Research by scientists from the Institute of Environmental Science and Research backs the importance of influenza shots during pregnancy after finding pregnant women with the flu are three to five times more likely to end up in hospital.

ESR epidemiologist Namrata Prasad says her study using data from the institute's world-leading Shivers (Southern Hemisphere Influenza and Vaccine Effectiveness Research and Surveillance) project emphasised the importance of seasonal vaccinations among pregnant women.

Those findings were recently published in the Journal of Infectious Diseases, a leading global journal for original research on infectious diseases.

Prasad said although pregnant women are prioritised for seasonal flu vaccines, evidence on the risk of flu during pregnancy has been limited.

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Most of the evidence of flu risk during pregnancy comes from the 2009 influenza A(H1N1)pdm09 pandemic, and therefore may be of limited application for seasonal influenza recommendations.

Prasad said there was also limited information about outcomes of flu in pregnant women compared with non-pregnant women of reproductive age, making it difficult to accurately assess whether pregnancy was a risk factor for severe flu.

In the ESR-led study, individual-level national administrative datasets and Shivers study data were linked together to estimate the rates of flu-associated hospitalisation and outpatient visits among reproductive-age women.

"We found pregnant women with flu experienced rates of hospitalisation three to five times higher than non-pregnant women," Prasad said.

"That risk was present throughout the entire pregnancy."

Rates of hospitalisation were significantly higher for Māori women compared with women of European and other ethnicity. Rates for Pacific women were also increased, but were not as marked as for Māori women.

"Improving access and advice relating to influenza vaccination, particularly among Māori and Pacific pregnant women, may be useful in improving health outcomes among pregnant women and in reducing health inequalities," Prasad said.

"Our findings from a robust, active, population-based study emphasise the importance of seasonal flu vaccination."

The ESR-led international Shivers project won a team award in last year's Science New Zealand Awards.

The project resulted from a successful $9 million application to the United States department of Health and Human Services through the Centers for Disease Control and Prevention in 2011 for a funding over six years.