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Zika epidemic could burn out in 3 years but return after 10

By Debora Mackenzie

25 July 2016

An infant with microcephaly caused by zika infection

Zika infection during pregnancy can cause babies to be born with microcephaly

Mario Tama/Getty

The Zika epidemic could be over in three years – but by then up to 93.4 million people may have been infected, including 1.65 million women of childbearing age, and tens of thousands of babies could be affected. But even if the epidemic fizzles out, this could be a temporary relief: after a decade, the virus may well be back.

Zika arrived in the Americas from Asia in 2013. Since then it has spread rapidly, as people in the region had no previous exposure or immunity to the virus. The babies of women infected early in pregnancy have been born with microcephaly and other birth defects; a recent World Health Organization estimate suggests there have been 1781 brain-damaged babies born in countries recently infected with the virus.

Meanwhile, two people in Florida are expected to be confirmed as the first to catch the virus from mosquitoes in the US. Surveillance is so sparse that the virus could already be spreading widely, undetected, in the southern US, says Peter Hotez at Baylor College of Medicine in Houston, Texas.

Novel infections spread unhindered only until so many people have been exposed and become immune that the virus is unlikely to encounter people who are still susceptible. Not everyone needs to be immune for this “herd immunity” to happen and the epidemic to burn out.

“We wanted to know how much of a given population will have been infected when herd immunity kicks in,” says Alex Perkins at Notre Dame University in Indiana. Using data for the rate at which Zika and the closely related dengue and chikungunya viruses spread, Perkins and colleagues used mathematical modelling to calculate how many people would get Zika before it burned out in each 5 kilometre-square parcel of Central and South America.

Worst-case scenario

Importantly, the team varied the rate of viral spread according to local conditions. One was climate: the virus matures and spreads more slowly in cooler mosquitoes. The other was prosperity: richer people tend to have air-conditioning and window screens, reducing exposure to virus-carrying mosquitoes.

As their model epidemic unfolded in all the 5-kilometre squares, 93.4 million people became infected before herd immunity kicked in. But the number who get infected in reality probably won’t be quite as many, says Perkins, because the virus may never arrive in some places, and in others it may randomly fizzle out before spreading far.

So 93.4 million is a worst-case scenario, although it is impossible to say how much better the reality might be. What Perkins really wants to know is how closely epidemics in local regions match the model’s predictions, so he has made his team’s full results available to health authorities in the region.

Over in three years

In a separate study, Neil Ferguson and colleagues at Imperial College, London, have modelled herd immunity across the region as a whole, and calculated that the current epidemic will be largely over in three years.

But if Zika can hang on after that, by spreading among people who aren’t yet immune, it could explode again after 10 years. That’s because most children born in those years wouldn’t have been exposed to the virus, so would form a large non-immune population – just right for an epidemic.

Ferguson’s model predicts that it is possible that Zika will hang on in humans. But a recent study by Ferguson and others found that the virus doesn’t seem to hang on in people in Asia and Africa unless it is also spreading among forest monkeys, which occasionally pass it to people. No one knows if monkeys in South America can do this, but some have been found to be infected with Zika.

In any case this boom and bust cycle means the epidemic could fizzle out just as vaccines and drugs are ready for testing. Ferguson’s team is calling for trials of Zika treatments and vaccines to be designed so that even if the epidemic burns out, they are ready to kick in quickly should Zika roar back again.

Journal references: Local modelling from Nature Microbiology, DOI: 10.1038/nmicrobiol.2016.126 ; Regional modelling from Science, DOI: 10.1126/science.aag0219

Read about the latest Zika findings and efforts to stop the virus

Article amended on 26 July 2016

The timing of Zika's return after the current outbreak ends has been clarified

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