New Delhi, Jan. 25 -- A fear of Chinese polio sneaking in has forced the apex drug regulator to slam the door on human trials of a paediatric vaccine, as it fights to preserve India's hard-won status as a polio-free nation.

The Central Drugs Standard Control Organisation (CDSCO) has rejected Indian Immunologicals Ltd's request for human trials of a six-in-one vaccine, according to two officials and documents seen by Mint. The regulator worries that an unintended leak of the polio strain could revive the crippling disease, which was banished from the country 14 years ago.

Documents reviewed by Mint showed that the IIL product uses antigens (weakened viruses) from Sinovac, China's largest private-sector vaccine maker. Many vaccines use we...