The United States government has begun the first clinical testing of an experimental vaccine designed to guard against Zika virus. The National Institute of Allergy and Infectious Diseases made the announcement earlier this week. The experimental vaccine has already been tested in animals. Anthony Fauci, the institutes director, said on Wednesday that investigators should know soon whether the vaccine is safe for use in human beings. He said the first human tests involve 40 human volunteers in the U.S. If we get a good immune response and there are no safety red flags by the end of December 2016, we should know if its okay to move on to phase II. He said the second phase could begin early next year. Those trials will be larger studies, involving more volunteers. They would be performed in countries where Zika has spread. There, researchers would look not only at the safety of the experimental vaccine, but its effectiveness in fighting infection.