02 August 2012
Is the basic reproductive number (R0) for measles viruses observed in recent outbreaks lower than in the pre-vaccination era?
- Public Health Agency of Catalonia, Barcelona, Spain
Citation style for this article: Plans Rubió P. Is the basic reproductive number (R0) for measles viruses observed in recent outbreaks lower than in the pre-vaccination era?. Euro Surveill. 2012;17(31):pii=20233. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20233
Date of submission:
26 July 2012
To the editor: In their recent article on the large outbreak of measles in Merseyside, England, Vivancos et al.  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 . The basic reproductive number obtained in the study is however the effective basic reproductive number.
The basic reproductive number R0 is the average number of individuals directly infected by one infectious case (secondary cases) during the entire infectious period, when the infectious agent has entered a totally susceptible population . The effective basic reproductive number R, on the other hand, is the reproductive number observed when of a part of the population is immunised (I) . In this situation, the reproductive number decreases from R0 to R=R0−R0I . Outbreaks can be interrupted when R=1.
The basic reproductive number R0 in the Merseyside outbreak can be determined from R0=R/(1-I), where I is the prevalence of protected individuals in the population. Assuming that prevalence of protected individuals was at least equal to 81–87% (85–92% vaccination coverage (V) x 95% vaccine effectiveness (VE)) the value of R0 necessary to generate the outbreak was 6.2–9.5, only slightly lower than in the pre-vaccination era. The lowest value is obtained taking into account a vaccination coverage of V=85% (two doses of measles-mumps-rubella (MMR) vaccine at five years) and vaccine effectiveness of E=95%: R0=R/(1-I)=R/(1-VE)=1.2/(1-0.8075)=6.2. The highest value is obtained taking into acount a vaccination coverage of 92% (first dose of MMR at 24 months) and 95% vaccine effectiveness: R0=1.2/(1-0.874)=9.5.
Measles is one of the most contagious infectious diseases, and outbreaks can only be prevented by means of achieving a high vaccination coverage. For a R0=11–18, the vaccination coverage required to prevent measles outbreaks is 96–99% .
- Vivancos R, Keenan A, Farmer S, Atkinson J, Coffey E, Dardamissis E et al. An ongoing large outbreak of measles in Merseyside, England, January to June 2012. Eurosurveillance, Volume 17, Issue 29, 19 July 2012. Available from: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20226
- Anderson RM, May RM. Infectious diseases in humans. Dynamics and control. Oxford: Oxford University Press; 1992.
- Plans-Rubió P. Evaluation of the establishment of herd immunity in the population by means of serological surveys and vaccination coverage. Hum Vaccin Immunother 2012;8(2):184-8.