Buy one, get one free: Vaccinate the mother, protect the child

When is the best time to vaccinate pregnant mothers to protect their babies against influenza?

Nov 21, 2018
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Flu vaccine the best way to protect

Infection with influenza virus, the causative agent of flu, is particularly severe in pregnant women and newborn children. If they do get infected, they are far more likely to get severe disease leading to hospitalisation. For the pregnant mother, the simplest approach to avoid this is to get the flu vaccine as soon as it becomes available. However, these vaccines are not licensed for children under 6 months of age – leading us to ask the question, how do we protect newborn children against influenza infection?

Maternal vaccination, protecting both mother and child

Luckily, the body has already come up with a solution. During pregnancy, mothers pass on immunity to their children. This passive protection is transferred in the form of antibodies, which are proteins made by the immune system that are highly specific for the molecules that make up the coats of viruses and can prevent the viruses from infecting our cells. This antibody transfer occurs in the second and third trimester of pregnancy and has evolved so that the newborn child has some early protection against whatever infections the mother has been exposed to. We can utilise this system with maternal vaccination. If we vaccinate the mother, she will make antibodies that recognise the virus in the vaccine and some of these antibodies will pass from her to her baby. This maternal immunisation approach has been seen to be very effective in reducing the burden of infection with tetanus and pertussis (whooping cough) in babies. Maternal immunisation has also been recommended as a method of reducing influenza infection in babies since 2005.

When is the best time to vaccinate?

One important question is when is the best point during pregnancy to vaccinate the mother to ensure the maximum transfer of antibody to the baby. It was originally thought that early in the third trimester (weeks 25-36 of pregnancy) was best as this was the peak of antibody transfer, but recent studies investigating pertussis vaccination of mothers saw higher levels in babies if the mothers were vaccinated in the second trimester (weeks 13-24 of pregnancy). We wanted to explore the best time to immunise mothers with influenza vaccine. In our latest paper, we measured the level of influenza virus specific antibodies in both mothers and babies at the time of birth. We compared babies born to mothers who were vaccinated in the first, second or third trimesters with babies born to unvaccinated mothers. We saw that there was significantly more influenza specific antibody in babies born to vaccinated mothers than in those born to unvaccinated mothers – demonstrating that maternal flu vaccination is highly effective at boosting the protection against influenza infection in the baby. We then investigated timing and observed that the high levels of antibody were seen in children born to mothers vaccinated in either the second or the third trimester, suggesting that either timepoint was equivalent, though there was less antibody transferred if the gap between vaccination and birth was less than four weeks.

Flu the ever changing

However, there is a complication with influenza virus; unlike the other pathogens for which maternal immunisation is recommended – pertussis and tetanus, the influenza virus changes. These changes in virus necessitate a new flu vaccine each year to match the viruses that are circulating. Flu is also seasonal – you are much more likely to get flu in winter months (in temperate climates). This seasonality had an effect on the levels of immune protection in our study: children born during the flu season had higher levels of antibody than those born outside it. 

The time is now

When we put the seasonality of influenza together with the best time to vaccinate mothers to pass antibody to children, we see that the current practice of offering flu vaccine to mothers as soon as it becomes available gives the best balance of protection to both mothers and their babies at the times when they need it most. This is because the flu season is 6 months long and pregnancy is nine months long. Whilst immunising mothers in the first trimester does not pass on the most antibody to the baby, immunising the mother at the start of the flu season gives the mother maximal protection for the whole flu season and they will give birth outside the flu season, so the baby requires less protection. Mothers who are in the second or third trimester at the start of the flu season will benefit from the protection of the vaccine themselves and pass antibody protection to their baby.

Therefore our study supports the current practice of offering influenza vaccine to mothers as soon as it becomes available.

John Tregoning

Senior Lecturer, Imperial College London

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