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Last week I wrote about influenza vaccines. I provided a link to a site summarizing the evidence that flu shots are effective and safe. Nevertheless, one reader commented “The flu vaccine does not work,” citing a Cochrane review. Elsewhere, a Twitter tweeter cited a study showing that flu vaccination in early pregnancy caused autism in babies. I thought it would be worthwhile to review those studies in case others are misled by them.

Does influenza vaccine in pregnancy cause autism in children?

The autism study didn’t say that at all. It was a large cohort study designed to investigate the association between influenza infection and vaccination during pregnancy and the risk of autism spectrum disorder (ASD). It looked at the records of 196,929 children born at Kaiser Permanente Northern California over a 10-year period. 23% of mothers (45,231 women) were vaccinated during pregnancy, and 1,400 mothers had clinical illnesses that were diagnosed as influenza infections by laboratory testing. 3,101 children were diagnosed with ASD. The study was published in 2017 in JAMA Pediatrics.

It concluded:

There was no association between maternal influenza infection anytime during pregnancy and increased ASD risk. There was a suggestion of increased ASD risk among children whose mothers received an influenza vaccination in their first trimester, but the association was not statistically significant after adjusting for multiple comparisons, indicating that the finding could be due to chance.

That single finding, which was not statistically significant, does nothing to tip the balance when weighed against this information from the CDC:

Flu is more likely to cause severe illness in pregnant women than in women of reproductive age who are not pregnant. Changes in the immune system, heart, and lungs during pregnancy make pregnant women (and women up to two weeks postpartum) more prone to severe illness from flu, including illness resulting in hospitalization. Flu also may be harmful for a pregnant woman’s developing baby. A common flu symptom is fever, which may be associated with neural tube defects and other adverse outcomes for a developing baby. Getting vaccinated can also help protect a baby after birth from flu. (Mom passes antibodies onto the developing baby during her pregnancy.)

Bottom line: flu shots are recommended for pregnant women.

The Cochrane review

The reader who asserted “the flu vaccine does not work” said, “Cochrane reviewed 90 flu vaccine studies, and less than 10 percent were considered good science.” He found this information here. That study didn’t say the flu vaccine didn’t work; it said it worked, with a number needed to treat (NNT) to prevent one case of influenza of 71.

The Cochrane review he cited was published in 2014 and has been superseded by a 2018 update. The 2018 review says:

Due to the uncertain quality of observational (i.e. non‐randomised) studies and their lack of influence on the review conclusions, we decided to update only randomised evidence. The searches for observational comparative studies are no longer updated.

They included 52 clinical trials of over 80,000 people. They said 15% of the studies they reviewed were well designed and conducted, but they found insufficient evidence to determine the likelihood of bias in about 70% of the studies they included in the review.

They found:

  • Moderate-certainty evidence that flu vaccines reduce the rate of influenza in healthy adults from 2.3% to 0.9%. (That’s percentages; think of how many individuals that corresponds to in our large population.)
  • 71 healthy adults need to be vaccinated to prevent one of them experiencing influenza. (This is actually not too shabby when compared to the NNTs for other widely accepted treatments, as reported here.)
  • Vaccination may lead to a small reduction in hospitalization, but the confidence interval is wide and doesn’t rule out a large benefit.
  • Low-certainty evidence that vaccines may lead to a small or no reduction in days off work.
  • A modest protective effect of vaccination in pregnant women and newborns.
  • A reduced risk of influenza-like illnesses.

Other relevant studies

A 2016 review in The Lancet found that the effectiveness rates for the specific strains of influenza virus covered in the vaccine ranged from 33 to 73%. For adults over 60, they found effectiveness rates of 24% to 63% depending on the strain.

The effectiveness varies with the accuracy of prediction of which strains will dominate in a given year. According to the CDC, the overall vaccine effectiveness for last year was 36%. The effectiveness for the H3N3 strain, the dominant strain circulating that season, was notoriously virulent and less receptive to vaccination than other types of influenza; its effectiveness was approximately 25%. The effectiveness for the less severe H1N1 was 67% and for influenza B viruses it was 42%.

As a TIME article pointed out:

36% effectiveness may not seem very impressive, but the CDC emphasizes in the report that even small increases in immunity can have a large impact on public health. CDC data has shown that even in 2014-2015, a year when vaccine effectiveness didn’t even hit 20%, immunizations prevented as many as 144,000 flu-related hospitalizations and 4,000 deaths. Plus, people who get the shot, but who still end up getting sick, tend to have less severe illnesses than unvaccinated people.

Lessons to be learned

  • Never rely on a single study.
  • Do your research carefully. Read what the study actually said rather than relying on a second-hand report from a possibly biased or ill-informed source.
  • Look for studies that refute the claim, not just ones that confirm it.
  • Make sure you have found the most recent information available.
  • When you find indications of a possible harm, weigh it against the known benefits.
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Posted by Harriet Hall

Harriet Hall, MD also known as The SkepDoc, is a retired family physician who writes about pseudoscience and questionable medical practices. She received her BA and MD from the University of Washington, did her internship in the Air Force (the second female ever to do so),  and was the first female graduate of the Air Force family practice residency at Eglin Air Force Base. During a long career as an Air Force physician, she held various positions from flight surgeon to DBMS (Director of Base Medical Services) and did everything from delivering babies to taking the controls of a B-52. She retired with the rank of Colonel.  In 2008 she published her memoirs, Women Aren't Supposed to Fly.