Is the basic reproductive number (R(0)) for measles viruses observed in recent outbreaks lower than in the pre-vaccination era?

To the editor: In their recent article on the large outbreak of measles in Merseyside, England, Vivancos et al. [1] obtained a basic reproductive number R0 of 1.2 in week 3 after the start of the outbreak. This result could suggest that measles viruses are less infectious in recent outbreaks than in the pre-vaccination era, when the basic reproductive number R0 ranged from 11 to 18 [2]. The basic reproductive number obtained in the study is however the effective basic reproductive number.