Measles epidemics 1949-91: the impact of mass immunisation in New Zealand

Mass immunisation of children against measles was introduced in New Zealand in 1969. Although measles is not a notifiable illness in this country accurate hospital discharge data stratified by age and diagnosis are available. These were used to examine the impact of mass vaccination on measles epidemics.

Official public hospital discharge tables for the period 1949-92 were analysed.

There were seven epidemics in the 20 year period from 1949-69 yielding an average interepidemic period of 2.86 years. Since 1969 epidemics have been separated by 3, 5, 3, 5 and 6 years. There has been no significant change in the number of patients discharged from hospitals in epidemic years since 1969. However, there has been a significant shift in their age distribution. Prior to 1969, in epidemic years, the proportion of patients with a discharge diagnosis of measles aged under 10 was 0.823, and aged 10-19 was 0.084. In 1991, the year of the last epidemic the corresponding figures were 0.643 and 0.28.

These results are predictable from standard compartment models. From these models a simple equilibrium relationship is derived between the interepidemic period (Ti) and a change in the vaccination rate (Pi). This is T2 = T1 (1-p1)/(1-p2) and it is consistent with the New Zealand data. It is noted that measles epidemics could be prevented by periodic boosters or population based revaccination campaigns that prevented the number of susceptibles ever attaining the epidemic threshold.