During pregnancy, the use of influenza vaccines moderately protected against laboratory-confirmed influenza-linked hospitalization over the course of 6 influenza seasons, according to a Centers for Disease Control and Prevention-coauthored study published in Clinical Infectious Diseases.1

A group of international investigators conducted a retrospective test negative design study to analyze the effectiveness of influenza vaccination against laboratory-confirmed influenza-associated hospitalization during pregnancy, using data from the Pregnancy Influenza Vaccine Effectiveness Network (PREVENT), which was composed of public health or healthcare systems with incorporated laboratory, medical, and vaccination records.

The data included in PREVENT were taken from pregnant women aged 18 to 50 years whose pregnancies took place during the local influenza seasons between 2010 and 2016.

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The outcomes measured included acute respiratory or febrile illness (ARFI) and the confirmed presence of influenza viruses in real-time reverse transcription polymerase chain reaction.

A total of 19,450 ARFI hospitalizations were reported, 6% of which had real-time reverse transcription polymerase chain reaction influenza virus testing. The analytic sample used included 1030 hospitalizations, 25 of which were repeated hospitalizations. Results suggested that 79% of ARFI hospitalizations occurred in women younger than 35 years; 65% of ARFI incidents occurred in women in their third trimester, and 66% had no high-risk medical conditions.

The researchers also noted that 58% of ARFI hospitalizations with polymerase chain reaction testing were positive for influenza virus: 13% of influenza-positive and 22% of influenza-negative women had been vaccinated, which correlates to an unadjusted influenza vaccine effectiveness of 48% (40% adjusted) against influenza-associated hospitalization during pregnancy.

“In addition to the ample data on the safety of inactivated influenza vaccination during pregnancy, mounting evidence that influenza vaccination reduces the risk of mild to moderately severe [laboratory-confirmed influenza] disease during pregnancy, and evidence that maternal vaccination offers secondary protection to infants during the first months of life, our finding of 40% [influenza vaccine effectiveness] in preventing [laboratory-confirmed influenza] hospitalization during pregnancy further strengthens the rationale for influenza vaccination programs for pregnant women,” the investigators concluded.

In a press release, the Centers for Disease Control and Prevention noted that influenza vaccination during pregnancy may not only protect pregnant women, but “helps protect the baby from flu infection for several months after birth, before he or she is old enough to be vaccinated themselves.”2

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The press release also added that “[f]lu shots have been given to millions of pregnant women over many years with a good safety record,” and that women who are pregnant are recommended to receive vaccination at any trimester of their pregnancy.

Disclosure: Some of the authors of this study receive grants from Pfizer, MedImmune/Astra Zeneca, and Merck, and 1 author is a content advisor to Jannsen Pharmaceuticals.


  1. 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 October 11, 2018]. Clin Infect Dis. doi: 10.1093/cid/ciy737
  2. Flu vaccine reduces risk of flu hospitalization among pregnant women [press release]. Centers for Disease Control and Prevention. https://www.cdc.gov/media/releases/2018/p1011-flu-vaccine-reduces-risk-pregnant-women.html. October 11, 2018. Accessed October 11, 2018.