Health professionals do not understand mathematical models.

EDITOR—The science commentary on herd immunity that accompanied the article by Panagiotopoulos et al on an increase in the occurrence of congenital rubella after immunisation seemed irrelevant.1 In this letter we attempt to explain the relevant issues. Immunising a proportion of the population reduces the risk of infection (not necessarily disease) among those who are not immunised. This indirect protection from infection is termed herd immunity. It can be manifested in two different ways. Firstly, if the level of vaccine coverage is high …