Researchers have begun the second phase of testing of a Zika vaccine developed by U.S. government scientists in a trial that could yield preliminary results as early as the end of 2017.

Dr. Anthony Fauci, director of the National Institutes of Allergy and Infectious Diseases (NIAID), said on Friday the $100 million trial has already been funded and will proceed, irrespective of the $7 billion in cuts to the National Institutes of Health (NIH) budget proposed by the Trump Administration over the next 18 months.

In a conference call with reporters, Fauci would not comment on the proposed cuts because it is not clear yet what the actual budget will be. Both Democratic and Republican lawmakers oppose cuts to the NIH, which funds 21 institutes, including NIAID.

NIH Director Dr. Francis Collins is scheduled to speak with President Donald Trump later on Friday. "I will certainly be talking to Francis Collins when he returns from the White House," Fauci said.

Zika typically causes mild symptoms, but when the virus infects a pregnant women, she can pass it to her fetus, causing a variety of birth defects including microcephaly, in which the baby's head is abnormally small.

Fauci said the current Zika vaccine candidate had cleared preliminary safety hurdles, and would now enter testing for efficacy, which would occur in two phases.

The first phase will continue testing for safety and evaluate the vaccine's ability to stimulate the immune system to develop antibodies to fight Zika. It will also test different doses to see which works best.

The second phase, set to begin in June, will attempt to determine if the vaccine can actually prevent Zika infection.

Several companies are developing Zika vaccines, including Sanofi SA, GlaxoSmithKline Plc and Takeda Pharmaceuticals.

In the NIAID study, researchers aim to enroll at least 2,490 healthy volunteers in areas with confirmed or potential active transmission of Zika by mosquitoes. These include parts of the continental United States, Puerto Rico, Brazil, Peru, Costa Rica, Panama and Mexico. They will receive either the vaccine, or a placebo, and be followed for two years.

If enough people are exposed to the virus, Fauci said they could get an effectiveness signal as early as the end of this year. The trial is expected to be completed by 2019.

Fauci said the government is already in discussions with pharmaceutical companies that would share the costs of the final stage of testing and handle manufacturing.

Zika is primarily transmitted by mosquitoes, but it can also be transmitted sexually. According to Centers for Disease Control and Prevention, 5,182 people in the continental United States have been infected by Zika either locally or through travel to places where the virus is spreading. Another 38,303 cases have been reported in U.S. territories, including Puerto Rico.