Sex May Spread Zika Virus More Often Than Researchers Suspected

An outbreak of the Zika virus in the continental United States could begin any day now. But while there is plenty of discussion about mosquito bites, some researchers are beginning to worry more about the other known transmission route: sex.

Intimate contact may account for more Zika infections than previously suspected, these experts say.

The evidence is still emerging, and recent findings are hotly disputed. All experts agree that mosquitoes are the epidemic’s main driver.

But two reports now suggest that women in Latin America are much more likely to be infected than men, although both are presumed to be equally exposed to mosquitoes.

The gender difference appears at the age at which sexual activity begins, and then fades among elderly men and women.

Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, called the evidence “striking.” Like other scientists, he had doubts about aspects of the data, but thought the results justified a more rigorous study, probably in Puerto Rico, of the role of sex in transmitting the Zika virus.

“I can’t say it’s not true that women are more at risk,” he said.

The Zika virus can persist for months in semen, even in men who have had very mild infections. That’s why women who are pregnant or trying to conceive are routinely warned not to have unprotected sex with men who have been in areas where the virus is spreading.

Ten countries — Argentina, Canada, Chile, France, Germany, Italy, New Zealand, Peru, Portugal and the continental United States — have reported infections that were almost undoubtedly passed via sex. No case of female-to-male transmission has been documented.

If sexual transmission is more common than believed, efforts to protect women may draw health officials in many of these countries into conflict with those who oppose greater access to birth control or more explicit discussion of sexual practices.

In most parts of the United States, including New York City, health officials have presumed that the risk of Zika infection is low, except possibly at the peak of summer, the height of the mosquito season.

But wider sexual transmission may alter that calculus. Prevention campaigns, for instance, would have to be retooled with a greater emphasis on protected sex.

Thousands of men return to the United States every week from countries in which the virus circulates. New York State alone has a quarter of the country’s travel-related cases.

The most disputed piece in this medical puzzle is a relatively obscure studyreleased in May by Brazilian and European biostatisticians. In Rio de Janeiro, a city of 6.4 million, they found “a massive increase of Zika in women compared to men.”

The authors, from the Getulio Vargas Foundation and other Brazilian, French and Scottish research organizations, adjusted their figures for two confounding factors: Pregnant women are tested for Zika more frequently than anyone else, and women generally visit doctors more often than men do.

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