New Strain Added to Flu Vaccine

A new strain of influenza virus will be included in next season's influenza vaccine, following the recommendations of a panel of experts convened twice a year by the World Health Organization's Global Influenza Program.

The new strain was first identified in California in January and has already spread widely, according to WHO. Known as A/California/7/2004(H3N2), the strain was identified in more than 20 percent of U.S. flu patients tested in early 2005. It has also appeared in Canada, Mexico, Europe, Asia, Africa and the Pacific Islands. WHO predicts it will be the dominant virus circulating in the Northern Hemisphere during the next flu season.

Otavio Oliva, regional advisor to the Pan American Health Organization (PAHO) on viral diseases, notes that new strains are only included in the vaccine when they are significantly different from the previous year's strains. "There is no evidence that A/California is more virulent than any of the recent H3N2 strains of Influenza A," he says.

Researchers are currently working on a prototype A/California vaccine to allow manufacturers to begin growing it in chicken eggs, the first step toward production. The standard flu vaccine is composed of three strains of influenza virus selected from among thousands of candidates identified by national influenza centers in WHO member countries around the world. For this year's vaccine recommendations, the centers isolated and characterized more than 10,000 viruses from all six WHO regions.

The recommended Northern Hemisphere vaccine for the next flu season will contain A/New Caledonia/20/99 (H1N1)-like virus (which has been included for the last six years), and B/Shanghai/ 361/2002-like virus (which has been included for two years). The California strain will replace A/Fujian/411/2002 (H3N2)-like virus which was included in both the northern and southern hemisphere vaccines for 2004).

The Southern Hemisphere vaccine for 2005 is already in production and consists of A/New Caledonia/20/99(H1N1)-like virus, B/Shanghai/361/2002-like virus, and A/Wellington/1/2004(H3N2)-like virus. Recommendations for the 2006 Southern Hemisphere vaccine will be made in September.

PAHO's member countries use both southern and northern hemisphere versions, depending on their geography. In addition to recommending the viral strains for flu vaccines, WHO provides manufacturers with prototype strains and materials to ensure and verify that their products meet global standards.