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 older adults.
Dr. Anthony S. Fauci, 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.
The most disputed piece in this medical puzzle is a relatively obscure study released 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.
Even after removing pregnant women from the data, the researchers found women were 90 percent more likely than men their age to be infected. To adjust for doctor-visit differences, the team compared the current Zika outbreak to two outbreaks of dengue, which is not sexually transmitted.
Even after that adjustment, said Flavio C. Coelho, a Vargas Foundation biostatistician and the lead author, women were still 60 percent more likely than men to be infected with the Zika virus. Sexual transmission, he said, “was the most probable cause.”
The paper’s “very intriguing” conclusions “merit further study,” said Dr. John T. Brooks, an expert on the sexual transmission of disease at the Centers for Disease Control and Prevention.
But other experts, including Donald A. Berry, a leading biostatistician at the University of Texas M.D. Anderson Cancer Center, dismissed the study. Women’s fear of Zika is so great, and confusion over dengue, which has similar symptoms, so common that these variables alone could have accounted for the difference in observed infections between men and women, Berry said.