A study found the shot offers 'moderate' protection. Only about half of pregnant women got their flu shot last year.
This article first appeared October 14, 2018 on Medpage Today.
By Molly Walker
The influenza vaccine offered pregnant women moderate protection against flu-related hospitalizations, researchers found.
Adjusted influenza vaccine effectiveness was 40% (95% CI 12%-59%) against influenza-related hospitalization during pregnancy, reported Mark G. Thompson, PhD, of the CDC, and colleagues.
Moreover, from 2010 to 2016, 13% of pregnant women who tested positive for influenza were vaccinated compared with 22% of pregnant women who tested negative, according to the study in Clinical Infectious Diseases.
Recent CDC data suggest that only about half of pregnant women got their flu shot last year, but the authors cited the "paucity of data" on influenza vaccine effectiveness in preventing severe influenza-related outcomes, given that randomized trials would be "unethical."
Researchers examined data from pregnant women ages 18-50 with records of live births or stillbirths with gestations ≥20 weeks, whose pregnancies overlapped with influenza seasons from 2010 to 2016.
The team analyzed records from the Pregnancy Influenza Vaccine Effectiveness Network (PREVENT), which contained laboratory, medical, and vaccination records in Australia, Canada, Israel, and the U.S. The authors estimated influenza vaccine effectiveness using the test-negative design, and adjusted for confounders.
A discharge diagnosis of "acute respiratory or febrile illness" was identified using ICD-9 and ICD-10 codes, and the authors noted that acute respiratory or febrile illness hospitalizations were included only if a clinician ordered real-time reverse transcription polymerase chain reaction (rRT-PCR) testing for influenza within 3 days prior to admission through the discharge date.
Overall, there were a little under 19,500 hospitalizations with an "acute respiratory or febrile illness" discharge diagnosis, and of these, only 6% had rRT-PCR influenza virus testing.
Thompson and co-authors noted that most acute respiratory or febrile illness hospitalizations were among women younger than 35, two-thirds were in their third trimester, and two-thirds had no high-risk medical conditions. Acute respiratory or febrile illness was the primary discharge diagnosis for around half of the hospitalizations, the authors wrote.
Among these hospitalizations, about 60% were positive for influenza virus infection. The authors found that compared with influenza-negative pregnant women hospitalized with acute respiratory or febrile illness hospitalizations, influenza-positive pregnant women were more likely to be in their third trimester and less likely to have a high-risk medical condition.
The researchers also found an influenza vaccine effectiveness rate of 55% among women who were hospitalized in their first or second trimester. This could contribute to discussions about the benefits of early maternal vaccination, particularly in the first trimester, the investigators noted.
The authors also said that the findings were potentially relevant to "several public health policy and research debates," including the lack of evidence for influenza vaccine effectiveness against severe laboratory-confirmed influenza during pregnancy, which has been "described as an obstacle to the expansion of maternal influenza vaccination programs" in low- and middle-income countries.
The study was supported in part by the CDC. Thompson disclosed no conflicts of interest; other co-authors disclosed support from Johnson & Johnson (Jannsen Pharmaceuticals), Pfizer, MedImmune/AstraZeneca, Merck, GlaxoSmithKline, Sanofi Pasteur, Protein Science, Novartis (now GSK), and Dynavax.