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LONDON (Reuters) - Fighting pneumococcal disease with immunisations is like trying to hit a moving target because vaccines that zero in on certain strains also make space for new types to develop and dominate, Dutch scientists said on Tuesday.Experts say there are as many as 90 different types of Streptococcus pneumoniae bacteria, or pneumococcus, which can cause serious illnesses such as pneumonia and meningitis. Dutch researchers who studied children vaccinated with Pfizer's Prevnar 7 pneumococcal vaccine found they were more likely than unvaccinated children to develop a strain of the disease known as 19A, which is not covered by the shot.The research suggests that while vaccinating children against dramatically reduces the burden of pneumococcal disease caused by the strains targeted by the vaccine, constant surveillance is needed to check for new strains, or serotypes, which are likely to take the opportunity to fight through.Pneumococcal disease is one of the world's biggest killers of children, claiming up to 1.6 million lives a year. Around 95 percent of deaths are in Africa and Asia but pneumococcal infections also affect many thousands in developed countries.Pfizer has already developed a next generation pneumococcal shot called Prevnar 13, which was approved by U.S. regulators in February and targets 19A as well as 12 other strains. Rival drugmaker GlaxoSmithKline has a 10-strain vaccine called Synflorix, which has shown in lab tests it can also tackle 19A.OTHER STRAINS FIGHT THROUGHHowever, the researchers, whose study was published in the Journal of the American Medical Association (JAMA), said their work showed that immunising populations against pneumococcal disease is an almost constantly evolving process."It seems as though eradicating the seven types that are targeted by the vaccine makes room for new types," said Lieke Sanders of Utrecht's University Medical Centre, who conducted the study. "Overall the net benefit is still good and there is a substantial decrease in the amount of disease, but new types are coming up and one of the most prominent types is 19A."Wealthy countries have already started switching to the new generation of broad-protection pneumococcal shots, and a deal agreed in March by the Global Alliance for Vaccines and Immunisation paved the way for modern pneumococcal vaccines to be introduced in 47 developing nations by 2015. Data for the United States show that the introduction of Prevnar 7 there in 2000 led to a 99 percent drop in the number of pneumococcal infection cases in children under 5 years old by 2007, according to the U.S. Food and Drug Administration (FDA). However, serotype 19A is now the leading cause of invasive and respiratory pneumococcal disease in the United States and the one most frequently found in the noses and throats of patients."We have to be aware that this type of vaccine that eradicates the pneumococci types that are in it will make room for new pneumococci that may take over," Sanders said. "It is a constantly moving target."Sanders and colleagues analysed links between pneumococcal vaccination and acquisition of serotype 19A pneumococci in the nose and throats of 1,003 healthy newborn babies in the Netherlands during follow-up to the age of 24 months.They found that at 24 months and after having jabs at 2, 4, and 11 months, the cumulative proportion of children who had the serotype 19A was 16.2 percent. This compared with 9.2 percent of children in a comparison group who had not been vaccinated."Other serotypes with similar characteristics and disease potential (to 19A) may be the next in line to proliferate," the scientists wrote.