Vaccine herd effect

Abstract

Vaccination ideally protects susceptible populations at high risk for complications of the infection. However, vaccines for these subgroups do not always provide sufficient effectiveness. The herd effect or herd immunity is an attractive way to extend vaccine benefits beyond the directly targeted population. It refers to the indirect protection of unvaccinated persons, whereby an increase in the prevalence of immunity by the vaccine prevents circulation of infectious agents in susceptible populations. The herd effect has had a major impact in the eradication of smallpox, has reduced transmission of pertussis, and protects against influenza and pneumococcal disease. A high uptake of vaccines is generally needed for success. In this paper we aim to provide an update review on the herd effect, focusing on the clinical benefit, by reviewing data for specific vaccines.

Figure 1

Schematic presentation of the herd effect: The index patient transmits an infectious agent to a given number (basic reproductive number R₀) of susceptible persons in the community. Black circles are infected persons and white circles are uninfected susceptible persons (panel A). After a mass immunization programme in the community (panel B), a proportion of the population is immunized (grey circles with ‘I’), thus directly protected from the infectious agent (direct effect). The immunized individuals further protect the susceptible population (white circles with ‘H’) by stopping the transmission within the social networks. This extra protection effect provided by a vaccine is called ‘herd immunity’.

Figure 2

The vaccine herd effect on Haemophilus influenzae type b diseases in Finland after introduction of the Haemophilus influenzae type b conjugate vaccine in 1986 (adapted from Peltola et al. [5]).

Figure 3

The vaccine herd effect on invasive pneumococcal diseases in the USA after introduction of the 7-valent pneumococcal conjugate vaccine in children aged < 2 y in 2000 (adapted from Whitney et al. [21]).

Figure 4

The vaccine herd effect on influenza in a randomized control trial in Canada (adapted from Loeb et al. [32]).