The real story on the flu vaccine during pregnancy


by Tara Haelle

A recent study about the flu vaccine during pregnancy has added to the continuously growing pile of evidence showing the vaccine’s safety and importance for pregnant women and their unborn babies. And yet, the fear, confusion and misinformation about pregnancy and the flu shot maddeningly persists.


According to multiple medical organizations and the CDC, one of the best things you can do for your unborn baby is to offer him or her the protection of the flu vaccine. Photo by Scott Snyder

I’ve written about getting the flu vaccine during pregnancy several times, including when I got my own flu shot several months back while in my first trimester and in my lengthy myth-busting post about the flu vaccine. However, articles claiming that the flu vaccine is dangerous during pregnancy continue to pop up, posted on social media or emailed to me by frightened, confused, misinformed or angry women. I won’t link to any of these articles, but they aren’t hard to find — and not a one of them contains accurate information.

The worst part about these inaccurate articles is that they prey on women’s fears and increase their anxiety while encouraging women to make a riskier decision – to skip the flu shot. Some of the women I’ve seen ask about this issue are first-time moms or have already lost pregnancies, perhaps multiple times, so they are especially sensitive to anything they might do that they fear would increase miscarriage risk. Even women who intellectually recognize that miscarriage is incredibly, incredibly common, and know that very, very few miscarriages can be attributed to something a woman “did” or “didn’t do,” these articles hit a nerve, and can lead them to avoid a vaccine that’s actually likely to *decrease* their risk of miscarriage.

Let me state this in no uncertain terms: the consensus of peer-reviewed evidence about the influenza vaccination and pregnancy is that it reduces the risk of flu among pregnant women and leads to better outcomes for their unborn babies – fewer preterm births, fewer underweight babies and fewer miscarriages and fetal deaths. More simply, if you’re pregnant, getting the flu shot is one of the best prenatal care decisions you can make, regardless of what trimester you’re in.

Among the claims you might hear are that the flu shot causes miscarriage and that the “mercury” in the shot is a danger to your child. The first is patently false. The second is both false and irrelevant (discussed in my flu vaccine myths post here), given that you can get a thimerosal-free (preservative-free) flu vaccine if you choose. (I personally got what was available, which happened to have thimerosal, and that is also fine. Prenatal exposure to thimerosal is also not associated with autism.)

I only know two ways to combat the misinformation and fear-mongering in these articles: to provide accurate information for the former, and to offer an anecdote about losing a fetus while sick with influenza for the latter. Read them in whatever order you wish, but both are important.

The anecdote is from Johanna Holmes, a friend who became sick with the H1N1 flu while she was pregnant in 2009. There are doubtless many other stories just like Johanna’s, and as the footnote makes clear, it’s impossible to know if the flu *caused* her loss or not, but her story is worth reading to drive home the risks associated with the flu during pregnancy. You can read about Johanna and Nicholas at the Voices for Vaccines site.

As for the research (all citations are linked to in the text), the most recent article came out just two days ago in the Canadian Medical Association Journal. A study of 12,223 women in Nova Scotia found that only 16% of pregnant women between November 2010 and March 2012 received the flu vaccine. (The CDC reported last month that 44% of pregnant women across 21 US states received the flu vaccine during the 2010-2011 flu season, a percentage that was similar or higher than in the previous season, so progress is occurring.)

But the women who were vaccinated had 25% lower odds of having a preterm birth and 27% lower odds of having a baby with a low birth weight (less than 5.5 lbs), after taking into account other differing characteristics of the women. There were too few fetal deaths to calculate risk differences among vaccinated and unvaccinated women. (This was a non-industry, internally funded study at Dalhousie University in Nova Scotia, and two of the five authors have previously conducted studies or attended medical panels for pharmaceutical companies.)

That study is one of the most recent ones. Two smaller studies published in the journal Vaccine last September (both funded by the US Department of Health and Human Services) found similarly encouraging results. The first followed 1,032 pregnant Canadian women, 81 percent of whom received an H1N1 flu vaccine. Among these women, 2.7% had a baby with a major birth defect, compared to 3.2% of women who did not get the flu vaccine. There were no differences in miscarriage rates or babies’ weights or lengths between the two groups. There actually was a slightly higher risk of preterm birth among those who got the vaccine – but those births occurred just an average three days earlier than in the unvaccinated group, hardly cause for concern and potentially due to chance.

The second study compared 4,191 babies with or without any one of 41 different birth defects. The rates for all but eight defects were essentially the same between the two groups. The eight defects that differed occurred in a tiny number of babies and did not reach statistical significance (so the findings were likely due to chance). During the 2009-2010 season, women receiving the vaccine had a slightly higher risk of preterm birth – by two days of gestation. During the 2010-2011 season, vaccinated women had a lower risk of preterm birth. (Again, the different results during different seasons reveal how any differences are likely due to chance.)

Meanwhile, a study published last January in the New England Journal of Medicine (funded by the Norwegian Department of Public Health by authors with no pharma ties) followed 117,347 pregnant women and found that “vaccination during pregnancy substantially reduced the risk of an influenza diagnosis.” In fact, the vaccine reduced the risk of flu among pregnant women by 70 percent — and the study found the miscarriage risk among women who had the flu was about double that of women who didn’t have the flu while pregnant.

Women who were unvaccinated during the 2009 H1N1 epidemic were 25% more likely to experience a fetal death (miscarriage after 12 weeks of pregnancy) than women who received the flu vaccine. These findings took into account the following differences among the women: age, number of previous children, marital status, use of nutritional supplements during pregnancy, smoking during pregnancy, previous miscarriages and eight medical conditions (asthma, high blood pressure, heart disease, kidney disease, rheumatoid arthritis, epilepsy, thyroid disease and diabetes).

