Cullen R M, Walker W J
Department of Mathematics, University of Auckland.
N Z Med J. 1996 Oct 25;109(1032):400-2.
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.
1969年新西兰开始对儿童进行大规模麻疹免疫接种。尽管麻疹在该国并非法定报告疾病，但可获得按年龄和诊断分层的准确医院出院数据。这些数据用于研究大规模疫苗接种对麻疹流行的影响。
分析了1949 - 1992年期间官方公立医院出院表格。
1949年至1969年的20年间有7次流行，平均流行间期为2.86年。自1969年以来，流行间隔分别为3年、5年、3年、5年和6年。自1969年以来，流行年份出院患者数量没有显著变化。然而，其年龄分布发生了显著变化。1969年之前，在流行年份，出院诊断为麻疹的10岁以下患者比例为0.823，10 - 19岁患者比例为0.084。在1991年，即最后一次流行的年份，相应数字分别为0.643和0.28。
这些结果可从标准的房室模型预测得出。从这些模型中推导出流行间期（Ti）与疫苗接种率变化（Pi）之间的简单平衡关系。即T2 = T1 (1 - p1)/(1 - p2)，这与新西兰的数据一致。值得注意的是，通过定期加强免疫或基于人群的再接种运动可预防麻疹流行，这些措施可防止易感人群数量达到流行阈值。
