WASHINGTON, Sept. 13 (Xinhua) -- A U.S. study published Wednesday found a possible link between the flu vaccine and miscarriage early in pregnancy.
The study, which appeared in the journal Vaccine, however, was quick to note that it "does not and cannot establish a causal relationship," meaning it don't necessarily show the vaccine directly causes a pregnancy loss, but "further research is warranted."
The findings were based on data from 485 pregnant women aged 18 to 44 who had a miscarriage and 485 women of similar ages who had normal deliveries during the flu seasons of 2010-2011 and 2011-2012.
"SAB (spontaneous abortion) was associated with influenza vaccination in the preceding 28 days," the study concluded. "The association was significant only among women vaccinated in the previous influenza season" with a shot that included protection against the 2009 H1N1 flu pandemic that killed more than 18,000 people around the world.
However, when vaccination was given either later in the first trimester or in the second or third trimester, there was no association seen with pregnancy loss or any other adverse pregnancy outcomes, it said.
"I think it's really important for women to understand that this is a possible link, and it is a possible link that needs to be studied and needs to be looked at over more (flu) seasons," Amanda Cohn, senior adviser for vaccines at the U.S. Centers for Disease Control and Prevention, which funded the study, told the Washington Post.
"We need to understand if it's the flu vaccine, or is this a group of women who were also more likely to have miscarriages," Cohn said.
The American College of Obstetricians and Gynecologists said in a statement it continues to recommend that pregnant women receive the flu vaccine.
"Multiple published studies, as well as clinical experience, have all supported the belief that the flu vaccine is safe and effective during pregnancy," the American College of Obstetricians and Gynecologists said.
"In evaluating all of the available scientific information, there is insufficient information to support changing the current recommendation which is to offer and encourage routine flu vaccinations during pregnancy regardless of the trimester of pregnancy."