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t’s a question that has been asked of experts repeatedly: Will infection with Zika now raise a woman’s risk of delivering a child with birth defects in the long run?

Health officials have hedged their responses because the issue hasn’t been thoroughly studied. But to people who know viruses, there’s almost certainly no reason to think infection raises a woman’s long-term risk.

It’s the virus that causes the birth defects, and the virus clears from a person’s bloodstream within about two weeks. No virus, no possibility that it can cause birth defects.

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To learn more, STAT spoke with Dr. Denise Jamieson, a senior member of the CDC’s Zika response team. The conversation has been edited and condensed for clarity.

Why shouldn’t a previous Zika infection affect a future pregnancy?

It’s only when you’re pregnant or around the time of conception, and there’s virus circulating, that it can then be passed to the fetus. That’s the time of concern. What happens is that virus is circulating in the mom’s blood, and we think it gets passed to the fetus. That has been linked to the defects in the fetus, in the baby.

The Zika virus only stays in the blood a relatively short amount of time, and once the virus has cleared from the blood, there’s no concern about future pregnancies. And in fact, based on what we know about other similar flaviviruses, we think that once you’re infected, you’re protected from future infections. So we don’t believe you are at risk for future pregnancies.

There’s still a lot to be learned about the virus, but from what’s known now, how long does it take for someone who contracts it to typically clear the virus from their blood?

A week or two.

There was a recent report in the literature of a pregnant woman where the virus stayed in the blood for a longer period of time, and it’s not clear why that is. Is it because pregnant women’s immune systems are temporarily altered somewhat? Or is it because, if the fetus is infected, is it possible that the virus is being passed from the fetus to the mom? It’s not clear.

If a woman contracts the virus or shows symptoms, how long should she wait before trying to get pregnant?

If she has Zika, she should wait eight weeks to get pregnant. It’s a very careful estimate to make sure all the virus has cleared from her blood.

And men are a different story?

Men who are infected, we are more concerned about. We are more concerned about the virus persisting in semen, so in that case, we say they should wait six months (before having unprotected sex).

[Update: On Sept. 6, the World Health Organization updated its guidance to recommend that people who travel to places where Zika is spreading should wait six months before trying to have a child, regardless of whether they showed Zika symptoms.]

[On Sept. 30, the CDC updated its recommendations to say that men who have traveled to a place where Zika is spreading wait at least six months after returning home before trying to have a child, regardless of whether they showed symptoms or not. The agency also said that all men returning from areas where Zika is transmitting should use condoms for six months to prevent passing the virus to a sexual partner.]

Why can a virus survive in semen longer than in blood?

The testes are thought to be an immunologically privileged site. There are certain parts of the body where a virus can persist for longer periods of time with some viral diseases, and one of those sites is the testes.

If your recommendation is that men who contract Zika wait six months, how long has the virus been shown to persist in semen?

It has been isolated for up to two months. And so when we made our estimates, we basically tripled that to six months. This is based on very limited information, so we’re trying to be, not overly cautious, but cautious.

Is there a way to know if the virus has cleared a man’s semen? Does the test that can identify the virus in blood work in semen?

It has not been validated in semen. We’re testing different body fluids such as saliva, urine, blood, and semen to try to determine if we can reliably detect virus in semen and for how long. Right now, we’re not recommending that anybody have their semen tested for virus.

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