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Ask the Harvard experts: The H1N1 flu vaccine is safe

Q: I've had my seasonal flu shot, but I'm not so sure about getting the swine flu shot. How can we be sure that it is safe?

A: The H1N1 flu vaccine should really not be considered a "new" flu vaccine. It's made the same way flu vaccines are made every year. Experts predict what the most likely strains of flu will look like during the next flu season, so the virus in the seasonal flu vaccine varies from year to year. This was done early in 2009 for this year's seasonal flu vaccine. The manufacturers devised the seasonal flu vaccine according to the experts' recommendations.

Most of the seasonal flu vaccine had already been produced by the late spring when the new H1N1 flu virus emerged. No one predicted the sudden emergence of this particular flu strain, especially during warm weather. Because there was no time to add killed H1N1 virus particles to the seasonal vaccine, manufacturers needed to make a second flu vaccine containing these particles.

Manufacturers have followed exactly the same procedures to make H1N1 vaccine as they have used to make seasonal flu vaccines. So, apart from the type of virus in the vaccine, the recipe is the same as for regular flu vaccines. It is therefore just as safe as the seasonal flu vaccine already available and carries similar risks. Some people have been worried that manufacturers have included extra ingredients (adjuvants) to stretch the supply of H1N1 vaccine. The H1N1 vaccine available in the United States does not contain any adjuvants.

The same companies that have been making flu vaccines are the ones producing the H1N1 swine flu vaccines. The process is identical. The vaccine has been tested on thousands of volunteers. The side effects have been the same as all other flu vaccines. The H1N1 vaccine did provide an immune reaction in most people within 7 to 10 days that predicts good protection against this virus. We won't know if the immunity seen in blood tests is enough until after the flu season has passed. But this is true with influenza in any given year. The U.S. Food and Drug Administration used the same measures to license this H1N1 flu vaccine as it has for past vaccines. No shortcuts.

Howard LeWine, M.D., is a practicing internist at Brigham and Women's Hospital in Boston. He serves as chief medical editor of Internet Publishing at Harvard Health Publications, Harvard Medical School.

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