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When Zika infections flared throughout Latin America, there was enormous concern that men carrying the virus could transmit it to women through sex — potentially infecting developing fetuses, if the women were pregnant. But it was unclear how long the risk might last.

A new study from scientists at the Centers for Disease Control and Prevention does not definitely answer the question, but it offers public health agencies data to assess — and suggests infected men might not shed the virus in their semen for as long as was once thought.


The study, published in the New England Journal of Medicine, analyzed more than 1,300 semen specimens from 184 Zika-infected men. The scientists found that very few men actually emitted whole — and potentially infectious — Zika viruses in their semen.

In fact, viruses could only be grown — a process called culturing — from three semen samples, all of which were taken within the first 30 days after those men developed symptoms of Zika infection. (Scientists tried to grow Zika viruses from only 78 of the semen samples.)

Zika virus RNA — in other words, fragments of viruses — were found more commonly, and for longer periods. Semen from 33 percent of the men contained viral fragments; one man’s semen tested positive for Zika RNA for 281 days — nine months.


While the scientists could not rule out that there might be some risk of transmission when semen tested positive for Zika virus fragments, they noted sexual transmission generally is believed to occur soon after infection. In all known cases, sexual transmission of Zika took place within 41 days of the onset of illness; most happened within 20 days, the authors said.

“We’re not getting transmission later, and we’re not getting positive cultures later,” said lead author Dr. Paul Mead, a medical officer in the CDC’s division of vector-borne diseases. (Zika, which is transmitted mainly by mosquitoes, is a vector-borne disease.)

The presence of Zika RNA may be deceptive, and “may overstate the duration and magnitude of the risk of sexual transmission,” Mead and his colleagues wrote in the journal article.

Dr. Heinz Feldmann, an Ebola expert who runs the virology laboratory at the National Institutes of Health’s Rocky Mountain Laboratories, agreed. In an editorial in the journal, he suggested that the discovery of viral RNA in the semen of men long after they had been infected with Zika and Ebola viruses may not signal there is a risk of transmission.

“This finding suggests that there is a short period during which [Zika virus] infected men might transmit this virus through sexual contact,” Feldmann wrote. “Likewise, the fact that sexual transmission could rarely be confirmed for [Ebola], despite the detection of RNA in the semen of survivors more than one year after acute infection, further shows the shortcomings of molecular detection alone in understanding transmissibility.”

Current public health guidance aimed at preventing sexual transmission of Zika errs on the side of caution. Both the CDC and the World Health Organization urge people who have been infected with Zika or have traveled to a place where Zika was spreading to put off trying to have a child for six months.

Both organizations also advise men who have traveled to a place where Zika was spreading — even if they don’t believe they have been infected — to wear condoms during sex or to abstain from sex for at least six months, if their partner could become pregnant. In cases in which a man’s partner was already pregnant, the couple should use condoms or refrain from sex for the duration of the pregnancy.

So do the new findings suggest these guidelines are excessive and could be loosened in some cases? Mead wasn’t ready to make that leap.

An expert committee at the CDC will likely assess these and other findings in the next few months and decide if the advice should be changed, he said.

Meanwhile, Feldmann argued that new tests are needed to determine when semen is actually infectious following illnesses, suggesting Zika and Ebola are unlikely to be the last viral diseases to pose questions about sexual transmission risks.