The Epidemiology of an Influenza Outbreak in Leeds

Abstract

1. This paper deals with the epidemiology of an outbreak of influenza which started in Leeds late in November 1932 and continued until the end of February 1933.2. Returns of the actual cases which occurred each week in certain official departments and large firms made possible the investigation of a selected population. This consisted of 17,195 persons, and among its members there were 2772 cases of influenza with five deaths. The case-rate was therefore 16·1±0·2 per 100 of the selected population. It is shown that in this popu lation factory workers were less liable to contract influenza than were assistants in large shops, outdoor workers, clerks and office workers, and school teachers; and it is suggested that propaganda relating to respiratory infections may have had some effect in lessening the incidence of the disease among the workers in these factories. The mean duration of the outbreak in the selected establishments was 11·55±0·29 weeks.3. The total cases which occurred each week in the selected population were plotted as a graph; the resulting curve was of the typical epidemic type. A Pearson's Type IV curve was fitted to these data. The available evidence points to the fact that the outbreak consisted of a single wave.4. From the data for the city as a whole it is shown that the Leeds out break took approximately twice as long to reach its peak as did the Glasgow epidemic, though both outbreaks started practically simultaneously. Never theless, in actual epidemicity, the Leeds outbreak did not fall far short of that which occurred in Glasgow.5. An investigation of the age distribution of deaths from influenza in Leeds since 1900 shows that, though the 1918 epidemic produced a sudden change in the age distribution, in that there was an unprecedented toll of young adult life, the return to thestatus quowas not sudden, as is often assumed, but was effected gradually over a period of about ten years.6. It is shown from an investigation of influenza deaths in different parts of the city that there was no direct association between these deaths and the density of population in the wards in which they occurred. Neither was there any association between the number of influenza deaths and the average number of persons per room in each ward. From a study of the distribution of the deaths in time and space it appeared that, with certain exceptions discussed in the paper, the outbreak started in the centre of the city and extended outwards from the centre with decreasing potential.7. An investigation of the meteorological conditions suggested that there was a definite relationship between a fall of temperature, especially if accompanied by fog, and an increase in the incidence of influenza and of pneumonia.8. The clinical features of the outbreak are discussed fully in the text. The disease was essentially of the respiratory type. Two forms of this were in evidence, (a) a form resembling the classical picture of influenza, and (b) a form in which the predominant feature was tracheitis. The gastric type of influenza was not common. Complications were not very frequent, but the commonest were influenzal pneumonia and acute otitis media. The duration of incapacity for work varied according to the social status of the individuals. Brain workers and persons in the middle classes were generally off duty for about nine days. On the other hand, the duration of incapacity in artisans and factory workers was usually about a fortnight or three weeks.9. From the evidence which is adduced in this paper as a whole, including a consideration of the tuberculosis statistics of the city, it would seem that, though the outbreak of influenza in Leeds during the winter of 1932–3 was of a mild and non-killing type, infection was widespread, and the total incapacity must have resulted in a considerable economic loss to the community.I am indebted to Prof. J. Johnstone Jervis for suggesting this investigation. My thanks are also due to the heads of the various departments and commercial firms, and to the Staff of the Public Health Department, without whose co-operation the data for this study could not have been obtained.