Last month, the CDC downgraded its warning; a spokeswoman said the WHO will soon follow with similar, less-restrictive travel recommendations. Officials said the disease has died down in most of the world — although they think it is still circulating at a much lower level.

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The new CDC guidelines urge women who are pregnant or thinking about getting pregnant to talk to their health-care providers about potential Zika risks before traveling. But with spring and summer travel season approaching, clinicians say it is not clear how women and their doctors can get the best information.

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Until Zika, there had never been a mosquito-borne virus that could cause serious birth defects on such a large scale. Among the most devastating consequences of Zika is microcephaly, a condition in which babies are born with abnormally small heads and underdeveloped brains. The danger to a developing fetus is most severe during the first and second trimesters of pregnancy.

The Zika pandemic in the Americas and the Caribbean exploded in 2015. The virus is believed to have spread from Uganda, traveling eastward across Asia and the South Pacific until it reached South America. It was introduced into a region that was highly infested with mosquitoes capable of transmitting Zika and a population that was “uniformly susceptible since it never experienced the disease,” said Albert Ko, an infectious disease expert at the Yale School of Public Health who is studying Zika.

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The virus infected hundreds of thousands of people and resulted in about 2,200 babies born with microcephaly, the majority in Brazil, Ko said.

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Academic researchers and CDC officials say Zika has waned in most places in the Western hemisphere because so many people became infected — more than 70 percent of the population in parts of Brazil. After being infected, people are immune to the virus and cannot pass it to other people either directly, via sex and bodily fluids, or through mosquitoes. Women who have been infected well before pregnancy are not thought to be at increased risk of having a baby with birth defects.

In the continental United States, mosquito-control measures helped stop the spread of the virus, and there has been no local transmission since 2018.

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There is no vaccine or treatment for Zika. People can protect themselves from mosquito-borne diseases by wearing pants and long-sleeve tops and using insect repellent or by staying in areas that are screened or air-conditioned.

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The virus is still circulating in Southeast Asia and South Asia, but large numbers of new infections and Zika-related birth defects are not being reported, Ko said. The only region reporting an active Zika outbreak is in India’s northwestern state of Rajasthan. The new CDC recommendations urge people to avoid travel only to places where there are active Zika outbreaks.

Although public health officials are fairly certain the Zika epidemic has faded overall, they say the virus may be circulating undetected in some areas, making it difficult to assess the risk.

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For the 88 countries and regions that have previously reported cases of the virus, and where there is currently no outbreak, “Zika virus has probably fallen below the radar,” said Martin Cetron, director of CDC’s Division of Global Migration and Quarantine, which tracks disease outbreaks overseas and in the United States.

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The virus has the potential to cause Guillain-Barré syndrome, which occurs when individuals’ own immune system damages their nerve cells, resulting in muscle weakness and sometimes paralysis. However, most Zika virus infections have mild or no symptoms, and many countries either have limited or no systems in place for routine surveillance and detection of the virus. As a result, “we can’t tell you exactly what the risk is,” Cetron said in an interview.

“Where there are those big outbreaks, we’re definitely going to tell you not to go,” Cetron said. “Where there is a range of possibilities, from no Zika to low-level background Zika, we’re going to tell you there’s been virus there before; it could still be there. If you’re a zero-risk person, don’t go. If you’re not, you decide.”

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The new recommendations have raised concerns among clinicians, who are getting questions from patients.

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“I get about four or five calls a week from concerned couples who are thinking of conceiving in the future,” Ko said. Risk still exists in much of Southeast Asia and South Asia. Though it is much lower than at the height of the epidemic in the Americas, he said, “that’s hard to tell a woman or a couple who wants to conceive and who wants to have it be absolutely 100 percent risk-free.”

Much of the onus for assessing risk will now fall on health-care providers, who may have no way to keep up with the level of threat.

“Once you dump this responsibility in the doctor’s lap, then [the pregnant woman] is at the mercy of the physician knowing about the topic,” said Laura Riley, chair of obstetrics and gynecology at Weill Cornell Medicine and chief obstetrician at New York Presbyterian Hospital. “The vast majority of doctors will look to see if there isn’t any active disease and say it’s probably okay to go.” That puts the doctor in a difficult situation for counseling patients.

Getting infected with the Zika virus during the first or second trimesters of pregnancy can lead not only to microcephaly, but a range of defects associated with congenital Zika syndrome. They include brain abnormalities, eye defects, hearing loss and problems moving limbs.

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“That’s the reason for such conservative concern,” Riley said. The underdeveloped nervous system seen in congenital Zika syndrome is “not like premature babies. We’re talking no brain.”

Riley recently spoke with a patient who was 26 weeks pregnant and planning to travel to the Bahamas in a few weeks. She wanted to know Riley’s assessment. On the CDC website, the information notes that the Bahamas has reported past or current Zika transmission, but it also says there is no accurate information on the current level of risk. It tells pregnant women and couples trying to become pregnant within the next three months to “work with their health-care providers to carefully consider the risks and possible consequences of going to areas with risk of Zika.”

Riley shared that information with the woman. She also warned the patient the doctor had no way of knowing there is not any circulating virus. “So if you decide to take that risk,” Riley recalled telling her, “wear long sleeves and use mosquito repellent, for God’s sake.”

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A spokeswoman for the American College of Obstetricians and Gynecologists said the group is taking the CDC’s updated travel guidance into account as it updates it own clinical guidance to its members.

The WHO ended its designation of Zika as an international health emergency in November 2016. Its current recommendations advise pregnant women to avoid travel to certain Zika-affected areas. Those will be replaced in coming weeks with a recommendation that pregnant women and couples seeking to conceive base their decision on information from individual country health departments and health-care providers, WHO spokesman Tarik Jasarevic said.

CDC officials said they spent about six months reviewing global Zika data. Some of the hardest-hit countries during the epidemic also rely heavily on tourism revenue. They were among the countries urging CDC to update the travel guidance. Within the agency, there were also initial differences, Cetron said.

In the end, CDC reached a consensus internally, he said. Regarding lobbying from affected countries, “the issue that you need to be clear about is that we do not base the decisions on their tourism volume,” Cetron said. “The decisions are based on evidence.”