Pediatricians have an abundance of opportunities to help their patients have safe childhoods. The most straightforward of these is the chance to keep children safe from diseases from polio to meningitis. And even though there are always parents who don't understand, or agree, regarding the clear lifesaving benefits of immunization, most parents do choose to immunize, especially when their pediatrician strongly recommends it.
Unfortunately, viruses don't conveniently wait until a child has had their first immunizations at age two months - or in the case of the influenza vaccine, six months - before they hit. For some of these diseases, specifically whooping cough and influenza, we rely on vaccines given during pregnancy: the Tdap and flu shots. These shots have been shown to reduce the chance of dangerous illness in our youngest patients, but since they are given during pregnancy, pediatricians don't always get the chance to advise parents on the importance of these vaccines. Nor, for that matter, are we the experts in what pregnant women are and are not supposed to do.
But infants are affected by these decisions, and infant health (like many issues) is distinctly in my lane. So I interviewed a certified nurse midwife at UnityPoint Health - Des Moines, about the importance of immunizations in pregnancy, and the influenza vaccine in particular. She was very interested in making sure pregnant mothers knew that immunizing duing flu season was important. During flu season, according to Larsen, “a pregnant mom should obtain a flu vaccine any time throughout their pregnancy.” “If you would compare to a woman who is not pregnant, the pregnant woman actually has an increased risk of not only obtaining influenza, but to have more severe effects from influenza virus,” Larsen said. “so it is really important for that special population to obtain a vaccine to help not only prevent them from becoming ill, but to help pass antibodies towards their baby, so when they're born they have some passive immunity for the first six months of life when they can not get a vaccination.”
I would guess that most expectant mothers, if they refuse the flu shot, do so because they are wary of its safety. Larsen says she commonly addresses the myth that flu vaccines aren’t studied in pregnancy. “The safety factor is there, she said. "There are no concerns about higher risk for autism, miscarriages, or any other harm to themselves or their baby.”
Let’s look at some of those studies. While the package insert for flu vaccines does usually say something like “Safety and effectiveness have not been established in pregnant women or children less than 6 months of age” Larsen explains that the insert is describing what the drug company itself has done for licensure. The CDC, ACOG, and other groups look at all the studies that have been done looking at vaccines and pregnancy by entities other than drug companies, and make their recommendations based on the totality of the research. And there are a lot of studies, she says.
Let’s find out. With Larsen's help, I pulled together all the safety studies we could find on influenza vaccines in pregnancy. Take a look - the body of evidence is pretty convincing. Not only do many large studies show that the vaccine is safe for mother and baby, but a good number find reduced numbers of adverse outcomes when mothers are immunized.
Studies Looking at the Risk of Influenza Disease in Pregnancy
- A 2013 study by the Norwegian Department of Public Health (Haberg et al), looking at over 100,000 pregnant women, found that an influenza diagnosis significantly increased the risk of fetal death, and that immunization was associated with a lower, though not statistically significant decrease in fetal death.
- A 1998 study by Neuzil et al found that about one in four hundred women in their third trimester during flu season will be hospitalized from influenza.
- A 2009 CDC study by Jamieson et al found that H1N1 influenza was more likely to cause hospitalization in pregnant women than in the general population, and noted six related deaths over a two month period.
- A CDC review of the risks of influenza disease among pregnant women and infants less than six months old.
- A 2013 study by Parboosing et al found that gestational influenza was associated with development of bipolar disorder later in life.
Studies Supporting the Safety of Influenza Vaccines in Pregnancy
- A 2014 Canadian study by Legge et al, looking at 12,223 women, found “The odds of preterm birth was lower among infants of vaccinated women than among those of nonvaccinated women (adjusted odds ratio [OR] 0.75, 95% confidence interval [CI] 0.60–0.94). The rate of low-birth-weight infants was also lower among vaccinated women (adjusted OR 0.73, 95% CI 0.56–0.95).”
- A large 2013 study using the Vaccine Safety Datalink by Kharbanda et al found no increased risk for adverse obstetric events such as chorioamnionitis, pre-eclampsia, or gestational hypertension) for pregnant women who received the flu vaccine.
- Another study in 2017 by Kharbanda et al found that the flu shot during the first trimester did not increase the risk of major birth defects.
- A study of 4191 subjects by Louik et al found a significantly decreased risk for PTD [pre-term delivery] in the 2010–2011 season. It also found no increased risk for congenital malformations in either in the 2009-2010 season or 2010-2011 season. In the 2010 season, the immunized subjects had a decrease in gestation length of less than two days.
- A 2013 study using Vaccine Safety Datalink data by Irving et al found no increased risk of miscarriage among pregnant women who received flu shots across two flu seasons.
- A Vaccine Safety Datalink study by Nordin et al in 2014 looked at two flu seasons and did not find an increase rates of premature delivery or small for gestational age infants by immunized mothers.
