UPDATE: FDA Sends Letter To Doctors On H1N1 Vaccine Safety

U.S. Food and Drug Administration Commissioner Margaret Hamburg on Tuesday sent a letter to doctors explaining the manufacturing process of the H1N1 flu vaccine as part of an effort to address concerns some people have about the safety of the new swine-flu vaccine.

While many Americans want the vaccine and haven't been able to get it yet amid an ongoing shortage, several polls suggest up to half of Americans have no plans to be vaccinated. There is also anecdotal evidence that some doctors are advising patients to skip the vaccine until more is know about its safety.

In 1976, more than 40 million Americans were vaccinated with a different swine-flu vaccine and there were several hundred reports of Guillain-Barre Syndrome, a serious neurological disorder. While it isn't clear if the vaccine caused the illnesses, federal health officials are aware of the concerns and will be studying whether there are increased reports of GBS and other side effects from this year's H1N1 vaccine.

Hamburg said so far there have been no reports of serious side effects from on-going clinical studies of the vaccine. More than 3,600 patients are enrolled in studies that are being overseen by the National Institutes of Health.

In the letter, Hamburg explained that the H1N1 vaccine is being made by the same companies in the same manner the seasonal vaccine is made. The composition of the seasonal flu vaccine changes almost every year, and the FDA typically signs off on those strain changes without requiring human clinical studies.

The U.S. Centers for Disease Control and Prevention and the World Health Organization routinely monitor influenza viruses and report on which strains are predominant.

U.S. health officials use that information to pick the three strains that are most likely to offer protection against dominant strains of influenza expected to be circulating during the upcoming seasonal flu season. More than 10,000 viruses can cause the flu.

The three strains included in influenza vaccines are grown in chicken eggs. It takes about eight months for the entire influenza-vaccine production process to be completed, meaning manufacturers usually start making one part of the vaccine in January in order for the finished product to start being shipped in late August.

Because the H1N1 virus wasn't discovered until April it was too late to include it as one of the strains in the seasonal vaccine so the U.S. asked five companies to make a separate H1N1 vaccine. The companies are: Novartis AG (NVS); Sanofi-Aventis SA (SNY, SAN.FR); CSL Ltd. (CSL.AU); GlaxoSmithKline PLC (GSK, GSK.LN); and MedImmune, which is a unit of AstraZeneca PLC (AZN, AZN.LN).

The FDA approved Glaxo's vaccine on Tuesday.

"The benefits of preventing serious consequences from infection with the 2009 H1N1 influenza virus far outweigh the risks associated with vaccination," Hamburg wrote in the letter. "All Americans, and especially pregnant women and others at high risk of severe influenza infection, should seriously consider the recommendation for vaccination to help protect themselves and their loved ones."

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; [e-mail]