TORONTO - Pregnant women in Ontario who received a flu shot during the 2009 H1N1 pandemic were less likely to give birth to a very preterm baby or to lose their baby shortly after birth, a new study shows.

They were also less likely to give birth to a baby who was born small for its gestational age, said the study, published in the American Journal of Public Health.

The study suggests — but cannot prove — that flu shots had a beneficial impact for fetuses as well as for the women who carried them during the pandemic.

The study is based on records of 55,570 women who gave birth to a single child in Ontario between Nov. 2, 2009 and April 30, 2010.

It's been known for some time that pregnant women can experience severe illness if they contract seasonal flu while pregnant. And accounts from earlier pandemics suggest pregnant women were among those at highest risk during these rare outbreaks caused by new viruses. Despite that, progress has been slow in getting this group to accept flu shots.

But in 2009, early reports of severe and fatal illnesses in pregnant women, along with a big promotional push from public health authorities led to a high vaccination rate among pregnant women in Ontario. In fact, 23,340 women or 42 per cent of the women who gave birth to a single infant during the study period received an H1N1 flu shot during their pregnancy.

The researchers — from hospitals in Ottawa, Kingston and Toronto — looked to see if there were variations in the rates of problems in the births or the babies born to women who had flu shots as compared to the women who were not vaccinated against H1N1.

On some key points, they saw differences. For instance, there were 139 fetal deaths (a rate of 4.31 per 1,000 births) among babies born to women who didn't get a flu shot. Among those born to women who did get a flu shot, only 60 infants died (2.57 per 1,000 births).

Women who got the flu shot were 27 per cent less likely to give birth to a baby born very preterm, a classification used to describe babies born at less than 32 weeks gestational age.

But there was no statistical difference between the groups when all babies born preterm — less than 37 weeks gestational age — were considered.

There are limitations to the study. The authors acknowledge they have no way of knowing whether the women contracted influenza during their pregnancies, so they cannot say with certainty that the different rates of outcomes were because flu shots averted illness in the women who received them.

Nor can they be certain that there isn't something fundamentally different about the two groups that might explain the higher rates of good outcomes in the women who got flu shots.

It is known, for instance, that efforts to study flu shot effectiveness in seniors can be confounded by what's known as the "healthy vaccinee" effect. Seniors who get flu shots can tend to be more health conscious than those who don't.

The authors don't think a "healthy vaccinee" bias can explain their findings; they note most pregnant women are healthy. But they say they cannot discount the possibility that some similar type of bias — a factor that makes the two groups different from one another — might be at play here.

They also note that findings from one flu season — especially a pandemic year — cannot foretell what might happen in other years. Flu seasons differ year to year, they say. The risk for pregnant women might be higher in some years than others. As well, a flu shot covering viruses that are not well matched to those circulating might not offer as much protection to this group as seen in 2009, they suggest.