Subacute sclerosing panencephalitis in England and Wales: transient effects and risk estimates.

Data on cases of subacute sclerosing panencephalitis (SSPE) with onset in 1970-89 show a marked increase in the interval from initial measles attack to onset of SSPE by year of onset. It is shown that this increase is a transient effect resulting from the decline in measles incidence following the introduction of mass vaccination in 1968. The risk of SSPE after measles is estimated to be 4.0 x 10(-5) (18 x 10(-5) after measles under one year of age) and the risk after vaccination to be no greater than 0.14 x 10(-5), thus confirming that vaccination is effective in reducing the incidence of SSPE. The paper makes use of methods developed to estimate the incubation period of AIDS.