When public health officials briefed President Obama about the alarming and rapidly evolving Zika virus situation this week, the message that emerged from Washington was clear: The world needs a vaccine.

The same message has emerged from the World Health Organization, which announced Thursday that it would be convening an emergency committee of outside experts to advise it on the extraordinary outbreak “spreading explosively’’ through the Americas.

“The level of concern is high, as is the level of uncertainty. Questions abound,’’ WHO’s director general, Dr. Margaret Chan, said during a special session on Zika in Geneva. “We need to get some answers quickly.’’

But public health experts warn that developing a vaccine for Zika — which is believed to have caused a surge in cases of babies born with abnormally small brains in Brazil — will be remarkably challenging.

As Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy, said: “We’re going to be living for quite a few years in a Zika vaccine-free world.’’

The challenge is reminiscent of the fight against Ebola, experts say. When that virus started ripping through West Africa in 2014, there was no licensed vaccine waiting in warehouses that could be deployed to contain the devastating spread. Vaccines only started to be used in the region in 2015, after the peak of the outbreak.

But in the case of Ebola, scientists had been working on vaccines for more than a decade, giving them a foundation to build on. Zika, by contrast, has been virtually ignored by the scientific community, in part because the virus was never seen as a serious, widespread threat to public health.

Zika virus causes mild, flu-like illness in one out of every five people infected. The rest don’t even know they’re sick.

Experts also note that in the case of any vaccine, developing, testing, and commercializing a product takes years — a process that helps ensure products are safe and effective.

Scott Weaver, head of the Institute for Human Infections and Immunity at the University of Texas Medical Branch in Galveston, has been designing experimental vaccines to protect against chikungunya virus, which shares some features with Zika virus. He is hopeful that scientists will be able to design a vaccine for Zika using what is known about viruses related to it.

“We can be fairly confident that we can make a good vaccine for Zika fairly quickly,’’ he said, because scientists have learned how to attenuate, or weaken, similar viruses for the purposes of inclusion in a safe vaccine.

Dr. Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases — part of the National Institutes of Health — made a bolder prediction. He told STAT this week that an experimental Zika virus vaccine should be ready for Phase 1 testing — the earliest stage of human testing for a vaccine or drug — this calendar year.

But Phase 1 trials are preliminary and small, designed to show that something is safe enough to continue testing it in people. They don’t tell you whether a vaccine works.

Larger studies known as Phase 2 and Phase 3 trials would have to be designed, conducted, analyzed, and reported before a company intent on licensing the vaccine could apply to the Food and Drug Administration — or some other country’s drug regulatory agency — for approval of the vaccine.

Even if everything goes according to plan, this represents years of work.

One of the world leaders in vaccines, the British company GSK, has said in a statement about the prospects for a Zika vaccine that the process normally takes 10 to 15 years. Fauci spoke in expedited terms. But even he admitted: “It might be five or seven years away, a commercial product.’’

And to get there on that kind of expedited timeline will take boatloads of money.

Dr. Ernesto Marques, a Zika virus researcher who splits his time between the University of Pittsburgh’s Center for Vaccine Research and Brazil’s FioCruz research institute, said if funders were willing to put up $2 billion right now — not part now and maybe more later — he thinks the work could be done in under eight years.

“It’s not going to be three years. That’s impossible.’’

The economics of developing a Zika vaccine may also prove challenging.

Because the virus seems to pose the most severe threat to newborns, pregnant women who live in or plan to travel to parts of the world where Zika circulates might race to line up if a vaccine becomes available. Women who want to become pregnant would probably join them in line.

But most people who are infected with Zika don’t even realize it. Given that reality — and the fact that new vaccines are generally not cheap — the market among people who aren’t women in the child-bearing phase of life may not be strong.

Marques thinks that won’t be a problem. He envisages girls getting the vaccine before they reach reproductive age — which is the original marketing approach for human papillomavirus or HPV vaccines.

Osterholm notes there is a long list of recently emerged pathogens that the world would have liked vaccines for, but none have made it to market yet. The list: SARS, West Nile virus, Ebola, MERS.

“I think this one is clearly in that same lineup of potential vaccines, but what’s the model for actually bringing it forward in a viable, economic, and scientific way?’’ Osterholm asked.

Helen Branswell can be reached at helen.branswell@statnews.com Follow her on Twitter @helenbranswell.