In the article by K. D. Ricketts, A. Charlett, D. Gelb, C. Lane, J. V. Lee and C. A. Joseph [Reference Ricketts, Charlett, Gelb, Lane, Lee and Joseph1] presented in Epidemiology and Infection, there was an error in the equation on page 1005. The correct equation is published below:
![](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20160921000648771-0136:S095026881100135X:S095026881100135X_eqnU1.gif?pub-status=live)