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3 August 2016

There could already be 40,000 people carrying Zika in the US

People in an airport
What will they bring back from their holiday?

Mark Waugh/Alamy Stock Photo

As many as 40,000 people in the US could already be carrying the Zika virus, having caught it while travelling abroad, a team analysing the epidemic has warned.

This figure was calculated by extrapolating from the numbers of pregnant women who have been confirmed to be carrying the virus after visiting countries where it is established, like Brazil and Colombia. In most people, Zika has no symptoms or causes only mild illness, but babies born to women infected during pregnancy can have microcephaly, which can cause brain damage.

Infected pregnant women who have travelled are the best proxies for the total number of travel-related cases, says Alessandro Vespignani at Northeastern University in Boston, Massachusetts. This is because pregnant women see medical professionals more regularly, so any infections they have are more likely to be detected.


Taking airline transit numbers and epidemic data from South America into account, Vespignani’s model estimates that pregnant women account for around 1 per cent of the total numbers of infected travellers returning to the US.

Newly released figures from the US Centers for Disease Control and Prevention reveal that the number of infected pregnant returnees to the US has now risen to 433, bringing Vespignani’s estimate for the total number infected up to 43,300.

Local transmission

Oliver Brady at the London School of Hygiene and Tropical Medicine says that Vespignani’s predictions are supported by multiple data sources, but that the extrapolation should be treated only as a ballpark figure. It may be an overestimate because many travellers from the US can afford to stay in air-conditioned hotels and use insect repellants, factors that reduce the risk of infection.

Nevertheless, Vespignani’s estimate makes it seem likely that many people are already carrying the virus in areas of the US that are inhabited by the Aedes aegypti mosquitoes that are capable of spreading the virus between people.

The first locally spread cases of Zika in the US were confirmed by the CDC last week, in Miami, Florida, and the number who have been infected there has since risen to 14. The CDC is now advising pregnant women to avoid visiting the “transmission area” in the Wynwood neighbourhood of Miami.

But sooner or later, we are likely to see Zika transmission in other sites in the US. “On any given day, there are hundreds, if not more, people entering from Central and South America and the Caribbean,” says Peter Hotez at Baylor College of Medicine in Houston, Texas. “That’s why we can expect multiple foci of transmission on the Gulf Coast and Florida, where the Aedes aegypti mosquitoes are widespread now.”

Cold-like symptoms

Vespignani’s team originally estimated that there were around 30,000 people carrying Zika in the US, but have revised their numbers based on the latest CDC data. “It’s changing day by day,” says Vespignani.

The estimate is important for giving an idea of how widespread the virus may already be. Many people who have Zika don’t become ill, or have only mild symptoms that could be mistaken for a cold. “Around 80 per cent of cases are asymptomatic, and even among those who do get ill, only around 20 per cent get diagnosed,” says Vespignani.

Although thousands may be carrying the virus, the breeding season of the Aedes mosquitoes will soon come to an end, limiting their capacity to disperse Zika further.

Journal reference: bioRxiv, DOI: 10.1101/066456

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