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Flu shot may prevent pre-term births

Pregnant woman

US study suggests women who have a flu jab are less likely to have a preterm or underweight baby.(Source: iStockphoto)

Pregnant women who get a flu jab are less likely to give birth to premature or small babies during the peak flu season, suggests a US study.

Researchers, led by Assistant Professor Saad Omer from Emory University Rollins School of Public Health, made the finding after analysing 4326 births in the US state of Georgia between mid-2004 and mid-2006.

According to the report in PLOS Medicine, the odds of a premature birth during the eight-week period of widespread influenza activity were around 70 per cent lower for vaccinated mothers compared to mothers who did not receive the influenza vaccine.

They also found that vaccinated mothers were 69 per cent less likely of giving birth to a baby in the lowest percentile weight range during the peak flu season.

"Our finding suggests that influenza vaccination may be a potential tool for reducing adverse birth outcomes, particularly prematurity," says Omer.

"Seasonal and pandemic influenza infection has significant adverse consequences for pregnant women and their newborns."

The researchers analysed data from the Georgia Pregnancy Risk Assessment Monitoring System (PRAMS) survey, which contains information such as maternal influenza vaccination during any part of the pregnancy, maternal attitudes, and birth certificate details.

They then compared the births of women who had been vaccinated to those who hadn't at different times of the influenza cycle to try to rule out confounding factors such as attitudes to vaccination, socioeconomic status and lifestyle.

Premature births were lowest during the pre-influenza season, when the vaccine was available but the virus was not widely circulating, and highest during the peak flu season. No differences in birth weight between vaccinated and unvaccinated women were seen outside the flu season.

"It is plausible that the impact of vaccines on birth outcomes would vary with the underlying influenza epidemiology and demographic characteristics," the researchers write.

But they admit the observational study is limited by a lack of data about how many pregnant women actually had influenza infections, and the precise time the women recalled receiving their vaccination.

"Studies in other populations, particularly randomised controlled trials, are needed to confirm our results," says Omer.

Reminder of vaccination impact

Professor Anne Kelso from the WHO Collaborating Center for Reference and Research on Influenza in Melbourne, says the study is a "reminder that something that people think of as trivial, like flu, could actually have a more significant impact than you would expect."

"Of course this work needs to be replicated … [but] looking at it superficially there's some quite strong evidence that influenza vaccination does protect babies from prematurity and being small at birth."

Kelso says that studies suggest that between 5 and 15 per cent of the population is exposed to the seasonal flu each winter, but pregnant women and young mums are more likely to be exposed than other groups to the virus.

She says this was highlighted during the 2009 flu pandemic when several pregnant women became seriously ill, delivered preterm or lost their babies.

"That was one of the strongest signals that we had that pregnant women should get vaccinated against influenza."

Associate Professor Peter Richmond from the School of Paediatrics and Child Health at the University of Western Australia agrees.

"This is another piece in the puzzle that highlights the importance of giving flu vaccination in pregnancy," says Richmond, who also heads the Vaccine Trial Group at the Telethon Institute for Child Health Research.

"Less people get infected during seasonal influenza than in a pandemic, but the risk is still high for mums in that second and third trimester during the influenza season."

He says the flu vaccine not only protects against infection, a known trigger for preterm delivery, but may also offer some protection for children under six months.

"I think we now have a number of pieces of evidence that flu vaccine works, it's safe and there's lots of evidence around the use of it in pregnancy," he says, adding that last year's vaccination scare was related to one brand.

Along with more research in the Australian context, Richmond says vaccination rates in pregnant women need to improve.

Australian guidelines recommend the flu vaccination is offered to women planning a pregnancy and those who will be in the second and third trimester during the peak flu season, including those in the first trimester at the time of vaccination.

Prior to the 2009 pandemic, around 13 per cent of pregnant women got a seasonal flu shot.

Early last year, pregnant women as well as Aboriginal women over 15 and women with chronic health conditions became eligible to receive free flu shots, but it is unknown how many women have taken up the offer.

The next Australian Vaccination Survey is due in 2012.
