Swedish researchers report that the vaccine against the H1N1 "swine flu" strain of influenza doesn't seem to have a link to birth defects.
One obstetrician who reviewed the research said the findings should ease concerns women might have about the vaccine.
"This year's flu vaccine includes protection against an H1N1-like virus," noted Dr Jennifer Wu, an ob/gyn with Lenox Hill Hospital in New York City.
She pointed out that the H1N1 strain made headlines in 2009-2010 as "swine flu" reached pandemic levels in the United States.
But the new Swedish study "indicates that first trimester administration of H1N1 vaccine does not seem to increase congenital birth defects," Wu said.
Current recommendations from the US Centres for Disease Control and Prevention advise all pregnant women to receive a seasonal flu vaccine since they are especially vulnerable to complications from influenza.
The new study was led by Dr Jonas Ludvigsson of the Karolinska Institute in Stockholm. His team looked at the risk of birth defects – overall and in terms of congenital heart disease, cleft palate and limb abnormalities – in more than 40,000 children of mothers who were exposed to the H1N1 vaccine, Pandemrix.
Embryo especially vulnerable
The researchers compared these children to their siblings as well as to the general population.
Ludvigsson's team found no sign that maternal vaccination boosted the overall risk of birth defects in babies.
The study wasn't able to completely rule out a connection between the vaccine and any specific type of birth defect, however.
Wu pointed out that examining risks for vaccines given in the first trimester of pregnancy is important, since this is when the early organ development occurs "and the embryo is especially vulnerable" to agents that could cause a birth defect.
Dr Leonard Krilov is chair of paediatrics at Winthrop-University Hospital in Mineola, New York. He agreed with Wu that the study "provides evidence that there was no increased risk of congenital malformations in infants born to [vaccinated] pregnant mothers".
Krilov believes that the flu vaccine remains very safe.
"The bulk of the influenza vaccine supply is inactivated and purified," he explained. "You cannot get the flu from the vaccine and reactions are primarily to local pain at the site of injection and low-grade fever."
The new study was published in the Annals of Internal Medicine.