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Influenza Vaccine Effectiveness During Pregnancy

Clin Infect Dis; ePub 2018 Oct 11; Thompson, et al

Influenza vaccines offer moderate protection against laboratory-confirmed influenza-associated hospitalization during pregnancy, a recent study found. The multi-country (US, Canada, and Israel) retrospective test negative design study identified pregnant women aged 18-50 years whose pregnancies overlapped with local influenza seasons from 2010 through 2016. Researchers noted hospitalizations with acute respiratory or febrile illness (ARFI) and clinician-ordered real-time reverse transcription polymerase chain reaction (rRT-PCR) testing for influenza vaccine. Overall influenza vaccine effectiveness (IVE) was estimated using the test-negative design and adjusting for site, season, season timing, and high-risk medical conditions. Among the findings:

  • Among 19,450 hospitalizations with an ARFI discharge diagnosis, 1,030 (6%) of the pregnant women were tested for influenza viruses by rRT-PCR.
  • 54% of these women had pneumonia or influenza discharge diagnosis.
  • Across sites and seasons, 13% of rRT-PCR-confirmed influenza-positive pregnant women were vaccinated vs 22% of influenza-negative pregnant women.
  • The adjusted overall IVE was 40% against influenza-associated hospitalization during pregnancy.


Thompson MG, Kwong JC, Regan AK, et al. Influenza vaccine effectiveness in preventing influenza-associated hospitalizations during pregnancy: A multi-country retrospective test negative design study, 2010‒2016. [Published online ahead of print October 11, 2018]. Clin Infect Dis. doi:10.1093/cid/ciy737.


In assuming that this was a relatively young population, one would expect the vaccine to be more effective than in an older population but, remember, pregnant women in the last part of pregnancy are more likely to be admitted secondary to some restrictive lung abnormalities secondary to pregnancy. Data from studies including and after the 2009 H1N1 pandemic show that pregnant women with influenza are more likely to be hospitalized and to be admitted to an intensive care unit compared with the general population. During the H1N1 pandemic pregnant women made up 5% of deaths while being only 1% of the population. A recent CDC study on vaccine efficacy from 2004-2018 showed an overall efficacy in all populations of around 40%. This lines up with this study’s results. We should continue to encourage this vaccine to our expectant mothers. — John Russell, MD