Flu shots in pregnancy protect babies from being born too soon, Canadian studies show
Canadian studies show that pregnant women who are vaccinated against the flu are less likely to deliver premature or low-birth-weight babies.
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Pregnant women who are vaccinated against the flu are significantly less likely to deliver premature or low-birth-weight babies compared to unvaccinated expectant mothers, new Canadian research finds.
Based on more than 12,000 women in Nova Scotia who gave birth in the immediate aftermath of the H1N1 flu pandemic, the study adds to mounting evidence that the flu can have “really detrimental effects for both mothers and their babies,” said first author Alexandra Legge, a fourth-year medical student at Dalhousie University in Halifax.
Yet flu shots remain a notoriously tough sell among pregnant women: of the 12,233 women who gave birth to a live-born or stillborn infant between November 2010 and March 2012, a “disappointingly low” 16 per cent received the flu vaccine during their pregnancy, the researchers said.
As women get closer to their due dates, their immune systems change, making them more vulnerable to serious illness from flu and other infections. That can put stress on the fetus. An earlier study from Nova Scotia showed that women who are admitted to hospital with respiratory illnesses during flu season while pregnant are more likely to deliver babies that are small for their gestational age or have a low birth weight.
The new findings suggest that flu shots help prevent pregnant women from going into premature labour.
Urinary tract infections are known to be triggers for pre-term labour, Legge said. The same may hold true for more widespread infections.
The theory is that infection with influenza viruses increase the production of cytokines — chemicals secreted by immune cells. These elevated levels of circulating cytokines, in turn, increase the production of prostaglandins, naturally occurring hormones that cause the muscles in the uterus to contract “and that are widely known to play a key role in the initiation of labour,” Legge said.
For their study, the researchers looked at all women who delivered an infant at any hospital in the province of Nova Scotia during the two flu seasons immediately following the 2009 H1N1 pandemic.
Overall, the odds of preterm birth (defined as deliveries at less than 37 weeks’ gestation) and lower-birth-weight infants were lower among the babies of vaccinated women.
Babies born prematurely are at higher risk of respiratory and heart abnormalities, intracranial hemorrhage (bleeding within the brain) and infection, all of which increase the risk of death.
Low-birth-weight babies are also at higher risk of dying in infancy than normal weight babies.
Women who lived in rural areas, as well as those with underlying health problems, were more likely to be vaccinated against the flu than single women, women with more than one child and those who smoked during their pregnancy, the researchers found.
Given mounting evidence of the benefits, “both Canadian and World Health Organization guidelines now recommend routine seasonal influenza vaccination of all pregnant women in any trimester,” Legge and her co-authors write in this week’s edition of the Canadian Medical Association Journal.
But while vaccination rates in pregnancy increased during H1N1 (government data suggests that 64 per cent of pregnant women in Nova Scotia alone received the H1N1 vaccine during the pandemic, the authors write) experts worry that it hasn’t translated into higher rates of flu vaccination since.
Many expectant women worry about the safety of flu shots, even though the vaccine, which is made of inactive or killed influenza virus particles, is considered safe in pregnancy, Legge said.
“Another reason I think is that a lot of pregnant women aren’t aware of the risks of influenza itself in pregnancy,” she said. “If there was more awareness of the evidence we have for the potential consequences, maybe pregnant women would be more willing to accept the vaccine.”
Doctors and other pre-natal care providers also need to do a better job of recommending, and offering the annual flu shot to pregnant women during routine pre-natal visits, the authors said.