A new vaccine that has proved highly successful in protecting children against seven of the worst strains of pneumococcal bacteria appears also, unfortunately, to be allowing new members of this disease-causing clan to surge to the fore and infect children.
'…I think the writing is on the wall that there is likely to be a slow shift in other areas like Canada towards these replacement strains.'—Dr. David Scheifele, Canadian vaccine expert
Two new studies from Alaska and from Spain suggest that serotypes — or strains of pneumococcal bacteria not covered in the vaccine (sold as Prevnar)— are stepping into the void left by those the vaccine protects against.
"I think we knew there was that potential for that to occur, but there was no way to predict if it would occur or to what extent," Dr. Rosalyn Singleton, the lead author of the first study, said from Anchorage, Alaska.
Singleton's study was done on native children in Alaska, a group particularly prone to infection with pneumococcal bacteria, which can cause illnesses ranging from ear and sinus infections to serious bloodstream infections, meningitis and pneumonia.
But she believes what she and her co-authors are seeing in Alaska has the potential to spread elsewhere.
"This is a phenomenon which may increase and be seen in other populations," said Singleton, a pediatrician and immunization consultant with the Alaska Native Tribal Health Consortium in Anchorage and a visiting researcher with the U.S. Centers for Disease Control.
It had been known that this phenomenon, called replacement disease, can occur with the widespread adoption of a new vaccine that targets only some strains of a pathogen. In fact, public health authorities will be looking to see if uptake of the new human papillomavirus or HPV vaccines produces this effect.
But replacement disease isn't inevitable. So public health officials always hope it won't rear its head and undermine the impact of new and often expensive vaccines.
These new studies— Singleton's appears in Wednesday's edition of the Journal of the American Medical Association, the Spanish study is slated for the June issue of Clinical Infectious Diseases— suggest that within a couple of years of Prevnar hitting the market, there was a renewed surge in disease caused by pneumococcal serotypes not included in the vaccine.
Upswing of cases in U.S. not yet seen in Canada
In Alaska, rates of invasive or serious pneumococcal disease among native children went from 403 per 100,000 children a year before the vaccine to 134 per 100,000 after the vaccine was introduced in 2001.
But that figure rose again to 245 per 100,000 by 2004-2006, with the bulk of cases— 229 per 100,000— due to pneumococcal strains not included in the vaccine, the Singleton article reports.
The Spanish study, by researchers at the University of Navarra, also showed an upswing of cases caused by non-vaccine strains after immunization with Prevnar began.
Canadian vaccine expert Dr. David Scheifele said researchers in Calgary and Vancouver are studying what strains of pneumococcal bacteria are causing infections in those cities, looking for this type of shift here.
Prevnar is provided free of charge by all provinces and territories in Canada.
But most jurisdictions only started offering the vaccine in 2005, so an upswing of disease caused by non-vaccine strains wouldn't be expected for awhile in Canada, he said.
"I think it's unlikely to happen to the same extent that it's happened in Alaska," Scheifele said, referring to the fact that native children there have three times more pneumococcal infections than non-native children.
"But I think the writing is on the wall that there is likely to be a slow shift in other areas like Canada towards these replacement strains."
Despite this disappointing development, Singleton and others were quick to point out that rates of pneumococcal disease in children and adultshave dropped dramatically since the vaccine went into widespread use.
And fewer sick kids have meant fewer prescriptions for antibiotics, a development that has led to a decline in rates of antibiotic resistance in pneumococcal bacteria.
"The vaccine is highly effective and it remains effective," saidDr. Katherine Poehling, a vaccine expert at Wake Forest University Baptist Medical Center in Winston-Salem, N.C., who wrote a commentary on the Alaska study.
"We're identifying changes but these children are still receiving great benefits from the vaccine… and so it gives us the opportunity to intervene while they maintain benefits," she said in an interview.
But Dr. Allison McGeer, an infectious disease expert at Toronto's Mount Sinai Hospital, said the findings point to the need for a vaccine that protects broadly against the pneumococcal family— saying a gradual increase in strains in the vaccine could drive the price of this vaccine beyond the capacity of publicly funded programs.