- Several CDC reviews of reports to the Vaccine Adverse Reporting System (VAERS) (Moro et al, 2011, Moro et al, 2011, Moro et al, 2017) found no evidence to suggest a link between pregnancy complications or adverse fetal outcomes among pregnant women and flu shots.
- A 2013 study of 1032 pregnant women by Chambers et al found no we found no increased risk of birth defects, spontaneous abortion, or small for gestational age infants in immunized women, and a mean gestational length difference of less than three days.
- This 2011 Canadian study by Fell et al looked at over 50,000 pregnant mothers, and found that women immunized against influenza had significantly lower chance of having a small for gestation age baby, premature delivery before 32 weeks, or fetal death.
- A 2005 study by Munoz et al looking at mother-infant pairs in Texas found “No serious adverse events occurred within 42 days of vaccination, and there was no difference between the groups in the outcomes of pregnancy (including cesarean delivery and premature delivery) and infant medical conditions from birth to 6 months of age.”
- A 2010 study by Lim et al looked at adverse reactions that occurred after immunization and found that those that occurred “were mild and resolved within several days without requiring any treatment or hospitalization.”
- A 2010 study by Yamaguchi et al looked at 182 immunized women in Japan and found “No adverse events were seen in pregnancy or fetal medical condition regardless of the pregnancy stage at which vaccine was administered.”
- A prospective study by Tavares et al of 265 pregnant women and a new, adjuvanted flu vaccine found “The results of this analysis suggest that exposure to the AS03 adjuvanted H1N1 (2009) vaccine during pregnancy does not increase the risk of adverse pregnancy outcomes including spontaneous abortion, congenital anomalies, preterm delivery, low birth weight neonates, or maternal complications.”
- A 2012 Danish study by Van Santen et al found that maternal influenza vaccine “was not associated with a significantly increased risk of major birth defects, preterm birth, or fetal growth restriction.”
- A 2010 study by Price et al looked at 256 children with ASD and 752 matched controls, and found no increased risk of autism associated with prenatal and infant thimerosal exposure, a preservative that is in a minority of influenza vaccines.
- A 2013 study by Ludvigsson et al looked at over 21,000 women in Sweden with singleton pregnancies found immunizing had no difference in low birth weight, preterm birth, small for gestational age births, or Apgar score. Influenza immunization was associated with a slightly reduced risk of cesarean section.
- Finally, this 2009 review published in the American Journal of Obstetrics and Gynecology assessed a number of safety studies on influenza vaccine in pregnancy, dating back as far as 1964.
Studies Finding Better Outcomes When Mothers Get Flu Shots During Pregnancy
- A 2018 CDC co-authored study by Thompson et al looked at pregnant mothers hospitalized with confirmed influenza across four countries, and found that getting a flu shot reduced hospitalizations for influenza by about 40%.
- A 2016 study by Regan et al looked at over 5000 pregnant mothers and found “Mothers who received seasonal TIV during pregnancy were significantly less likely to experience stillbirth compared with unvaccinated mothers.”
- A 2013 study by Adedinsewo et al looking at over 8000 live births in the state of Georgia found “reduced odds of small for gestational age and preterm births during the widespread influenza activity period.”
- A 2014 study by Steinhoff et al of over 12000 women found fewer low birth weight infants as well as fewer preterm births in those immunized against influenza.
- A 2011 study by Omer et al looked at over 4000 births in Georgia and found that immunization was associated with a significant reduction in premature delivery during flu season.
- A 2013 retrospective cohort study of live births within Kaiser Permanente Georgia and Mid-Atlantic States by Richards et al found “women who received H1N1 influenza vaccine were less likely to give birth preterm, and gave birth to heavier infants.”
- A 2012 study by Kallen et al looked at all births in Sweden over a two year period and found H1N1 influenza immunization was a lower rate of preterm birth and low birthweight.
- A 2012 study by Sheffield et al studied over 10,000 women in Texas and found a decreased stillbirth rate in those who were immunized against influenza.
- A 2012 randomized, controlled study by Steinhoff et al found better birth weights and fewer SGA infants in those whose mothers were immunized with influenza vaccine vs. pneumococcal vaccine.
- A 2013 study of over 9800 mother-infant pairs found that maternal influenza immunization, when coupled with infant pneumococcal immunization, provided greater protection against infant ear infections a over the pneumococcal vaccine alone.
- A 2010 Yale Pediatrics study by Benowitz et al looking at 247 infants found “Influenza vaccine given to pregnant women is 91.5% effective in preventing hospitalization of their infants for influenza in the first 6 months of life.”
- A 2008 study by Zaman et al gave 340 mothers either influenza vaccine or pneumococcal vaccine, and found that those that receive the influenza vaccine had a significant reduction in respiratory illness in mothers as well as a reduction in influenza in the infants.
As I said, this list is not exhaustive, and I will add to it as I find more studies. But it's clear that not only has the research been done, but that the consensus is that influenza vaccines during pregnancy are safe, and likely lead to better outcomes for mothers and babies.