# Meningococcal vaccines and herd immunity: lessons learned from serogroup C conjugate vaccination programs

Caroline L. Trotter; Martin CJ Maiden

Disclosures

Expert Rev Vaccines. 2009;8(7):851-861.

### Herd Immunity

In its simplest sense, 'herd immunity' refers to the prevalence of immune individuals in a population, but the term is commonly used in a broader sense to relate to the concept that the presence of immune individuals in a population can indirectly protect those who are not immune against infection[5,6]. The rate of transmission of an infection depends upon the characteristics of the infectious agent, the frequency and patterns of contact between hosts, and the proportion of individuals who are susceptible to infection in the host population. Transmission will be highest in a fully susceptible population. The basic reproduction number (R0) describes the number of secondary infections resulting from a single infection introduced into a fully susceptible population, and is an intrinsic measure of transmissibility. If immunological experience, as a consequence of vaccination or natural exposure, induces some degree of immunity against infection, then ongoing chains of transmission of the infection can be interrupted. The net reproduction number (Rn) describes the actual number of transmissions per infection, and is equivalent to the basic reproduction number R0 multiplied by the proportion susceptible in the population. In order for the infection to persist in the population, each infected individual must in turn infect at least one other person (i.e., Rn must be ≥1). If the proportion of the population who are immune is sufficiently high (i.e., >1-1/R0) then Rn will fall below 1 and the infection incidence will decline; this is known as the herd immunity threshold. A substantial body of mathematical theory underpins and extends these ideas (e.g., to cover more credible scenarios where populations are not homogeneous and do not mix at random), as reviewed in detail elsewhere[5,6,7].

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