MEASLES EPIDEMICS 1949-91 - THE IMPACT OF MASS IMMUNIZATION IN NEW-ZEALAND

Aims. Mass immunisation of children against measles was introduced in New Zealand in 1969. Although measles is not a notifiable illness in t his country accurate hospital discharge data stratified by age and dia gnosis are available. These were used to examine the impact of mass va ccination on measles epidemics. Methods. Official public hospital disc harge tables for the period 1949-92 were analysed. Results. There were seven epidemics in the 20 year period from 1949-69 yielding an averag e interepidemic period of 2.86 years. Since 1969 epidemics have been s eparated by 3, 5, 3, 5 and 6 years. There has been no significant chan ge in the number of patients discharged from hospitals in epidemic yea rs since 1969. However, there has been a significant shift in their ag e distribution. Prior to 1969, in epidemic years, the proportion of pa tients 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 c orresponding figures were 0.643 and 0.28. Conclusion. These results ar e predictable from standard compartment models. From these models a si mple equilibrium relationship is derived between the interepidemic per iod (T-i) and a change in the vaccination rate (p(i)). This is T-2 = T -1 (1-p(1))/(1-p(2)) and it is consistent with the New Zealand data. I t is noted that measles epidemics could be prevented by periodic boost ers or population based revaccination campaigns that prevented the num ber of susceptibles ever attaining the epidemic threshold.