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A new study suggests at least half of men who have been infected with Zika will emit traces of the virus in their semen, but in most cases that viral shedding stops after about three months.

The research, conducted in Puerto Rico, found that 56 percent of men who had been infected had traces of virus in their semen but about half of them stopped emitting those viral traces by about a month after they first became ill.

And by three months after the onset of symptoms, only 5 percent still had virus in their semen.


The findings underscore the prudence of the the current guidance from the World Health Organization and the Centers for Disease Control and Prevention aimed at preventing sexual transmission of the virus from a man to a female partner who is or might become pregnant.

Both agencies recommend that men who have been infected should wait at least six months before trying to father a child and should practice safe sex or abstinence for that period of time if their partner is pregnant.


“What we’ve seen with this study is that the existing guidelines are supported by what the science is showing,” said Tyler Sharp, senior author of the paper, which was published in the New England Journal of Medicine. “So certainly for the time being, I think the guidelines are going to stand.”

Sharp is acting head epidemiologist with the CDC’s dengue branch in Puerto Rico. The study was conducted there with the help of other CDC staff and researchers from several institutions in Puerto Rico.

Because of the nature of the tests used, the group cannot say if the specimens that tested positive contained whole, live viruses that could have infected a sexual partner, or degraded viral fragments that would not have the power to infect.

“That’s one of the big questions we still have remaining. And we’re still working on techniques, both here at the dengue branch and in many other places, to best assess infectivity,” Sharp said.

These are the preliminary findings involving the group’s research on how long Zika virus is found in a variety of bodily fluids — serum (a component of blood), saliva, semen, urine, and vaginal secretions.

The aim was to track how long traces of the virus could be detected in these fluids, either to determine the optimal way to test for Zika infection or to see how long a risk of sexual transmission of the virus persists.

The researchers chose not to study whether and for how long tears and sweat contained traces of virus, concluding that neither fluid posed much of a transmission risk.

The group is looking for the presence of the virus in breast milk. But to date few pregnant women have been enrolled, and those findings will be reported in another study.

These results are based on the first 150 people enrolled in the study. Sharp said the group ultimately expects the study to involve 350 individuals.


The question of sexual transmission of Zika — a virus normally transmitted to people by infected mosquitoes — was a new and unexpected wrinkle for a flavivirus, the family to which Zika belongs.

A case was reported in 2008, by an American researcher infected in Africa who infected his wife on his return to the US. But before the current Zika outbreak in the Americas, it was thought to be a rare event. It is now believed to happen more frequently, though mosquitoes are still thought to be the major driver of Zika spread.

Given that the virus inflicts its worst toll on the developing fetuses, quantifying the risk has been an urgent line of inquiry for scientists, whose work informs the public health recommendations issued by agencies like the CDC.

There have been reports of viral traces being found in the semen of infected men for months after infection. Currently the longest time on record is 189 days, Sharp noted, In the Puerto Rico study, one man’s semen tested positive for 125 days.

The researchers saw very little evidence of Zika virus in vaginal secretions — only one woman among 50 tested was positive, three days after developing symptoms.

Saliva tested positive more commonly, but still only in a minority of cases. Urine testing turned up a high degree of positive tests in the first week, but very few after a month.

Serum testing appeared to be the best way to find evidence of Zika. “What we’re seeing is that urine has some utility [for diagnostic testing] but at least in this cohort of individuals … serum is better for a diagnostic specimen,” Sharp said.