In a study of two new vaccines, “carriage rates” of meningococcal bacteria were reduced by up to 39%.
This was likely to curb person-to-person transmission of the bugs long after the vaccines ceased to be active in the blood.
Lead scientist Professor Robert Read, from the University of Southampton, said: “We have shown that vaccines modify the way the bacteria are carried, so even when the antibodies are no longer present in the blood, the carriage in the throat is still prevented, and so is onward transmission of the infection to others.
The research stems from a curious observation that followed the introduction of the meningitis C vaccine in the UK in 1999.
A year after babies were immunised they no longer had antibodies against the bacteria in their bloodstream, yet still seemed to be protected.
Scientists speculated that the reason might be because bacteria were being kept out of their normal reservoirs in the nose and throat.
“The effect on the population greatly outweighed what we saw from the antibody protection in the blood,” said Prof Read.
The latest study, published in The Lancet, focused on two new meningitis vaccines — MenACWY-CRM and 4CMenB — and involved almost 3,000 young people aged 18 to 24 at 10 centres across the UK.
One month after the vaccines were administered neither of them had any effect on carriage rates, the research showed. But over the course of a year, MenACWY-CRM lowered carriage rates by 39% while 4CMenB reduced them by 20% to 30%.
Prof Read said: “The standard practice is to vaccinate with the aim of inducing high levels of antibodies in the blood to protect against the disease, but we know these antibodies can disappear. This study is telling us the vaccines also have an effect on carriage in the throat and explains why they can be so effective across the population.”
How the vaccines lowered carriage rates is not clear.