"Cadilla Pharmaceuticals of Ahmedabad and Serum Institute of India in Pune have approached the ICMR to carry out clinical trials in the country. The vaccines produced by them is based on alternative approach," Dr V M Katoch, director general, ICMR, said.

"Unlike the conventional method of using killed virus, Serum Institute has used live virus and Cadilla Pharmaceuticals has used VLP technology to develop vaccines," he said.

Bharat Biotech in Hyderabad and Panacea Biotec in Delhi are also asking permission to make the swine flu vaccine, but theirs is the conventional approach. "They are using killed virus to prepare vaccine," he said.

Detailing about live virus vaccine, Dr N K Ganguly, President, Jawaharlal Institute of Postgraduate Medical Education and Research (JIPMER), said it contains an attenuated virus wherein the virulence of the pathogen used is reduced but still alive.

"As such, it produces natural immunity but does not produce the disease and causes a weakened infection that lasts for a few days to a week or more, depending on how much immunity the recipient has".

Research has revealed that live virus vaccine gives somewhat better protection than the killed virus vaccine in smaller children. So far it is only recommended for people between two and 49 years of age.