India’s Bharat Biotech International Ltd. and the U.S. National Institutes for Health appear to be the most advanced in developing possible vaccines for Zika, the World Health Organization said, although it will take at least 18 months for any shots against the virus to be ready for clinical trials.

Closely held Bharat Biotech, which has done research on other mosquito-borne viruses, earlier this month said it has two potential Zika vaccine candidates under development. While several companies are working on Zika vaccines, the current absence of standardized animal models and reagents is slowing down development, the WHO said in a statement Friday.

"The landscape is evolving very rapidly and numbers change daily.," the WHO said. "About 15 companies/groups have been identified so far, most have only just started work."

Vaccines can take years to develop, and global companies including France’s Sanofi have said they are looking into possible vaccines for the virus that is spreading in Latin America. The mosquito-borne disease has been linked to microcephaly, a condition that causes babies to be born with abnormally small heads and potential developmental problems.

The Hyderabad, India-based Bharat Biotech sequenced the genome for Chikungunya, another mosquito-borne virus, in India a decade ago, Bharat Biotech’s chief executive officer, Krishna Ella, said in an interview earlier this month. The company started looking at Zika after assessing other potential threats to India that would arise from the same vector mosquito. Bharat Biotech, which received an $18.5 million grant last year from the Bill and Melinda Gates Foundation to support construction of a vaccine manufacturing facility, says it’s delivered more than three billion doses of vaccines more than 65 countries.

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