Maternal Flu Vaccination and Effect on Flu Infection in Young Infants
A new study, published online at the Archives of Pediatrics & Adolescent Medicine, looks at the effects of mom’s flu vaccine on young infants.
Study Summary – The objective of this study was to assess the effect of seasonal influenza vaccination during pregnancy on laboratory-confirmed flu infections in infants up to 6 months of age. A total of 1160 mother-infant pairs were included in the study. The women gave birth during the regular flu season. Some of them received the flu vaccine, some didn’t. The assignment to either receive the flu vaccine or not was not random. The study authors looked at actual lab-confirmed influenza illnesses (ILI), and ILI hospitalization rates of the infants as the main outcomes. They compared ILI confirmed rates, and ILI hospitalization rates between the infants born to vaccinated mother and infants born to unvaccinated mothers.
Results – Infants born to vaccinated mothers were less likely than infants born to unvaccinated mothers to contract ILI. Specifically:
- 41% reduction in the risk of laboratory-confirmed influenza virus infection (relative risk, 0.59; 95% confidence interval, 0.37-0.93)
- 39% reduction in the risk of ILI hospitalization (relative risk, 0.61; 95% confidence interval, 0.45-0.84)
- Significantly higher hemagglutinin inhibition antibody levels at birth and at 2 to 3 months of age
Conclusion – Methodologically, the main design issue is that test subjects were not assigned randomly to either the vaccine or no-vaccine. Furthermore, it appears from the abstract at least, that the mothers in the no-vaccine group did nor receive a placebo shot. The implication is that they simply did not receive a shot and were aware of it, which would affect the blinding as well.
When taken together these two facts should lower our reliance on the results, although not negate it entirely. The differences of 41% and 39% are too big to be due simply to bias introduced by these design issues. So the conclusion should be that given the large sample size and the large differences between the two groups, this study is highly indicative that babies born to vaccinated mothers do receive a tangible benefit from the vaccine, but the actual reduction in infection and hospitalization rates may be a little less than the numbers reported in this study. These conclusions should be compared to other studies, hopefully studies that had better blinding and randomization.
The author’s own conclusion is as such;
Maternal influenza vaccination was significantly associated with reduced risk of influenza virus infection and hospitalization for an ILI up to 6 months of age and increased influenza antibody titers in infants through 2 to 3 months of age.