Nurse Floreliz Mendoza instructs study participant Andrea Vaught, 25, on how to monitor herself as part of a Zika vaccine clnical trial at the National Institutes of Health campus in Bethesda, Md. (Allison Shelley for The Washington Post)

Several companies and U.S. government institutions are racing to develop a vaccine to prevent infection from the Zika virus. The vaccine candidates to date, which use a variety of approaches, are in different stages of development.

Some efforts rely on the traditional method that requires growing or purifying a weakened or killed virus. Others are employing newer technology, using pieces of DNA or RNA, that allows a faster timeline. But no results from the new technologies have reached the market for human use.

Three candidates are furthest along:

● Two DNA vaccines are in preliminary safety testing, one developed by the National Institute of Allergy and Infectious Diseases (NIAID), the second by Inovio Pharmaceuticals and GeneOne Life Science and its U.S. and Canadian academic collaborators.

● An inactivated vaccine, developed by the Walter Reed Army Institute of Research, is also in preliminary safety testing. Collaborating on this are NIAID, the Biomedical Advanced Research and Development Authority (BARDA), which is part of the Department of Health and Human Services, and the drugmaker Sanofi Pasteur.

Other candidates in the pipeline include:

● A weakened-virus vaccine developed by NIAID and the Butantan Institute, a Brazilian nonprofit.

● An RNA vaccine developed by Moderna Therapeutics in collaboration with BARDA.

● An inactivated-virus vaccine formulated with an adjuvant, a substance that enhances the immune response. This was developed by Takeda Group and BARDA.

● An RNA vaccine developed by NIAID and GlaxoSmithKline.

● A vaccine being developed by NIAID that uses a genetically engineered version of an animal virus primarily affecting cattle.

— Lena H. Sun

(Gillian Brockell,Jorge Ribas/The Washington Post)