New Delhi, Aug 11 - Developing vaccines against swine flu virus strains is a complex and time-consuming process and inoculations are usually effective only for a single season, says an expert at a leading vaccine-producing institute in Pune.

Influenza viruses change frequently and hence a vaccine made against a specific strain is useful only for vaccination during a single season, said Rajeev M. Dhere, senior director with the Serum Institute.

The seasonal vaccine production for the influenza viruses, expected to be active in the winter of 2009, is almost fully ready and stocked up in major consumption areas of the world, Dhere told the Indian edition of Technology Review, the 109-year old magazine of the Massachusetts Institute of Technology published by CyberMedia.

Experts like him also indicated that the current year's seasonal influenza vaccine has used three virus strains - A/Brisbane/59/2007 (H1N1), A/Uruguay/716/2007 (H3N2) and B/Florida/4/2006.

Dhere says experts at the Atlanta-based Center for Disease Control have determined the genetic sequence of the new H1N1 influenza virus.

'The genetic sequence is significantly different to the H1N1 contained in the current seasonal influenza vaccine. So the current seasonal vaccines will not provide any protection against the new H1N1 strain. Hence, a different vaccine is required.'

Several strains of the H1N1 viruses have been extracted from infected people in Mexico and California. The A/Mexico and A/California strains are the basis for the vaccine production.

Dhere also explains the process:

-First the 'seed' virus is made for which experts at the Atlanta-based institute have used A/Mexico and A/California H1N1 strains.