J. Hyg., Camb. (1980), 85. 451 451 Printed in Great Britain Changes in the seasonal incidence of measles in Iceland, 1896-1974 BY A. D. CLIFF Department of Geography, University of Cambridge AND P. HAGGETT Department of Geography, University of Bristol (Received 25 July 1980) SUMMARY The changing seasonal patterns of reported measles cases in Iceland during this century are analysed. These changes are related to increased population mobility following the development of external and internal transport links, particularly since 1945. The forging of such links has resulted in a shift in the seasonal distribu- tion of cases from one peculiar to the local social and economic conditions in Iceland to one broadly similar to that in other countries of northern temperate latitudes. INTRODUCTION Yorke et al. (1979) have argued that whether or not a virus persists in a human population is determined largely by the degree of coincidence between (1) seasonal upswings in the amount of infection and (2) increases in the susceptible population above a critical threshold level. Clearly, if these two things are in phase, wide- spread infection is likely to occur; if they are not, there is an increased probability of fade-out of the disease. If Yorke et al. are correct in suggesting that an under- standing of seasonal patterns of variation in disease intensity is a necessary pre- requisite for the design of adequate control strategies, then the changing seasonal patterns in disease incidence are worth monitoring. In this note, we describe the seasonal patterns of reported measles cases in Iceland, 1896-1974, and show that they are not stable through time. MATERIALS AND METHODS Materials The measles records for Iceland have been analysed at length by Cliff et al. (198 1) in a monograph on the spatial diffusion of epidemics and are summarized in a statistical appendix to that volume. Published monthly records of the number of reported cases of measles are available for more than 50 small geographical areas and go back in unbroken form to 1896. Although the reporting rate can be estab- lished only indirectly, it appears to be substantially better than that for England and W07ales over comparable time periods (Cliff & Haggett, 1979). Certainly, Black 0022-1724/80/0076-1980 $01.00 © 1980 Cambridge University Press