Amesh A. Adalja, MD, FACP, FACEP, FIDSA, September 4, 2015
One of the major ancillary benefits of vaccination is that a well-vaccinated population will create ripple effects in the community, providing herd protection for those who are not vaccinated, who are unable to be vaccinated, or whose immunity has waned. This has been well demonstrated for the measles vaccine as well as other vaccines.
One such vaccine is the pneumococcal conjugate vaccine, introduced for routine pediatric use in its 4-dose 7-strain form (PCV7) in 2000. Soon after its introduction, it was noted that a remarkable decrease in invasive pneumococcal disease (IPD) caused by vaccine-included strains occurred not only in those who received the vaccine, but also in older adults, for whom IPD is a serious event.
Pingali et al, in a paper just published in the Journal of Infectious Diseases, explore the hypothesis that geographic gaps in pediatric vaccination with PCV7 would translate into higher rates of IPD in adults in those areas.
The research team employed a database of Connecticut’s IPD cases from 1998 to 2009 to determine the trajectory of infections. Overall, 5,838 cases of IPD occurred, with 2,313 being caused by PCV7-targeted serotypes.
As expected, after vaccine introduction (with 4-dose PCV7 compliance reaching 88.4%), IPD caused by PCV7-targeted serotypes declined in every age group. Uptake of 3 or 4 doses of PCV7 was lower in zip codes with predominantly black or Hispanic populations, a higher proportion of children under age 5, and higher population densities.
Areas with lower than average PCV7 vaccination rates accordingly had higher rates of IPD caused by vaccine serotypes in adults. A multivariate model revealed that the best predictor for an adult having IPD caused by a vaccine strain was having a population of children with a lower than average rate of complete PCV7 vaccination (ie, missed 4th dose). A 10% drop off in 4th dose receipt increased the odds of adult vaccine strain IPD by 54.3%.
The study provides concrete evidence of the phenomenon of herd immunity and emphasizes the myriad benefits that accrue from vaccination. The study also has several policy implications, including the fact that vaccine uptake—even in an era with Vaccines for Children (VFC) operating—can be suboptimal, and geographic clustering of under-immunized populations can be dangerous for both under-immunized children and adults.
Since 2010, PCV7 has been replaced with PCV13, an improved vaccine that confers protection against 6 more strains of pneumococcal disease. This vaccine is recommended not just for the pediatric population but, since 2014, for adults older than 65 years as well. However, coupling the work of Pingali et al with the strong indirect protective effect of PCV vaccines, reducing rates of IPD in adults may best be approached by optimizing pediatric vaccination rates—a point strongly made by the authors.
Pingali SC, Warren JL, Mead AM, et al. Association between local pediatric vaccination rates and patterns of pneumococcal disease in adults. J Infect Dis 2015. http://jid.oxfordjournals.org/content/early/2015/08/26/infdis.jiv431.abstract. Accessed September 2, 2015.