MAY 15, 2007


Pneumococcus back with a vengeance

Vaccine leads to new strains in kids

A widely-used kids vaccine against pneumococcal bacteria has caused an upsurge in other strains of the bacteria, according to two independent studies in the US and Spain.

The 7-valent conjugate vaccine Prevnar, which had reduced the incidence of invasive pneumococcal diseases (IPD), including potentially life-threatening meningitis and pneumonia, has been given routinely to babies at two, four and 12 months in Canada since 2005.

The American study, which appears in the April 25 issue of JAMA, focused on the incidence of IPD among aboriginal Alaskan children who had received the vaccine. "Alaskan natives had triple the rate of pneumococcal disease compared to the rest of the US," says Dr Singleton, lead author of the report and member of the Center for Disease Control's bacterial surveillance team. The findings indicate these kids are experiencing IPDs from new serotypes of the bacteria not covered by the vaccine. The new strains are serious, but so far not life-threatening.

This phenomenon, known as replacement disease, is common and doesn't really come as a surprise to experts. "Pneumococcus is a bacteria that has the greatest disease replacement possibility," says Dr Caroline Quach, a pediatric infectious disease specialist at the Montreal Children's Hospital. Pneumococcus is responsible for most bacterial meningitis infections and can cause blood infections as well as ear infections; kids under two are at the highest risk.

Dr Singleton and her Active Bacterial Core Surveillance program have been monitoring the impact of the vaccine on overall IPD prevalence in children, as well as antibiotic resistance. "We looked specifically at pneumococcal serotypes to track what happened in children after vaccination," says Dr Singleton. "The strains covered by Prevnar were the ones that had the highest antibiotic resistance." The good news is Prevnar significantly decreased IPD incidence and antibiotic resistance, and even indirectly decreased IPD in adults by reducing contact infections, notes Dr Singleton.

Conjugate vaccines are effective against the most infectious, drug-resistant strains of Pneumococcus bacteria, but the sad truth is that a physician's choices are limited. "The 23-polysaccharide vaccine is available, but it is recommended only for a much older population," says Dr Quach. "Children under two years of age don't respond to it, and older kids respond but not as much as they do to the conjugate vaccine Prevnar," she adds.

Because Prevnar's only been used here since 2005, replacement disease has not been observed as much as in the US, where the vaccine has been used since 1998. However, while the phenomenon is not necessarily irreversible, you never know how bacteria will behave, Dr Quach warns. "An active and continuous surveillance is necessary."

Dr Singleton's study highlights the need to evaluate the emerging strains in order to target the development of new vaccines accordingly. Wyeth, which developed Prevnar, is currently working on a 13-valent vaccine. "The most important serotype it will cover is 19-A, which is currently causing IPDs," says Dr Singleton. "19-A is genetically speaking closely associated to 19-F, a serotype covered by Prevnar. We had hopes that Prevnar would cover it as well, but that was not the case, so a new vaccine would be welcome."

Good news for Dr Singleton: a new pneumococcal vaccine, this time by GlaxoSmithKline, should soon be on the market and will cover some of the emerging strains of the pathogen.

Dr Singleton hopes governments won't wait too long to add any new vaccines to the schedule. "The budget has to be taken into consideration, unfortunately," says Dr Quach. "We can make recommendations, but depending on the financial priorities, they may not be implemented."

In the meantime, infection control measures seem to be the only way to go. "Pediatricians should be aware that other strains can occur and they need to do culture tests," says Dr Singleton. "They should also monitor antibiotic administration and dosages." This should limit antibiotic prescription strictly to bacterial infections and avoid antibiotic resistance.



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