There is no association between exposure to the 2009 H1N1 "swine flu" vaccine during pregnancy and health problems in early childhood, concludes a study from Canada published by The BMJ today.

These results are reassuring and suggest the influenza vaccination during pregnancy is safe for both mother and offspring.

Pregnant women and their newborns are considered to be at high risk of serious illness during flu pandemics and seasonal epidemics. As such, many countries advise all pregnant women to have a flu vaccine, which can protect both mothers and their young infants from the flu.

But uptake is low, with safety concerns a common reason given for not being immunised, especially across Europe and in North America.

And although substantial evidence supports the safety of flu vaccination with respect to newborn health, few studies have assessed the health of older children who were exposed to flu vaccination in the womb.

So researchers based in Canada and the United States decided to evaluate the relation between 2009 pandemic H1N1 influenza vaccination during pregnancy and health outcomes in the children during the first five years of life.

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Using a provincial birth registry linked with health records, they identified all live born infants from November 2009 to October 2010 in Ontario, Canada and tracked the health of these children until 5 years of age.

Of 104,249 children, 31,295 (30%) were born to vaccinated mothers.

No elevated risk was found for cancer, infections, chronic diseases, hospital admissions or death in the children of vaccinated mothers.

One outcome, childhood gastrointestinal infections, was slightly lower in children born to vaccinated mothers and one other, childhood asthma, was slightly higher in children born to vaccinated mothers. But these associations were very small and the researchers say they cannot rule out the possibility that this may have been due to other unmeasured (confounding) factors that could not be fully accounted for in the analysis.

This is an observational study, and as such, can't establish cause. However, the results were largely unchanged after further analyses, and are consistent with results from other similar studies.

As such, the researchers say their results are reassuring and support the safety profile of 2009 pandemic H1N1 influenza vaccination during pregnancy.

Future studies in different settings and with different influenza vaccine formulations such as seasonal vaccines, "are important for developing the evidence base on longer term pediatric outcomes following influenza vaccination during pregnancy," they conclude.

The message is clear: influenza vaccination during pregnancy is, by all available evidence, safe for mother and offspring, say researchers in a linked editorial.

"Especially in this era of 'anti-vaxx' anxiety and misinformation, it is our duty to be clear: vaccination of pregnant women saves lives," they conclude.