When a woman is pregnant, her immune system is dampened to protect the unborn foetus. This is because the baby is recognised as a foreign being by the mother's immune system - the baby is genetically different to that of the mother.

Scientists at the Heinrich Pette Institute in Hamburg have shown in mice that the mother's suppressed immune system provides a unique opportunity for the influenza virus not only to infect the mother but to evolve into a more powerful strain.

"Pregnant women become more susceptible to influenza than non-pregnant women [because they] do not have an adapted immune system," said co-lead author Gülsah Gabriel. "Influenza seems to be particularly pathogenic for pregnant women because . . . the virus uses this opportunity of a 'weakened immune system' and continues to mutate."

There have been many studies to show that pregnant women suffer more readily from influenza than non-pregnant women. Statistics from the World Health Organization in 2010 highlighted that pregnant women were 4-5 times more likely to develop severe disease from influenza A, also known as influenza H1N1. Compared to non-infected pregnant women, pregnant women with influenza are more likely to develop complications such as adverse pregnancy outcomes like spontaneous abortion, preterm birth and fetal distress.

Some previous studies using pregnant mice have used fetuses that were genetically identical to the mother. This does not properly replicate what is seen in humans. Professor Gabriel and colleagues used mice with fetuses that were genetically different to the mothers to more fully replicate what is seen in humans. The results of the study showed that the immune system is more suppressed when the baby is genetically different to the mother.

The normal immune system is divided into two parts, the innate immune system and the adaptive immune system. The innate system is the first line of defense, it's the basic protection found in our bodies. The adaptive immune system is one that each body learns - it is created on the individual's response to exposure to a foreign substance. In the case of influenza, the innate immune system attacks viral infection with different types of cells, which release inflammatory factors called cytokines to help stop the spread of infection.

To understand immune suppression in the pregnant mice, the researchers examined gene expression patterns in these immune cells during infection. They found that the genes responsible for releasing the cytokines were suppressed which led to a weaker initial response to infection. In addition to the innate immune system being dampened, the researchers also found that cells associated with the adaptive immune system response are also dampened. Due to this dampening of the immune system the virus can mutate and produce a range of variants, some of which are likely to cause a severe infection.

This study highlights the need for an improved need in the education for pregnant women on the importance of vaccination against influenza to better protect themselves and their baby. This would also help prevent the development of more virulent strains of influenza, which could affect other vulnerable people.

"We hope that increasing evidence will allow the improvement of the currently poor vaccine uptake in pregnant women to protect the mother and the unborn," commented Gabriel.

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