colombia, zika, zika virus.jpg
A view through a microscope shows the Aedes aegypti mosquito, which transmits the virus Zika, at a laboratory at the National Institute of Health in Bogota, Colombia, 26 April 2016.
Photograph: EPA/LEONARDO MUNOZ

The Zika epidemic in Latin America may have already passed its peak, according to a study in the US journal Science published Thursday.

Public health researchers from Imperial College in London and the Johns Hopkins Bloomberg School of Public Health in Baltimore said there was "limited time" to start testing vaccines before Zika transmission rates fell too low to measure their impacts.

They expect the current Zika epidemic to be "largely over" in three years, they said.

From that point, disease models showed that herd immunity - in which a critical mass of people in a given population are immune to a disease, preventing transmission - would fend off further large epidemics of Zika for at least a decade.

People who have been infected with Zika become immune to future infection.

Researchers used models of secondary infection rates and time lags between outbreaks to look at the Zika epidemic's past and possible future.

But they had no clear answer for why the virus has hit so hard in Latin America, and whether other regions like Asia might be at risk for future epidemics.

An outbreak of the virus in 2015 has since spread throughout Latin America and the Caribbean as well as the United States. The US Congressional Research Service estimated 4 million people in the western hemisphere could be at risk of infection in 2016.

In Brazil alone 1.5 million people have been infected with the virus, which can cause severe neurological defects in babies born to mothers infected with the virus.

While fighting the virus by combating the Aedes aegypti mosquitos that carry it can be effective in reducing infection rates, researchers said, it might also be counterproductive in keeping the disease under control.

The fewer people infected, the less powerful herd immunity will be - and the less time it will take before the population of people not immune to the virus grows to the point at which a new epidemic can begin.

Researchers in the US and Brazil are developing possible Zika vaccines, which they hope to start testing this year.

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