A new study published in Vaccine on September 13 is causing some concern for pregnant women who have received the flu vaccine or are considering it. The study of pregnancies in the United States found that women who had miscarriages between 2010 and 2012 were more likely to have had back-to-back annual flu shots that included protections against the H1N1 virus (swine flu).
The Centers for Disease Control and Prevention (CDC) has reached out to a doctor’s group, the American Congress of Obstetricians and Gynecologists, to warn them the study is coming out and help them prepare for a potential wave of worry from expectant mothers, CDC officials said.
Increased Incidence After Repeat Vaccine
“We are not saying this is a causal relationship,” said James Donahue, DVM, PhD, MPH, a senior epidemiologist at the Marshfield Clinic in Wisconsin and the lead author of the study, meaning the data does not necessarily show that the flu vaccine causes miscarriages. “There’s no biological basis for this phenomenon, so the study represents something that wasn’t expected.”
The study was conducted over two flu seasons (2010 to 2012), and involved 485 women. These women who experienced spontaneous abortion, or miscarriage, were matched with those who delivered full-term or stillbirths. Donahue and fellow researchers were examining if miscarriages were more apt to occur if a woman received the flu vaccine in the 28 days prior to the spontaneous abortions. The case-control study was funded by the CDC, according to CIDRAP (Center for Infectious Disease and Research Policy) News.
It was found by researchers that no association between miscarriage and flu vaccine appeared if a woman had not received a vaccine in the previous year, but in women who had consecutively gotten a flu vaccine containing the 2009 H1N1 virus, the investigators found an adjusted odds ratio (aOR) of 7.7. The women not vaccinated in the previous season had an aOR of 1.3. This increased association was seen in both seasons studies.
In the 28-day window, the overall aOR was 2.0, or double the risk, however those findings, in contrast to the H1N1 subset, were not statistically significant. There was no visible association in any other exposure window, reports CIDRAP News.
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Previous Studies Involving Flu Vaccine and Miscarriage
“In a previous study conducted on flu vaccine and miscarriage, we did not see a risk,” Donahue said, referring to research done from 2005 to 2007. That study was performed after the CDC made the recommendation 2004 that all pregnant women in all trimesters get the seasonal flu shot. After the 2009 H1N1 pandemic, the CDC requested a follow-up study.
“We can speculate that what we see here is vaccination with H1N1pdmo9 for the first time is like getting primed, then boosted,” said Donahue. “But now H1N1 is just a circulating seasonal virus; most children and young adults have been exposed by infection or vaccination.” In other words, there may have been a real effect in the seasons on the heels of the 2009 pandemic, however, further study will fail to show this association. Donahue and his co-workers are planning a follow-up study that examines more recent flu seasons and triples the number of cases, reports CIDRAP News.
Further Investigation Necessary
A research professor of global health at George Washington University, Lone Simonsen, PhD, said the timing of the study is of interest. She has done her own research showing that in 1918, when the world had another H1N1 pandemic that was worse than in 2009, the number of women who miscarried were as many as one in ten.
“In this case getting the flu vaccine can be like being introduced to a virus that was evolutionarily quite similar to the virus from 1918, she told CIDRAP News.
However, she also said the new study included only women with documented miscarriage. “Who are the women that document a miscarriage at 5 weeks?” asked Simonsen. “Probably someone who is also likely to get a flu shot.” If this is true, that may throw off numbers showing an association.
In an accompanying commentary in the same journal, three United States experts not taking part in the study said that miscarriage is one of the hardest birth outcomes to study in observational research.
“Among other factors, the high proportion of [spontaneous] abortions [SAbs] that take place in clinically-unrecognized pregnancies and the lack of consistency in accurate capture of these events in medical records when SAbs do occur, make such research difficult to carry out,” they write. After pointing out what they see as limitations of the study, and the implausibility of the prime-boost hypothesis, the experts write, “One important take-away message from this study is that seasonal vaccine formulations are not all the same.”
Both Donahue and Edward Belongia, M.D., the director of the Center for Clinical Epidemiology & Population Health at the Marshfield Clinic and a study coauthor, said this study in no way suggests reversing or revising the CDC’s advisories relating to pregnant women and flu shots.
“It’s very well known that getting the flu is bad for pregnant women and bad for the baby, and we have a vast amount of safety data on the vaccine,” said Belongia.
Legal Information and Advice About Flu Shots
If you or someone you know has been negatively affected by a flu vaccination, you may have valuable legal rights. The personal injury attorneys at Parker Waichman offer free, no-obligation case evaluations. We urge you to contact us at 1-800-YOURLAWYER (1-800-968-7529).