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How does pandemic flu affect pregnant women and their babies?

Before the 2009 pandemic, it was already known that pregnant women are more susceptible to the complications of flu, such as pneumonia. NIHR research also showed that if women caught flu during the pandemic, their babies were at greater risk of being born prematurely or even stillborn.

The risk to unborn babies is greatest when mothers develop serious complications but treating mothers with antiviral drugs appears to reduce the chance of pneumonia. Therefore it makes sense to make them a priority for the prevention and treatment of flu.

The study made use of existing reliable data collection systems, including the UK Obstetric Surveillance System (UKOSS) and information systems in primary care and hospitals. The researchers were able to identify factors that made women more susceptible to serious illness and estimate the benefits of antiviral drugs. The study was not a ‘gold-standard’ randomised trial, but the results are likely to be reliable and are the best that could be achieved in the heat and urgency of a pandemic.

"It’s important to remember that there are some situations – and a flu pandemic is perhaps one of them – where it’s really difficult to do ‘gold standard’ randomised controlled trials. But we can learn a lot from simply collecting information and observing what happens to women who are treated or not treated, in this case with anti-viral drugs.”

Professor Marian Knight, Professor of Maternal and Child Population Health

Assessing risk

Pregnant women who get flu are more likely to need admission to hospital for complications if they smoke, have asthma, or are overweight or obese. In those women needing admission for flu, the risk of their baby dying was nearly four in a hundred whereas it was less than one percent in uninfected women.

Fortunately, there was evidence that antiviral drugs given in hospital can reduce the severity of the mothers’ illness. For instance, they reduced the chance of needing intensive care by 84 percent. While the numbers were too small to see a benefit to babies, it is possible that they too will benefit from their mothers being less unwell.

Changing policy

The emergence of these results during the pandemic was very important and enabled a change of policy to make antiviral drugs more easily available to pregnant women. This research strongly informs public health measures to make flu vaccine and drugs available to pregnant women in future pandemics. It also supports the need for effective public health measures to reduce smoking and obesity in this group as well as the population at large.

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