DNA study reveals how bacteria evaded childhood vaccine

The researchers from the University of Oxford and the US Centers for Disease Control and Prevention (CDC) used the latest DNA sequencing techniques to investigate how pneumococcus bacteria evolved after the introduction of a childhood vaccine in 2000 in the USA.

Although an improved vaccine is now used in the USA and the UK, the scientists say their study provides insight into why vaccines to combat serious childhood infections can eventually fail.

The research was funded by the Wellcome Trust and is published in the journal Nature Genetics.

Infections with pneumococcus bacteria (Streptococcus pneumoniae) 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.

The vaccines recognise pneumococcus bacteria by their outer coat. There are over 90 different strains of the bacteria, each with a different coating.

In 2000, the US introduced a vaccine against pneumococcus which targeted 7 of the 90 different strains of the bacteria. This vaccine was extremely effective and had a dramatic effect on reducing disease, seeing 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 this success, some bacterial strains managed to continue to cause disease by evolving to camouflage themselves from the vaccine. 

The Oxford and CDC researchers showed that the bacteria had evaded the vaccine by swapping DNA with a different strain not targeted by the vaccine.

The DNA that was swapped included genes responsible for making the outside coat of the bacteria. Taking on the coat of a different strain effectively disguised the bacteria, making it invisible to the vaccine.

Dr Rory Bowden of the Department of Statistics at the University of Oxford explains: 'Imagine that each strain of the pneumococcus bacteria is a class of schoolchildren, all wearing the school uniform. If a boy steals from his corner shop, a policeman – in this case the vaccine – can easily identify which school he belongs to by looking at his uniform.

'But if the boy swaps his sweater with a friend from another school, the policemen will no longer be able to recognise him and he can escape. This is how the pneumococcus bacteria evade detection by the vaccine.'

Dr Bowden, along with colleagues from the Wellcome Trust Centre for Human Genetics at Oxford University, used DNA sequencing to identify a number of newly emerged strains that had managed to evade the vaccine.

One in particular grew in frequency and spread across the US from east to west over a period of several years.

The original vaccine first introduced in the US has now been replaced by an updated vaccine, which targets 13 different strains of the bacteria instead of 7. This includes the new strain that swapped DNA to escape the original vaccine. The updated vaccine was introduced in the UK in 2010.

'Childhood vaccines are very effective at reducing disease and death at a stage in our lives when we are susceptible to serious infections,' says Professor Derrick Crook of the Nuffield Department of Clinical Laboratory Sciences at the University of Oxford, who is also an infection control doctor at the Oxford University Hospitals NHS Trust.

He adds: 'Understanding what makes a vaccine successful and what can cause it to fail is important. We should now be able to understand better what happens when a pneumococcal vaccine is introduced into a new population. Our work suggests that current strategies for developing new vaccines are largely effective but may not have long term effects that are as successful as hoped.'

Dr Bernard Beall, a scientist at the US Centers for Disease Control and Prevention, said: 'The current vaccine strategy of targeting predominant pneumococcal serotypes is extremely effective, however our observations indicate that the organism will continue to adapt to this strategy with some measurable success.'