When a Zika Virus Outbreak Is Considered 'Over'

— -- The outbreak of locally spread Zika virus in Florida has led to a large response by state and local health authorities to curb the outbreak. But despite all their work to kill the mosquitoes that spread the virus and identify those who are infected, it will likely take time before they can declare the outbreak "over."

Health departments have to look at multiple factors to declare an end to an outbreak, meaning that even if they haven't found a new infection in weeks, the outbreak may technically be considered ongoing.

In some viral outbreaks, health departments have been able to look at the incubation period of an illness -- the time between when a person is infected to when a person starts to show symptoms. When no new cases are found in an area after twice the incubation period has passed, the outbreak is considered over. During the Ebola outbreak, health officials estimated an outbreak was over if 42 days had passed with no new infections, since the incubation period for that virus is generally believed to be up to 21 days.

However, health officials in Florida have a more difficult job, since the Zika virus is spread mainly through infected mosquitoes, rather than from person to person, as with Ebola. In the case of a mosquito-borne virus, the incubation period is less important because the mosquitoes are a reservoir for the virus.

Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center, explained health officials will likely be extremely cautious and look at a variety of factors before declaring an outbreak over.

"They're going to keep their intense surveillance going not just going in the Wynwood area [the northern Miami neighborhood where the outbreak is located] but obviously all through southern Florida," he said. "They would like as much time as possible before they commit themselves and say it's over."

Schaffner explained that declaring an outbreak over too early can do serious damage to the public's view of the health department. Additionally, it could lead to additional cases if people stop taking preventative measures.

"It's a matter of judgment. They would like some period of time and if there are no cases discovered and if there is sustained evidence that the Aedes [aegypti mosquito] population has been reduced," he said.

So far, 16 people have been found infected from the local Zika outbreak in Miami, according to health officials. Aerial pesticide spraying began last week of the area where the outbreak is occurring to reduce the population of the Aedes aegypti mosquitoes that spread the disease.