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.
