A study described by some as puzzling, of pregnancies in the USA, found that women who miscarried from 2010 to 2012 were more apt to have had flu vaccines for two straight years that included protection from swine flu.
Vaccine experts think the results may reflect the older age and other miscarriage risks for the women, and not the flu shots. Health officials said there was no reason to change the recommendations by the government that all women who are pregnant receive a flu vaccination. They say the flu itself is a much greater danger to women and their fetuses.
The Center for Disease Control, which funded the study, said that it was only a possible link and would need to be studied over multiple flu seasons.
"I want the CDC and researchers to continue to investigate this", said Dr. Laura Riley, a Boston-based obstetrician who leads a committee on maternal immunization.
Flu vaccinations of pregnant women increased substantially during and after the pandemic.
The CDC is clear in its guidelines that pregnant women in any trimester of their pregnancies should get the flu vaccine.
Flu and its complications kill thousands of Americans every year.
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Flu and the complications from it kill thousands each year in the US with young children, the elderly and pregnant women under additional risk.
Researchers studying the flu vaccine in pregnancy have found a hint of a possible link between miscarriage early in pregnancy and the flu vaccine in women who received a certain version of the vaccine two years in a row.
Marshfield researchers conducted a similar study among pregnant women during the 2005-2006 and 2006-2007 flu seasons, and found no association between the flu vaccine and miscarriage.
The researchers from the study attempted to make adjustments to the statistics to level some of the difference out, but some researchers do not think there was success.
Other experts said they don't believe a shot made from killed flu virus could trigger an immune system response severe enough to prompt a miscarriage. Experts said that the actual exposure to swine flu, and not the vaccine, could have caused the miscarriages.
Some companies, especially those in the health care fields, require their employees to get flu shots each year. "Not at all", said Poland, who also is director of vaccine research at the Mayo Clinic.