The basic reproduction number, R nought (R0), is defined as the average number of
secondary cases of an infectious disease arising from a typical case in a totally
susceptible population, and can be estimated in populations if pre-existing immunity
can be accounted for in the calculation. R0 determines the herd immunity threshold
and therefore the immunisation coverage required to achieve elimination of an infectious
disease. As R0 increases, higher immunisation coverage is required to achieve herd
immunity. In July, 2010, a panel of experts convened by WHO concluded that measles
can and should be eradicated. Despite the existence of an effective vaccine, regions
have had varying success in measles control, in part because measles is one of the
most contagious infections. For measles, R0 is often cited to be 12-18, which means
that each person with measles would, on average, infect 12-18 other people in a totally
susceptible population. We did a systematic review to find studies reporting rigorous
estimates and determinants of measles R0. Studies were included if they were a primary
source of R0, addressed pre-existing immunity, and accounted for pre-existing immunity
in their calculation of R0. A search of key databases was done in January, 2015, and
repeated in November, 2016, and yielded 10 883 unique citations. After screening for
relevancy and quality, 18 studies met inclusion criteria, providing 58 R0 estimates.
We calculated median measles R0 values stratified by key covariates. We found that
R0 estimates vary more than the often cited range of 12-18. Our results highlight
the importance of countries calculating R0 using locally derived data or, if this
is not possible, using parameter estimates from similar settings. Additional data
and agreed review methods are needed to strengthen the evidence base for measles elimination
modelling.