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A study published earlier this month in JAMA Network concluding maternal influenza vaccination during pregnancy was not significantly associated with an increased risk of adverse early childhood health outcomes failed to consider several key points.

The study, “Association of Maternal Influenza Vaccination During Pregnancy With Early Childhood Health Outcomes,” was based on a cohort of 28,255 children born in Nova Scotia between 2010 and 2014. About  36% of the children were exposed to the influenza vaccine during gestation.

Researchers followed the children during gestation and after live birth, for an average of 3.6 years — an inadequate length of time to provide any real assurance that the vaccine administered to the mother during pregnancy would have no negative impact on the child’s long-term health.

The study’s authors claim to have found no relationship between maternal influenza vaccine exposure and childhood diagnoses of asthma, upper and lower respiratory infections, gastrointestinal infections, ear infections, neoplasms, hearing or vision loss, or urgent and in-patient health services utilization.

Diagnoses considered in this study were based on emergency room visits and hospitalizations only — even though most diagnoses are made in other, out-patient settings such as practitioner office visits.

According to the Asthma Fact Sheet, 9.5% of all Canadians have asthma. Yet the study authors report an incidence of 3.0 diagnoses per 1,000 person years, which translates to 1.1 cases of asthma per 100 children in the study or 1.1%.

Other diagnoses in the study are similarly under-ascertained and reflect only a small fraction of the actual number of diagnoses in each group.

The researchers’ claim that there was no significant difference between the vaccinated and unvaccinated groups studied is simply not true. Adjusted rates of ear infections (otitis media) were reported as significantly higher in Table 2 of the paper.

Association between seasonal flu vaccine and pregnancy 1

In addition, when including marginally significant results, adjusted rates of lower respiratory infections and urgent and in-patient health services utilization were higher in the vaccinated group as well as crude rates of gastrointestinal infections and urgent and in-patient health services utilization.

When looking specifically at the trimester of vaccination, third-trimester vaccination was significantly associated with lower respiratory infection when compared to the unvaccinated group (see figure below). These results were summarily ignored by the study authors, despite the level of significance achieved.

Association between seasonal flu vaccine and pregnancy

The control diagnosis (nebulously defined as “all-cause injuries”) shows a significantly higher incidence in maternally vaccinated children versus unvaccinated children. The control diagnosis in epidemiology is used as a study “barometer” to test validity of results obtained and should not be significantly related to vaccination status.

This casts a shadow of doubt on any conclusion made from this study. It also raises concerns that physicians will use the study to provide false assurance of safety regarding the risks of administering flu vaccines during pregnancy.