Flu Shot During Pregnancy Produces Unexpected Benefits

If this woman gets a flu shot, it may produce benefits for her child. (click for credit)
If this woman gets a flu shot, it may produce benefits for her child. (click for credit)
I try to keep up with the medical literature, but a chemist who reads this blog, Dr. Jonathan D. Sarfati, made me aware of a couple of studies that I had missed over the past couple of years. Those two studies led me to a third study, and when all three of them are put together, they make a strong case for pregnant women getting the flu shot. I know there is a lot of misinformation out there related to the flu shot, but the evidence makes it clear that the shot is not only safe but also very beneficial, especially for pregnant women.

The first study Dr. Sarfati sent me was published in The American Journal of Public Health back in 2012. The authors used used Ontario’s birth record database (called BORN) to analyze more than 55,000 single-child births that took place in Canada during what the authors call “the 2009–2010 H1N1 pandemic.” They specifically looked at which mothers got the H1N1 flu vaccine. They found that 42% of the mothers did get the vaccine, while the rest did not. They also found that the mothers who received the vaccine were 28% less likely to deliver before their full term and 19% less likely to have a child with a low birth weight. The most striking finding, however, was that mothers who got the vaccine were 34% less likely to have a stillborn child.1

Now this study was specifically focused on a time that health-care officials labelled as a pandemic. Does the flu shot offer the same benefits to the children of pregnant women during non-pandemic flu seasons? Another Canadian study indicates that it does. That study looked at a different database (one in Nova Scotia) for births that occurred from November 1, 2010 to March 31, 2012. There were more than 12,000 births during that time period, and only 19% of the mothers in the group had the flu shot. Nevertheless, they were 25% less likely to give birth to preterm infants and 27% less likely to give birth to infants with low birth weights. Unfortunately, only live births were considered in the analysis, so the risk of stillborn infants was not addressed.2

These two studies together make a very strong case that for the sake of their unborn children, pregnant women should get the flu shot. The other study that Dr. Sarfati sent me was a bit less conclusive (in my opinion), but it does suggest a link between the flu shot and autism. However, it’s not the link anti-vaccination advocates are looking for.

That study looked at more than 96,000 Danish children born from 1997 to 2003. It was motivated by the fact that animal studies indicate when a mother is pregnant and suffers an infection, the neurological development of her fetus is impaired.3 The authors studied mothers who had been recruited for the Danish National Birth Cohort, an initiative that collects data related to infants and their health. As a part of that initiative, the mothers were interviewed by telephone twice during pregnancy and once about six months after birth. Those interviews asked the mothers about their health, including whether or not they had a fever, were taking medicine for infections, etc. Here is how the authors state their findings:4

Overall, we found little evidence that various types of mild common infectious diseases or febrile episodes during pregnancy were associated with ASD/infantile autism. However, our data suggest that maternal influenza infection was associated with a twofold increased risk of infantile autism, prolonged episodes of fever caused a threefold increased risk of infantile autism, and use of various antibiotics during pregnancy were potential risk factors for ASD/infantile autism.

So they found that when the mother is ill for a long time during pregnancy, it can strongly increase the child’s risk of autism/ASD. Of course, one of the specific illnesses they mention is the flu. If the mother having the flu during pregnancy does increase the child’s risk of autism/ASD, then there is a connection between the flu vaccine and autism – if the mother gets the flu vaccine, she is less likely to catch the flu, and her child’s risk of autism/ASD decreases.

Now please note that while I find the first two studies in this discussion very convincing, I don’t think this one is nearly as convincing. First, only 1% of the children (976) in the study had autism. That’s a very low number. Thus, there is probably a lot of statistical error in the analysis. In addition, whether or not the mother was ill during pregnancy relies on the mother remembering the illness at the time of the call. The authors did try to check the mothers’ memories by comparing what they said to any medical records that were collected during the time of the pregnancy, but that’s a pretty hit-or-miss approach. In the end, then, I think this study suggests that prolonged illness (like the flu) during pregnancy can increase the child’s risk of autism, but a more detailed study must be done to confirm it.

The first two studies I discussed, however, seem very robust. When a mother receives a flu shot during pregnancy, the child benefits. Every pregnant woman should consider the results of those two studies and discuss them with her physician.


1. Deshayne B. Fell, Ann E. Sprague, Ning Liu, Abdool S. Yasseen III, Shi-Wu Wen, Graeme Smith, Mark C. Walker, and for Better Outcomes Registry & Network (BORN) Ontario, “H1N1 Influenza Vaccination During Pregnancy and Fetal and Neonatal Outcomes,” American Journal of Public Health 102 (6):e33-e40, 2012, doi:10.2105/AJPH.2011.300606
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2. Alexandra Legge, Linda Dodds, Noni E. MacDonald, Jeffrey Scott, Shelly McNeil, “Rates and determinants of seasonal influenza vaccination in pregnancy and association with neonatal outcomes,” Canadian Medical Association Journal, 2014, doi:10.1503/cmaj.130499
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3. Paul H. Patterson, “Maternal infection and immune involvement in autism,” Trends in Molecular Medicine 17(7):389-394, 2011
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4. Atladóttir HÓ, Henriksen TB, Schendel DE, Parner ET, “Autism after infection, febrile episodes, and antibiotic use during pregnancy: an exploratory study,” Pediatrics 130(6):e1447-54, 2012, doi:10.1542/peds.2012-1107
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6 thoughts on “Flu Shot During Pregnancy Produces Unexpected Benefits”

  1. Dr Jay, your conclusion is wonderful: after stating clearly your support for the obvious likelihood of benefit, you send the pregnant woman off to get a shot of knowledgeable advice from her doctor, rather than commanding then to get that flu shot. Your humbleness and transparency are always appreciated.
    Thank you.

    1. Thanks for your kind words, Bill. I really don’t try to advocate a position so much as to simply get the data out there. I want people to make their own decision regarding vaccines, but I want them to have good data to help them make that decision.

  2. Having not yet gone through the studies in detail, I can’t say for sure, but it seems to me that the benefits may not be due to the flu shot exclusively. It seems to me that those who do get the shot may be more likely to do other things that benefit their child as well, which may produce the same results.

    1. That is certainly a possibility, LtRommel, since it is nearly impossible to compare two sets of people absolutely equally. However, the authors did investigate several possible confounders, including “maternal age, parity, history of PTB, smoking during pregnancy, chronic hypertension, a composite measure of maternal medical comorbidity (any of the following preexisting medical conditions: asthma, chronic hypertension, insulin-dependent diabetes, non-insulin-dependent diabetes, or heart disease), pregnancy-induced hypertension, preeclampsia, a composite of obstetrical complications during the pregnancy (e.g., pregnancy-induced hypertension, preeclampsia, gestational diabetes, placenta previa), and a composite of intrapartum complications during labor or birth (e.g., cord prolapse, intrapartum bleeding, meconium).” They also looked at socioeconomic status.

  3. I find it interesting that you did not discuss the fact that most flu vaccine inserts state right on them that the safety and efficacy has not been established in pregnant women. This is somewhat dangerous not to mention to folks.

    1. But that’s precisely what the first two studies in this post address, Namk. In each case, the pregnant women who got the flu shot had no more negative health consequences than the pregnant women who didn’t get the shot. This indicates the shot is, indeed, safe for pregnant women. Of course, that makes sense, because if the flu shot had harmed the pregnant women, they would not have had fewer stillbirths, fewer premature births, and fewer babies with low birth weights!

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