Zika 'Very Likely' to Spread From Mosquitoes to Humans in US, Official Says

— -- Health officials stressed today that they are doing everything they can to minimize Zika outbreaks in the U.S.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institute of Health, said at a press conference today he thought it was "very likely" that Zika virus would spread from mosquitoes to humans in the U.S. in the future, but stressed that officials think the outbreaks will be constrained in the same manner as past domestic outbreaks of dengue fever and chikungunya.

And while a recent USA Today article categorized control of the mosquito that spreads Zika as a “lost cause,” Fauci cautioned against that mentality.

“Aedes aegypti is a very difficult mosquito to control and eliminate,” he said. “That doesn’t mean it’s impossible to have a significant impact on it -- but it will require a very aggressive, concerted effort.”

The USA Today article pointed out that Aedes aegypti cannot be eliminated as effectively as some other species by traditional insecticide-spraying methods. But there are still other protective measures that the Centers for Disease Control, the World Health Organization, and the Pan American Health Organization can and will engage.

These organizations are working to “raise public awareness, have cooperation at the community level to get people to eliminate and diminish standing water of any type, as well as to push and try and utilize environmentally friendly larvicides and insecticides,” Fauci said.

There is also a trial underway that involves the use of genetically modified mosquitoes to help control Aedes aegypti reproduction, which is being overseen by the FDA.

Fauci pointed out that Zika virus itself is usually mild -- but it is very dangerous for pregnant women and their fetuses, due to the risk that their babies will be born with a brain development defect known as microcephaly due to the virus.

“The focus is on pregnant women and making sure they’re not exposed to the virus,” CDC spokesman Tom Skinner told ABC News. “We want them to avoid traveling to countries with Zika and make sure they know about prevention of mosquito bites.”

Fauci also said that researchers are working on a Zika virus vaccine that will be given to humans in a study beginning in September, and that they would likely know if the vaccine is safe to use by the start of 2017 -- though it could take anywhere from one to three years.

Speaking at the PAHO press briefing, Fauci stressed that the NIH and CDC need "$1.9 billion dollars because it's critical," referring to the money requested from Congress to combat Zika.

"What I have had to do is move money, hopefully temporarily, from other areas I would have spent it [on]," said Fauci. "We need to get the Zika money to work with Zika, and we need to backfill the money to other" areas of research.

Dr. Sylvain Aldighieri, Zika incident manager for PAHO, estimated that about 500 million people in the Americas are at risk to be infected by the Zika virus.

There have been no locally transmitted cases in the continental U.S. as of yet, but there have been over 400 travel-related cases. And there have been close to 700 cases in Puerto Rico, with 65 pregnant women having been infected.

The mild nature of Zika virus for those who are not pregnant presents a challenge for officials trying to communicate its risk, Fauci noted.

“How do you communicate the danger and the threat of a disease that is fundamentally and historically mild?” he said.

Another unanswered question: Scientists do not yet know the risk of a Zika-infected pregnant woman giving birth to a baby with microcephaly. An ongoing study of pregnant women, largely in Brazil, will help to answer that question once enough data has been collected.

ABC News’ Dr. Sunjay Devarajan contributed to this report.