In this section, we discuss the possible range of values of the serological attack rate. During the preparation of this paper, a similar theory has been presented (Bootsma & Ferguson 2007 ), which we present in more detail. If social distancing is sufficiently effective (s!1/R 0 ) and can be maintained, then an epidemic will go extinct by the epidemic threshold theorem (Becker 1989) . In a large population, the fraction who become infected in this case is negligible. This may have contributed to the extinction of SARS virus (Riley et al. 2003) .