Diphtheria, pertussis, and measles in Portugal before and after mass vaccination: a time series analysis.

Techniques of time series analysis were used to examine historical records of the incidence of diphtheria, pertussis, and measles, and of deaths by measles in Portugal during the twentieth century. There are statistically significant seasonal and longterm oscillations in the incidence of these diseases. Seasonal oscillations appear to be in close association with the resumption of school classes in the fall in the case of diphtheria, but not in pertussis and measles. Long-term oscillations in pertussis (3.5-4 year period) and measles (3-year period), before vaccination, corroborate theoretical predictions about the dynamics of these diseases, whereas absence of long-term oscillations in diphtheria is probably due to the influential presence of carriers upon the dynamics of the disease. Mass vaccination strongly suppressed disease incidence, did not eliminate seasonal oscillations, and appeared to have acted to lengthen long-term periodicity in pertussis and measles.