems, also associated with the virus.
Because data from the Zika epidemic in South America indicate that just one out of 100 babies exposed to Zika in the womb develop microcephaly, a case linked to locally transmitted Zika is statistically unlikely to occur in the United States given the relatively few pregnant women expected to be exposed, Longini said.
Most people infected with Zika experience no symptoms, and symptoms are mild and flu-like for most of those who do develop them. The virus also increases the risk of developing Guillain-Barre syndrome, in which a person’s immune system attacks the nervous system, but chances of that remain very small.
The study, developed in collaboration with scientists at Northeastern University, the University of Washington and several Italian institutions, carries several caveats. Because of a lack of data, the number of actual cases of local transmission could be somewhat lower or a lot higher, but would not constitute a widespread epidemic even in the worst scenario. The model also doesn’t account for cases due to sexual transmission.
The projections are based on a statistical model and website the scientists created outlining Zika’s spread through Central and South America, Mexico and the Caribbean and forecasting its course into the future. They applied the model to the continental United States after public-health authorities confirmed the first cases of local transmission in the country in Miami’s Wynwood neighborhood last month.
The results show that Florida will constitute the epicenter of the outbreak by a large margin, with 395 cases. Significantly smaller numbers of infections, ranging from three to 16, are projected in a swath of eight Southern states stretching from South Carolina to Oklahoma and Texas. The study calls those numbers “low.”
The Florida Department of Health has confirmed 42 local Zika infections by mosquitoes so far this year. The infections have been traced to four counties, with one each in Broward, Palm Beach and Pinellas and the remainder in Miami-Dade.
Longini said he expects other clusters to emerge in Florida in large metro areas such as Tampa and Orlando, and possibly in other Gulf states. The projections will be updated and refined every couple of weeks based on new data, including confirmed cases of local transmission, he said.
Scientists caution that even if the current Zika outbreak ends by itself, the virus will probably be reintroduced by travelers carrying it from places with full-fledged epidemics underway, such as Brazil or Puerto Rico, resulting in new clusters after summer weather and mosquito season returns. Those outbreaks will still likely be small as well.