Routine pneumococcal immunization programs in America have demonstrated phenomenal vaccine health impact
April 25, 2007 – JAMA. Researchers from Alaska, Rosalyn Singleton, Tom Hennessy and colleagues report on the pre- and post- seven-valent pneumococcal vaccine introduction health impact findings in Alaska based on population-based surveillance data that span twelve years (Jan 1 1995 – Dec 31, 2006). Policy decisions to introduce the multi-valent pneumococcal vaccine center around two key scientific issues: serotype coverage and replacement disease. The study underscores the importance of surveillance pre and post vaccine introduction to monitor vaccine impact and in particular replacement disease.
Dr. Tom Hennessy spoke with PneumoADIP.
The findings reported in this paper are based on population-based data that was in place before and after routine pneumococcal vaccine introduction in Alaska.
Soon after the introduction of pneumococcal vaccines in 2001, there was a dramatic decrease in vaccine-type case rates and in overall invasive pneumococcal disease (IPD) case rates in both Alaska Natives and other Alaskan populations. This excellent trend continued until 2003 when we started to see yearly increase in non-vaccine type case rates that was especially high in the youngest (<2 years old) Alaska Native sub-population compared to the rest of the Alaska population. The rise in non-vaccine type disease is a phenomenon known as replacement disease or serotype replacement and we are continuing to monitor this phenomenon.
To put into perspective, since vaccine introduction, the overall case rate for invasive pneumococcal disease remains 40% lower prior to vaccine introduction. In other words, there is evidence of replacement disease that is especially higher in <2 year old Alaska Native population; however, routine vaccination has reduced invasive pneumococcal disease dramatically in the Alaska population.
Replacement disease was certainly a concern and was expected after vaccine introduction, however the extent of replacement disease was not known prior to vaccine introduction – which is why post-introduction surveillance is so crucial to closely monitor this phenomenon.
The relatively higher rates of replacement disease are observed in three U.S. sub-populations: <2 year old Alaska Natives, Alaska Native adults, and HIV-infected.
However, these higher rates of replacement disease among the three U.S. sub-populations are not observed in other populations with routine vaccination.
Despite the rise of replacement disease in one segment of the population in Alaska, it is important to note that the overall impact of the vaccine has been very beneficial – preventing hundreds and hundreds of invasive pneumococcal disease and meningitis in the Alaska population. The overall benefit has been so tremendous both in preventing invasive pneumococcal disease and in reducing antibiotic resistance.
One way to look at this is…If we had to go back in time and stand at the edge of 2001 when we were making the decision whether or not to introduce the seven-valent vaccine – and even if we had this replacement disease data, which we actually did not have at 2001 – the decision to introduce the vaccine would be the same due to the overall health benefits.
Now at 2007, the situation for Alaska is to continue to use the seven-valent vaccine given the tremendous vaccine impact, but bearing in mind that there is a limitation due to replacement disease. The introduction of an extended vaccine with additional serotype coverage that will address this replacement disease would be the next decision when the extended vaccines are available.
From a scientific point of view, how a country would decide whether or not to introduce a particular vaccine would depend on the vaccine(s) available and the health impact that the vaccine(s) would have in that country.
The value of population-based laboratory surveillance with serotyping cannot be underestimated. This provides a first-hand look at the full extent of the pneumococcal disease over time and it is the best way to look at the vaccine health impact. This is a powerful tool to both epidemiologists and clinicians; it provides a first-hand feel of the impact of vaccines on a real time basis. Epidemiologists can monitor serotypes and case rates of pneumococcal diseases over time and in different populations and sub-populations. Clinicians can rely on antibiotic resistant trends and lab results for appropriate antibiotic treatment.
It is important to note that the seven-valent pneumococcal vaccine is still highly effective and that replacement disease is not a failure of the vaccine. Ongoing surveillance is in place to closely monitor the replacement disease and the extended conjugate vaccines with additional serotype coverage will be very welcome in the Alaska Native population.