By Welcome Trust
Genetics has provided surprising insights into why vaccines used in both the UK and US to combat serious childhood infections can eventually fail. The study, published today in Nature Genetics, which investigates how bacteria change their disguise to evade the vaccines, has implications for how future vaccines can be made more effective.
Pneumococcus (Streptococcus pneumoniae) causes potentially life-threatening diseases including pneumonia and meningitis. Pneumococcal infections are thought to kill around a million young children worldwide each year, though the success of vaccination programmes has led to a dramatic fall in the number of cases in countries such as the UK and US. These vaccines recognise the bacteria by its polysaccharide, the material found on the outside of the bacterial cell. There are over ninety different kinds – or ‘serotypes’ – of the bacteria, each with a different polysaccharide coating.
In 2000, the US introduced a pneumococcal vaccine which targeted seven of the ninety serotypes. This ‘7-valent’ vaccine was extremely effective and had a dramatic effect on reducing disease amongst the age groups targeted. Remarkably, the vaccine has also prevented transmission from young children to adults, resulting in tens of thousands fewer cases of pneumococcal disease each year. The same vaccine was introduced in the UK in 2006 and was similarly successful.
In spite of the success of the vaccine programmes, some pneumococcal strains managed to continue to cause disease by camouflaging themselves from the vaccine. In research funded by the Wellcome Trust, scientists at the University of Oxford and at the Centers for Disease Control and Prevention in Atlanta studied what happened after the introduction of this vaccine in the US. They used the latest genomic techniques combined with epidemiology to understand how different serotypes of the pneumococcus bacteria evolve to replace those targeted by the initial vaccine.
The researchers found bacteria that had evaded the vaccine by swapping the region of the genome responsible for making the polysaccharide coating with the same region from a different serotype, not targeted by the vaccine. This effectively disguised the bacteria, making it invisible to the vaccine. This exchange of genome regions occurred during a process known as recombination, whereby one of the bacteria replaces a piece of its own DNA with a piece from another bacterial type.
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