Also, consider other studies that have found risks among pregnant women who caught the flu, such as a higher risk of hospitalization and death than the risk for those who don’t catch the flu. A study that investigated miscarriages among women who lived through the 1918 Spanish flu epidemic found a particularly high rate of losses – 1 in 10 pregnant women – that were attributed to the infection. (One case study reported by the CDC found evidence for a biological mechanism by which an influenza infection could cause a miscarriage.) Yet as this 2006 study from Epidemiologic Reviews notes in its abstract, “No study has demonstrated an increased risk of maternal complications or adverse fetal outcomes associated with inactivated influenza vaccine.”

Have you seen the false claim that reports to the Vaccine Adverse Event Reporting System were higher for pregnant women vaccinated against the flu than for those who skipped the shot? There is even a study that directly refutes that misinformation, finding that “no unusual patterns of pregnancy complications or fetal outcomes were observed in the VAERS reports of pregnant women after the administration of the trivalent inactivated influenza vaccine or live attenuated influenza vaccine” between 1990-2009. Another study looked specifically at VAERS reports in 2009 after the flu vaccine and found that “rates of spontaneous abortion, preterm birth and major birth defects in pregnant women who received live H1N1 vaccine were similar to or lower than published background rates.”

Another study, in JAMA in July 2012 (funded by the Danish Medical Research Council, conducted by authors with no pharma ties), looked specifically at outcomes among 53,432 babies. Overall, 13% (6,989 infants) of the babies’ mothers had received the H1N1 flu vaccine. Comparisons between these babies and those not exposed to the flu vaccine found no significant differences in rates of birth defects, preterm birth or being underweight (small-for-gestational-age). (Watch a video interview with this study’s lead author, Björn Pasternak, here.)

But wait! There’s more! You won’t find cherry-picking in this post. Consider the following list, and research to your heart’s delight…

An April 2012 study in the American Journal of Public Health tracked 55,570 pregnant women, 42% of whom received the H1N1 flu vaccine. Compared to unvaxed moms, the vaccinated moms were 10% less likely to have a baby weigh below the 10th percentile, 19% less likely to have a baby weigh below the 3rd percentile, 27% less likely to give birth before 32 weeks of pregnancy and 34% less likely to have a fetal death.

Another March 2013 study in Obstetrics and Gynecology compared 75,906 pregnant women who received the flu vaccine with 147,992 pregnant women who did not receive it: no differences in adverse events (including pregnancy complications) between the two groups were seen in the 42 days after immunization.

Similarly, a September 2013 study in Obstetrics and Gynecology compared 74,292 flu-vaccinated pregnant women with 144,597 unvaccinated pregnant women (matched by age, location and pregnancy start date). During the 42 days after immunization, there were no higher rates among the vaccinated women for hyperemesis (severe morning sickness), high blood pressure, pregnancy-induced high blood pressure, gestational diabetes, protein in urine or urinary tract infection. Through the end of the pregnancy, rates were no higher for the vaccinated moms for protein in urine, urinary tract infection, pregnancy-induced high blood pressure, pre-eclampsia, eclampsia, bacterial inflammation of the fetal membranes, bacterial infection during childbirth, pulmonary embolism (lung blood clot) or a weak heart muscle.

A February 2012 study in the Canadian Medical Association Journal compared 340 Bangladeshi women who received either the flu vaccine or a pneumococcal vaccine (in the control group). While the flu virus was circulating, 26% of the flu-vaccinated women had underweight babies compared to 45% of the control-group women. The average birth weight of babies born to flu-vaccinated moms was 7 pounds, compared to an average weight of 6.6 pounds among babies born to control-group women.

A December 2013 study in Vaccine, involving 3,393 live births in the state of Georgia, found lower odds for preterm birth and underweight babies among pregnant women who received the flu vaccine compared to unvaccinated pregnant women.

A July 2013 study in the European Journal of Epidemiology, involving 21,087 pregnant women, found no higher risk among women vaccinated against the flu than unvaccinated women for preterm birth, babies with low birth weight, underweight babies (small-for-gestational-age) or a low Apgar score.

A May 2011 study in PLoS Medicine, involving 4,326 babies, found that newborns were 69% less likely to be born underweight and 40% less likely to be born premature during flu season if their mothers had received the flu vaccine while pregnant than if the mothers were not vaccinated against the flu.

A May 2013 study in the journal Clinical Infectious Diseases, involving 3,327 babies born during the 2009 H1N1 flu season, found that pregnant women vaccinated against the flu were less likely to have preterm babies than unvaccinated women. The babies of vaccinated moms were also an average 45 grams (0.1 pounds) heavier than babies of unvaccinated moms.

A December 2012 study in BJOG: An International Journal of Obstetrics and Gynaecology compared 18,612 pregnant women who received the flu vaccine to two comparison groups of women without known vaccinations. Rates of underweight babies and birth defects were similar across all women, but rates for stillbirth, preterm birth and low birthweight were lower among the women vaccinated against the flu.

A September 2012 study in Obstetrics and Gynecology compared 8,690 pregnant women receiving the seasonal flu vaccine with 76,153 women who declined vaccination over five years (these numbers are only available in the full study, not the abstract). The researchers found no significant differences between the two groups of women in terms of major birth defects, stillbirth, fetal death or preterm birth, regardless of the trimesters when vaccinated women received their vaccines.

All these studies were mostly published within the past two years, and many (if not most) were conducted without any pharmaceutical or other industry funding. There are more studies than these — I could not possibly list all of them. But frankly, if this list does not convince you of the safety, effectiveness and importance of the flu vaccine during pregnancy, and Johann’s story does not offer the same food for thought that scary, unsubstantiated flu-vaccine-caused-my-miscarriage claims offer, then I suspect no study or story will satisfy you.