From Infectious Disease Special Edition

Why Do Vaccine-Preventable Disease Outbreaks Occur in the US?

James D. Cherry MD, MSc; Kathleen H. Harriman PhD, MPH

In This Article


Numerous outbreaks of vaccine-preventable diseases have occurred in the United States during the past several years. These outbreaks have occurred on national, regional, and local levels and have involved large numbers or only relatively small numbers of children, adolescents, and adults.

To gain understanding of the occurrence of vaccine-preventable diseases in the present era, 2 important concepts should be addressed: reproduction number and community (“herd”) immunity. Reproduction number (R 0) relates to the transmissibility of a particular pathogen and indicates the number of secondary cases that one infected person would produce in a completely susceptible population. Measles, for example, is an extremely infectious airborne disease with an estimated R 0 of 12 to 18.[1,2,3]

Community immunity describes a condition that occurs when a significant portion of a population (or herd) is immune—either through immunization or previous infection—and provides a measure of protection for individuals who are not immune (Figure 1.) It is directly related to the R 0 as well as the duration of protection following infection or vaccination. The level of protection conferred to individuals and, by proxy, society, following infection or vaccination varies based on the causative pathogen and the vaccine.

Figure 1.

The Relationship Between Community Immunity and Transmission of Infectious Diseases.