MONDAY, Nov. 2 (HealthDay News) -- During mass immunization with pandemic H1N1 influenza vaccines, awareness of background rates of disease is essential for assessing vaccine safety, and may help allay vaccine-associated fears among the general public, according to an article published online Oct. 31 in The Lancet.

Steven Black, M.D., of Cincinnati Children's Hospital, and colleagues identified background rates of selected medical events for several countries, and found a wide variation in the rates of disease events by age, sex, method of ascertainment, and geography.

Based on this data, the researchers estimated that the vaccination of 10 million people in the United Kingdom would be followed in six weeks by 21.5 coincident background cases of Guillain-Barre syndrome and 5.75 coincident background cases of sudden death. They also estimated that 397 out of every 1 million vaccinated pregnant women in the United States would have a miscarriage within a day of vaccination.

"Background rates can provide the media, the public, public-health officials, and politicians with important information about the expected number of events that can occur in the absence of any vaccination program," Black and colleagues conclude. "Additionally, they can be used to estimate the number of such events that will occur after immunization of any number of individuals."

Several authors reported financial relationships with GSK, Wyeth, Novartis, and Sanofi Pasteur.

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