The World Health Organization is now urging women who are pregnant to avoid travel to places where the Zika virus is spreading, if they can do so.

The Geneva-based global health agency now believes the link between Zika infection during pregnancy and microcephaly is sufficiently strong that it is prudent to warn pregnant women to try to avoid travel, Christopher Dye, director of strategy for WHO Director-General Dr. Margaret Chan, told STAT.

Dye said the WHO also believes the evidence is becoming very strong that the spread of Zika virus through the Americas is responsible for a spike in cases of Guillain-Barré syndrome (GBS), a form of progressive paralysis from which most, though not all, people recover.

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“Zika needs to be considered guilty unless proven innocent at this stage,” Dye said.

The WHO declared the suspected consequences of Zika infection — microcephaly and GBS — a public health emergency of international concern on Feb. 1, on the advice of a panel of outside experts. At the time, it refrained from issuing travel advice to pregnant women, even though the United States, Britain, and several other countries already had.

Dye couldn’t point to a single piece of evidence that prompted the agency to change its mind 11 days later. But he said there has been an accumulation of supporting facts that all point to Zika infections causing these neurological problems.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions. Alex Hogan/STAT

“There clearly is a chance for something going wrong for women, particularly in the early stages of pregnancy. So it seems prudent to warn them of that so that they can decide for themselves how much of a risk they want to take,” he said.

The WHO’s updated guidance released Friday also said men and women who have travelled to places where Zika is circulating — especially pregnant women and their partners — should use condoms while having sex to prevent sexual transmission of the virus.

It did not advise them on how long they should practice safe sex. Public Health England has suggested 28 days if a man has been to a place where Zika is spreading but he had no symptoms, and six months if he was sick during or after his travels.

There are two known cases of sexual transmission of Zika, and two reports of Zika virus being found in semen after a man has recovered from Zika. In one case, reported by Public Health England scientists, traces of the virus could be found in the semen of a man 62 days after he first became ill with Zika.

Zika virus has been around for decades at least. It was first discovered in Uganda in 1947 and for most of the intervening time was thought to be inconsequential. When it was seen to infect people, the infections were mild.

But an outbreak in French Polynesia started to change that view. And the virus’s arrival in Brazil — and now more than 30 other countries and territories in the Americas — has brought into focus significant side effects of infection.

Dye said it is still not possible to say how often infection during pregnancy leads to microcephaly in the fetus, or what proportion of people infected with Zika develop GBS.

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