In the most optimistic scenario, a Zika vaccine could prove its worth by the start of 2018, Anthony Fauci, head of the U.S. National Institute of Allergy and Infectious Diseases (NIAID) in Bethesda, Maryland, said today.

NIAID plans to begin tests of a vaccine made in its labs in September in 80 people, said Fauci, who spoke at a press conference held at a meeting on Zika virus risk communication challenges in Washington, D.C. If the vaccine proves safe and capable of stimulating relevant immune responses, he said NIAID plans in the first quarter of 2017 to launch what he called a phase 2b study “in a country that has a very high rate of infection.” That study would enroll thousands of volunteers.  “If in the early part of 2017 we still have major outbreaks in South America and in the Caribbean, we may show that it’s effective or not within a year,” Fauci said.

Several factors will determine how long it takes a trial to determine a vaccine’s worth, he cautioned. One is how well the vaccine works: It’s easy to see a difference between vaccinated people and unvaccinated controls if a candidate product is 100% safe and effective—which rarely is the case.

A second critical factor is how rapidly Zika virus is spreading in the communities taking part in the studies; a higher incidence makes it easier to collect more data quickly. But incidence, in turn, can be affected by the degree of immunity that a population naturally has acquired before the trial starts; specifically, the virus will spread more slowly in populations that have already been exposed to it and developed effective immune responses.

Fauci noted that at the time NIAID set out to do Ebola vaccine efficacy trials in West Africa in 2015, there was a steep drop off in new infections, making it more difficult to run effective trials. “If that happens when Zika vaccine efficacy trials start,” he said, “it may take 3 years to show whether it works or not.” But the massive Zika pandemic now underway, unlike the Ebola epidemic, is not expected to disappear from Latin America and the Caribbean any time soon.

Sylvain Aldighieri of the Pan American Health Organization in Washington, D.C.—which cosponsored the conference with the U.S. Department of Health and Human Services—said that, to date, the Zika pandemic has affected 37 countries and territories. “We think that about 500 million people in the Americas are at risk to be infected by the Zika virus,” Aldighieri said.

Zika triggers no detectable symptoms in roughly 80% of the people it infects, but rarely causes serious harm. “That’s one of the real issues we have with communications,” Fauci said. “How do you communicate the danger and the threat of a disease that’s fundamentally, historically mild?”

But there is at least one real threat, “which has thrust it upon the attention and concern of the entire world,” Fauci said: If the virus infects pregnant women, their babies can be born with small heads, called microcephaly, and other abnormalities.

Fauci stressed that “we don’t know exactly what the percent is” of Zika-infected women who will give birth to babies with related problems. One small study suggested it may be as high as 29%, but another study that used a very different methodology found it was only 1%.

To get more reliable numbers, NIAID, the U.S. Centers for Disease Control and Prevention, and Brazilian researchers are planning to collaborate on a study that hopes to follow thousands of pregnant women in Zika-infected countries. “We don’t know beyond microcephaly what the long-range effects are on babies who might look like they were born normal but might have defects that are more subtle—hearing, vision, intellectual and others,” Fauci added. “Although we know a lot about Zika, literally every week that goes by we learn more and more.”