By Marc Santora and Samantha Schmidt

New York Times

NEW YORK >> One by one, the women paraded into the research center in Midtown Manhattan. There were about 40 of them, pregnant or of reproductive age, brought together by New York City health officials for focus-group sessions in English and Spanish.

They were there to discuss the Zika virus. But not the mosquitoes known to carry it.

They were there to talk about sex.

Specifically, public health officials were trying to devise the most effective way of communicating the growing threat posed by sexual transmission of the virus — a threat that could significantly alter the course of the epidemic.

After two days, they had their message, aimed at men as much as women: “Do not put your child at risk.”

While mosquitoes are the primary carriers of the virus, which has swept across the Americas and the Caribbean, infecting tens of thousands of people and causing devastating birth defects in more than 1,800 newborns, health officials are concerned that the threat of sexual transmission remains little understood, largely underpublicized and worryingly underestimated.

Dr. Thomas R. Frieden, the head of the federal Centers for Disease Control and Prevention, in an article published this week in The Journal of the American Medical Association, said Zika represented “an unprecedented emergency.” He wrote, “Never before, to our knowledge, has a mosquito-borne virus been associated with human birth defects or been capable of sexual transmission.”

With 491 cases so far, four believed to have been contracted through sex, New York City’s total now accounts for roughly a quarter of all Zika cases in the United States. More than 3,000 pregnant women have been screened for the virus.

While sustained local transmission by mosquitoes is considered unlikely in the city, it has both a large population of people who travel back and forth to Zika hot zones and sophisticated public health surveillance systems, making it one of the best places to closely study the virus’ alternate means of transmission and how to prevent Zika from spreading through those means.

Because much about the virus is unknown, just how great the threat sexual transmission poses remains contested.

As long as mosquitoes that thrive in warm tropical environments are the primary carriers, Zika’s spread will be somewhat limited by geography and mosquito-control efforts.

Experts agree, however, that if sex becomes a common means of spreading Zika, the already difficult challenge of containing the virus will become exponentially harder.

It could also further stoke a debate about family planning and the availability of birth control, even as the risk to pregnant women increases.

All men and women who travel to places where Zika is spreading are now told either to abstain from sex for at least eight weeks or to use barrier protection — if they have symptoms or not. Men with symptoms should abstain from sex or use condoms for at least six months. Pregnant women who live where the disease is present or whose partners have been to a Zika zone should have their partners use condoms or abstain from sex for the duration of the pregnancy, health officials say.

“Those recommendations come as a pretty big shock to our patients,” said Dr. Gary Mazer, director of emergency management at the CityMD walk-in urgent care centers in New York City. “Many people have no idea.”

In a recent survey by the Annenberg Public Policy Center, people seemed aware of the risks of traveling to places where Zika is spreading and the need to take precautions from being bitten by mosquitoes.

But when asked what steps they could take to prevent contracting the disease, only 5 percent named using condoms or abstaining from sex with someone who lives in or travels to a Zika area.

Getting doctors and patients to talk openly about sexually transmitted diseases is often difficult, and part of the challenge with Zika is that it exploded so suddenly and that so much remains unknown. Only seven months ago, there was a debate over whether sexual transmission was even a real concern.

William Smith, the head of the National Coalition of STD Directors, said he had heard concern from health departments across the country.

“There has been an under-appreciation across the board and a lack of leadership in talking about and educating people — both citizens and providers — that sexual transmission is a real threat,” he said.

He pointed to the posters distributed around the country by the CDC and mirrored by local health departments, with big pictures of mosquitoes but no mentions of condoms.

In addition, the emergence of the virus is coming at a time when public health departments are strained and the three most commonly sexually transmitted diseases — syphilis, gonorrhea and chlamydia — are on the rise for the third year in a row in the United States, an indication that messages about protected sex are falling short.

The city is planning to start its new campaign in two weeks, building on its current educational effort that focuses largely on travel and mosquitoes. It will include fliers, posters, and radio and television advertisements drawing on suggestions from the focus group and years of battling other sexually transmitted diseases. Community outreach workers will spread out to gathering places like street fairs and transit hubs to communicate the risks of sexual transmission.

Dr. Mary T. Bassett, the city’s health commissioner, said that even as the city addressed concerns about sexual transmission, it did not want to lose focus on what remained the primary threat to women in New York: travel to places where the virus is spreading.

“What we do know right now is that mosquitoes are the most important source of transmission in Zika-affected areas,” Bassett said. “We do need every New Yorker to know that Zika can also be sexually transmitted, and we have adapted our messaging to reflect this. For New Yorkers, the two best ways to prevent Zika are to avoid traveling to Zika-affected areas and to use barrier methods when having sex with someone who traveled to a Zika-affected area.”

© 2016 The New York Times Company