The vaccine that protects against influenza A (H1N1), commonly known as swine flu, caused no increase in birth defects when given to pregnant women but did produce a very small increase in the risk of Guillain-Barre syndrome when given to people older than 50, according to two new studies reported Tuesday. Overall, the studies show that the vaccination campaign conducted against the pandemic of the winter of 2009-10 was very safe.
Pregnant women and the elderly, along with young children, are among those who are most susceptible to the swine flu, and to influenza in general. Those groups are thus given priority for vaccination, but there has been some concern about possible side effects of the vaccines that were used. An estimated 2.4 million pregnant women were vaccinated in the United States. The U.S. vaccine consisted of surface antigens from the H1N1 virus. The vaccine used elsewhere, such as in Denmark and much of Europe, used the same antigens but also contained what is known as an adjuvant to boost immune stimulation by the antigens. There have been concerns about the safety of such adjuvants.
Dr. Bjorn Pasternak of the Statens Serum Institut in Copenhagen and his colleagues used Danish medical records to study all infants born between Nov. 2, 2009, and Sept. 30, 2010, a total of 53,432 infants. About 13.1% of those, 6,989, were born to mothers who had received the swine flu vaccine during pregnancy.