Indian biotechnology firm Bharat Biotech says it has come up with a template for an immunization against the Zika virus now spreading across the Americas. But Indian public-health officials cautioned that any useable vaccine was likely years away.

The United Nations World Health Organization this week declared the Zika outbreak, which has been linked to a rash of birth defects in Brazil, a global public-health emergency and the hunt is on for ways to check its spread and treat those who fall ill.

Still, possible vaccines have to pass through rigorous screening for safety and efficacy. Soumya Swaminathan, director general of the Indian Council for Medical Research, which oversees clinical trials in India, said testing would take two to three years.

“We have to encourage indigenous drug manufacturers,” said Dr. Swaminathan. But “we cannot bypass any steps in clinical development.”

Bharat Biotech, based in Hyderabad, says it has worked since last year with a strain of the virus to make it weak enough that it can be safely used in a vaccine. Known as an attenuated version of a virus, it prompts the body to form protective antibodies without making a person sick.