Sanofi SA has formed a partnership with the U.S. Army to expand research and development of an experimental Zika vaccine that has shown promise in early laboratory studies and is among a few candidates expected to be tested on humans in the coming months.

At least 15 companies and entities, including Sanofi, are racing to develop vaccines against the Zika virus, which is behind an epidemic in the Americas that the World Health Organization says constitutes a public health emergency because the virus is linked to birth defects...