Researchers led by Saad Omer of Emory University in Atlanta, Georgia, found mothers who were vaccinated against influenza were less likely to have premature babies.
Compared with the newborns of unvaccinated mothers, these babies also had a lower risk of abnormally low birthweight when influenza was widespread in the population.
The observational study followed 4,168 mother-baby pairs. Researchers tracked birthweight and whether a child was born prematurely.
Babies born during the influenza season to vaccinated mothers had a 40 per cent reduced risk of premature birth.
The protective effect of vaccination increased as influenza outbreak severity rose, peaking at an 80 per cent reduced risk when a child was born during a widespread national outbreak.