The nasal flu vaccine that is often given to children causes a temporary rise in the number of pneumonia-causing bacteria in their noses, which they sometimes pass on to members of their household, a study has found. But this may not actually lead to more pneumonia cases among their relatives.
In fact, given that bacterial pneumonia in older people is often triggered by a bout of flu, vaccinating children could cause fewer pneumonia cases over the flu season, as they would catch the infection less often and so be less likely to pass it on.
The intranasal flu vaccine contains a live, weakened form of the virus, which multiplies inside the nose for up to two weeks, often causing cold symptoms. This is in contrast to the injected form of the flu vaccine, which contains virus that is completely inactivated and doesn’t reproduce.
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The immune response to the weakened live flu vaccine seems to cut the ability of the immune system to keep bacteria inside the nose in check.
One cause of bacterial pneumonia is a microbe called Streptococcus pneumoniae, which is often carried harmlessly in people’s noses, especially in young children. It is only when these bacteria spread beyond the nasal passages to the lungs that they cause pneumonia.
It was already known that if children get flu, or receive the intranasal vaccine, the pneumonia bacteria temporarily increase in their nose.
In the latest research, at the University of Bristol in the UK and her colleagues studied about 400 2-year-old children and their families for two months after they received the nasal flu vaccine.
Every two weeks, the researchers took nasal swabs from the children and about two household contacts per child. In half the cases, immunisation was carried out two weeks later so that families where the vaccine had already been given could be compared with those who hadn’t yet had it.
As expected, the team found that children who received the nasal spray were twice as likely to have a high density of pneumonia-causing bacteria two weeks after vaccination as those who hadn’t had it yet.
The new finding was that the vaccinated children’s household contacts were also 80 per cent more likely to have a high density of pneumonia bacteria, presumably because the children passed it on. This effect had largely gone by two weeks after the vaccine was given.
It is unclear how this affects pneumonia illness levels in the population, the researchers write in their paper. However, “given the well-accepted importance of children in the dynamics of [pneumonia bacteria] transmission across all age groups, this question deserves further attention”, they write.
at the Liverpool School of Tropical Medicine in the UK says the findings suggest that children with flu are even more likely than recently vaccinated children to pass on pneumonia bacteria to their families.
“If they encounter a flu virus, the effect could very likely be much more pronounced than what we’re seeing with the attenuated virus of the vaccine,” she says, as the vaccine contains just a weakened version of the virus.
Some older people may wish to avoid close contact with children who have had the flu vaccine in the past two weeks, says at the Bill & Melinda Gates Foundation, which funded the research.
But at the University of Birmingham in the UK says it is too early for the findings to start changing health advice. “This is a really interesting study and I think it’s an important first step,” he says. “But I don’t think it’s at the stage where you would issue any kind of health guideline on interaction with grandchildren after vaccination.”
MedRxiv
