Yet researchers at Dalhousie University in Halifax, where the study was conducted, say vaccination rates among pregnant women remain "disappointingly low."
Their study, published in Monday's issue of the Canadian Medical Association Journal, points out that during the 2009 H1N1 influenza pandemic, 64 per cent of pregnant women in Nova Scotia rolled up their sleeves for the flu shot.
Since then, the annual inoculation rate for that segment of the province's population has averaged about 16 per cent, despite public health recommendations that women get vaccinated, no matter which trimester of pregnancy they are in.
"We found among pregnant women in our cohort that women who were vaccinated against influenza during their pregnancy were less likely to have a pre-term or low-birth-weight infant, compared to women who did not receive the influenza vaccine while pregnant," said lead researcher Alexandra Legge.
"And we're certainly not the first study to show a protective effect of the influenza vaccine," Legge said Monday from Lunenburg, N.S.
The study used a database showing birth outcomes for more than 12,000 pregnant women between November 2010 and March 2012, of whom almost 1,960 had received a flu shot. The analysis found that women who were vaccinated against flu had a 25 per cent lower risk of having a premature or underweight newborn compared to unvaccinated women.
Researchers found pregnant women living in rural areas and those with underlying medical conditions were more likely to be vaccinated, while single women, those who had previously given birth and those who smoked during pregnancy were less likely to get the shot.
Legge, a fourth-year medical student who co-authored the study with a perinatal epidemiologist and several senior physicians, said there are a number of reasons why pregnant women choose not to get inoculated against flu.
Some are worried about deleterious effects of the vaccine on their developing fetus, she said. But Canada's National Advisory Committee on Immunization, or NACI, says studies have shown no evidence of harm to either mother or fetus from immunization.
"I think that lack of knowledge of the dangers of influenza illness in pregnancy is definitely another barrier to vaccination," said Legge, noting there is scientific evidence showing that getting the flu while pregnant can have adverse effects on both mother and fetus.
"Canadian studies have shown that pregnant women are at higher risk for serious influenza-related morbidity and mortality (illness and death) compared to non-pregnant women of a similar age and health status."
Given the ramp-up in flu activity in various regions across the country, Dr. Jeff Kwong called the study both relevant and timely.
During the 2009 H1N1 pandemic, a significant number of pregnant women were hospitalized after contracting the flu, said Kwong, a Toronto family physician and scientist with the Institute for Clinical Evaluative Sciences and Public Health Ontario.
"And it's possible we will see it again this year," he said, encouraging women who are pregnant to protect themselves by getting vaccinated.
Kwong also pointed out that the effects of being born prematurely are nothing to sneeze at: preemies may be born with lung, heart or other health problems and need to stay in hospital longer than full-term infants. They may also be at risk for potentially life-long complications.
"Kids not being born prematurely is definitely a benefit," he said.
The risk of pre-term or low-birth-weight newborns from maternal flu infection could be related to the virus inducing an inflammatory response in the body, which in turn causes the production of prostaglandins —hormone-like fatty acids that play a key role in the initiation of labour.
"And so the theory would be that perhaps the vaccine is exerting a protective effect on the fetus by averting infection and therefore avoiding the systemic inflammatory response," she said.
In the study, low birth weight and prematurity went hand in hand. The proportion of full-term babies that were underweight was similar for both mothers who got the flu shot and those who didn't.
"So this suggests that the greater likelihood of low birth weight among unvaccinated mothers is actually driven predominantly by the greater likelihood of pre-term birth."
Legge said the study underscores the need for all pregnant women to get the seasonal flu shot, in keeping with Canadian and World Health Organization guidelines.
"I think our focus needs to be on better elucidating what the reasons are for low vaccination uptake and to then look at strategies for improving that uptake."
That could include targeting family doctors, obstetricians and midwives, as the authors found that pregnant women often will agree to vaccination when it's recommended by their health-care provider.