The pneumococcal vaccine has reduced cases in children
A strain of bacteria which can cause pneumonia and meningitis in children is on the rise in England and Wales, figures suggest.
The introduction of a vaccine against pneumococcal disease in 2006 has dramatically cut the number of infections in children.
But cases caused by a pneumococcal type not covered by the vaccine seem to be increasing, say researchers.
Similar patterns have been seen in the US and new vaccines are in development.
The pneumococcal vaccine given to infants at two and four months with a booster dose at 13 months of age protects against seven types of Streptococcus pneumoniae.
After the introduction of Hib and meningitis C vaccines, pneumococcal infection became one of the most common causes of invasive bacterial infection in children and it can be fatal.
Figures from the Health Protection Agency show there has been a huge fall in the number of children suffering serious illness as a result of pneumococcal infection since immunisation began.
At the end of 2007, government officials predicted 300 children had avoided invasive pneumococcal disease, which includes pneumonia, septicaemia and meningitis.
But surveillance data presented at the Royal College of Paediatrics and Child Health annual scientific conference also shows serotype 1 pneumococcal may be coming in to "fill the gap".
Dr David Spencer, consultant respiratory paediatrician in Newcastle, has been monitoring cases of empyema in children - a complication of pneumonia which involves the cavity around the lungs.
He found that in most cases the disease is caused by serotype 1.
And he predicts there are probably about 1,000 cases a year in the UK now compared with a handful in the early 1990s.
"Serotype one is continuing to increase and the likely factors are better recording but also that there has been a genuine increase because it's not covered by the vaccine.
"Overall there have been very dramatic benefits from vaccination but we're dealing with constantly shifting sands and surveillance will help us plan for the future."
There are two vaccines in the pipeline which protect against serotype 1 as well as others and it is hoped they will be available within a couple of years.
A spokesperson for the HPA said experience from countries such as the US showed you would expect to see some non-vaccine strains of pneumococcal becoming more common.
"Currently there are more than 90 known pneumococcal types and vaccination protects against the seven most common types which circulate in the UK.
"Serotype 1 was increasing prior to introduction of a pneumococcal conjugate vaccine so it is too early to tell if this trend has been exacerbated by the introduction of the vaccine."
Professor Adam Finn, head of the Bristol Children's Vaccine Centre said the increase in serotype 1 may be a coincidence and not related to the introduction of the vaccine.
"In the US they're definitely seeing higher rates of invasive disease caused by non-vaccine types.
"So everyone's been expecting that to happen.
"You can pretty much guarantee that once one of the new broader vaccines becomes available we will switch to it."