The United States has two potential candidates for a vaccine for the Zika virus and may begin clinical trials in people by the end of this year, but there will not be a widely available vaccine for several years, U.S. officials said on Thursday.

Dr. Anthony Fauci, director of the National Institute for Allergy and Infectious Disease, said one of the vaccines was based on work done on the West Nile virus.

Fauci said that vaccine was never developed because a drug company partner could not be found, but he did not see this as an issue for Zika.

"We're already talking to a few companies who are able to partner with us in advanced development," he told a news conference.

Zika, a mosquito-transmitted virus, has been linked to severe birth defects in thousands of babies in Brazil. There is no vaccine or treatment for Zika, which is like dengue and causes mild fever, rash and red eyes. An estimated 80 percent of people infected have no symptoms, meaning it can be hard to tell if a pregnant woman has been infected.

Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention, said there have been 31 cases of Zika infection among U.S. citizens who traveled to areas affected by the virus. So far, there have been no cases of transmission of the virus through mosquitoes in the United States, she said.

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In Geneva, the World Health Organization (WHO) said on Thursday that Zika is spreading "explosively" and could affect as many as four million people in the Americas.