The vaccine was approved in India in January while the third phase of clinical trials was still under way, sparking some concern and criticism.
Bharat Biotech, which makes the vaccine, has since published data suggesting 78 percent efficacy.
The WHO said in a tweet it believed the benefits far outweighed the risks.
Some experts had pointed to a fast-track approval and incomplete data, but the firm's chairman, Dr Krishna Ella, said the vaccine was "200% safe".