Role of herd immunity in determining the effect of vaccines against sexually transmitted disease.

Vaccination programs provide both direct protection to those immunized and herd immunity, which is indirect protection of those who remain susceptible, owing to a reduced prevalence of infections. The well-understood impact of vaccination against ubiquitous childhood infections is compared with that of vaccination against sexually transmitted infections (STIs), and theoretical insights are derived from a review of mathematical modeling studies. Typically, a large fraction of cases of STIs are acquired by those with modest risk, and these cases could be prevented by low-efficacy vaccines. If coverage is good, vaccination of only one sex can protect the other sex. Candidate vaccines against human papillomavirus (HPV) and genital herpes are in the final stages of testing. The former is likely to be highly efficacious for a limited number of disease-causing HPV types, and the latter has provided protection against disease in women who initially were seronegative for both herpes simplex virus (HSV) type 1 and HSV-2, with 73% efficacy. In models, this vaccine had a substantial impact when infectiousness was assumed to be reduced along with incidence of disease. With such vaccines on the horizon, the requirements for vaccine delivery need to be considered, particularly who should be vaccinated and at what age.