1940 PUBLIC HEALTH

The Age Selection of Mortality from Tuberculosis in Successive Decades

By RALPH M. F. PICKEN, M.B., H.B., B.SC., D.P.H., Mansel Talbot Professor of Preventive Medicine, The University of Wales.

The title of this short article is borrowed with- out modification from the last, posthumously published, article of the late Dr. W. H. Frost of The Johns Hopkins School of Hygiene and Public Health. 1 The reason for this plagiarism is that it is simply an attempt to test against British data a method of examination applied by Frost to figures derived from the State of Massachusetts. His plan was to follow up the generation, or "cohort " as he called it, born in a decade, and examine its experience of mortality from tuberculosis in each subsequent decade. Frost quotes Andvord, a Norwegian, as having studied U.S. mortality statistics in this way in 1930, and it may be noted that Greenwood in discussing an article by Hill suggested this method z just about the time when Frost appears to have begun to be interested in it. It is, in fact, a specific appplieation of the general study of relative mortalities in age-groups made by Kermack, et al., in 1934, 3 from which these writers concluded that the improvement attainecl at any particular time in the death-rates of the various age-groups depends primarily on the date of birth of the individuals concerned, and that it is probable that the important determining factors upon the health of an individual during his whole subsequent life lie in his environment up to the age of, say, 15 years.

Age-Changes of Tubereulosls in the U.S.A. It should be noted that throughout the period

1880 to 1930 studied by Frost the peak of mortality from tuberculosis (all forms) among males moved consistently to higher and higher ages both in the Registration States and in Massachusetts. At the beginning of this period the curve was far from simple, for, in addition to the usual heavy mortality in early childhood and sharp decline followed by the main peak in adult life, it showed a fall in middle age and then another pronounced rise in old age. This latter feature is absent from the corresponding curves for England and Wales and disappears in America in more recent years with the advancing age of the main peak, at least so far as males are concerned. But when the cohorts are plotted out separately, that is to say, when we trace the rate of mortality from tuberculosis to which the survivors of those born between 1870 and 1879 were subject when they were aged 10 to 19 in 1890, 20 to 29 in 1900, 30 to 39 in 1910

and so on, and do the same for each subsequent generation or cohort, the form of the curve is remarkably constant and quite different from that obtained by charting a cross-section of the popula- tion at different ages at any one time. The main peak (for males) is now always in the relatively young age-group 20 to 29 and the only difference in the graphs of the successive decennial cohorts from 1870 to 1910 is the steadily falling mortality at all ages with the progress of the years. The same is true for the female cohorts, although the bias of the curve is much more toward youth. It is easy to see that a constant form of curve for each generation, combined with a steadily falling toll of mortality at all ages in the successive generations, would have the effect of pushing to higher ages the peak of mortality when it is charted against the population in age-groups living at any one time.

Changes in Age-Distribution The basic data are, of course, the age- and sex-

specific death-rates at different times, a series of cross-sections of the mortal effects of tuberculosis upon the population at different ages living at each of the times chosen for inspection. From the Reports and Reviews of the Registrar-General for England and Wales, Figs. 1 and 2 have been prepared in order to illustrate for males and females the well recognised changes which have. taken place in the age-distribution of the mortality from tuberculosis (all forms) when annual death returns are analysed in this way. The rates have been taken out for 1882 and each succeeding tenth year up to 1932, but some years have been omitted from the charts in order to make them less confusing to the eye. For the same reason the rates at ages 0 to 4 and 5 to 9, which have dropped steadily and very rapidly, have not been charted. From Fig. 1 it is apparent that the main peak of male mortality was moving to higher ages up to 1912, and that afterward there was a slight recession to younger ages and a quite new peak at ages 20 to 24. In Fig. 2, a similar process is revealed, the peak of female mortality definitely moving to a higher age up to 1902, but the trans- ference of the accent to ages 15 to 24 which is so characteristic of 1922 and 1932 began for females some time before 1912. That is the familiar story of the age-career of tuberculosis in this country.

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FIG. 1 .-England and Wales : Male death-rates from TubercuIosis (all forms) by age in certain years since 1882.

The age-groupings of tuberculosis in Massachusetts and the U.S.A. Registration States presented the different picture already indicated, and the slight post-war check to the fall of mortality in young adult females was evanescent and much less striking than it was here. Ape-Mortality in the Cohorts

The age-experience of tuberculosis when a group of persons born in one decade is followed iap throughout life can be traced in Table I. The calendar years examined are the same as before, but the age-groupings above age 10 are different, so that the mortality from tuberculosis at ages under 10 in 1882, 10 to 19 in 1892, 20 to 29 in 1902, and so on, represents roughly the subsequent experience of the generation born during the ten years before 1882. As neither the actual deaths from tuberculosis nor the death-rates were stated in such age-groups by the Registrar-General for the earlier years, these rates are estimates but they may be regarded as reasonably near the truth. A cohort can thus be followed diagonally downward from any age-group on the left-hand side of the table, as is indicated for the cohort of 1882 by

FIG. 2.-England and Wales : Female death-rates from Tuberculosis (all forms) by age in certain years since 1882.

black figures. Figs. 3 and 4 show graphically the result of this way of extracting the Brltish figures. It will be seen that for those cohorts born well before the end of last century, i.e., when the peak of age-mortality for both sexes appeared to be moving to higher ages, the age-groups consistently most affected were 30 to 39 for males and 20 to 29 for females, and the form of the curves remained fairly constant. Up to this point the British and Massachusetts charts resemble one another, except that 20 to 29 is the age-group most affected for both sexes in Massachusetts. In the latter case, however, this type of curve remains constant as far as it has been possible to follow the cohorts of the present century, whereas a very striking change has come over both the male and female mortality rates in the British cohorts of 1902 and almost certainly of 1912. The peak for males has retired for the first time to ages 20 to 29, and for females it belongs much more pronouncedly to this age than before. Discussion

Frost suggested as tentative conclusions from his study of the Massachusetts records that the

1940 PUBLIC HEALTH

TABLE I. DEATH-RATES PER 100,000 FROM TUBERCULOSIS (ALL FORMS), FOR ENGLAND AND WALES, 1882 TO 1932, BY AGE

AND SEX.

Age. 1882 1892 1,902 1912 1922 1932

Ma&s : 0-4 536 472 335 195 123 84 5-9 89 82 65 56 38 24

10-19 113 90 71 67 63 47 20-29 290 242 202 159 164 116 30-39 368 305 265 206 167 118 40-49 386 339 315 240 188 142 50-59 347 308 300 241 177 148 60-69 255 224 229 196 142 107

Fema~s : 0-4 417 384 286 170 103 67 5-9 87 85 69 54 40 25

10-19 170 18g 95 95 90 68 20-29 298 204 159 136 147 124 30-39 322 244 188 166 119 88 40-49 275 220 180 132 98 63 50-59 196 170 141 107 81 53 60-69 140 123 107 93 71 46

advancing peak of mortality of our time does not represent a postponement of risk, but is a residuum of higher rates in earlier life; that constancy of age selection in successive cohorts sug.gests rather constant physiological changes in resistance with age (cf. Kermack, et al., referred to above); and that, if it is right to suppose that the frequency and extent of exposure to infection has been diminishing throughout these decades, there is no indication that this has increased the risk of death in adult life from lack of acquired immunity. These appeared to be reasonable deductions from his data, and he thought that the mortality records of England and Wales bore them out. They do up to the end of the 19th century, but the rather sudden concentration on young adults in more recent times which, in the case of females at least, does not seem to be passing away very rapidly in spite of the fall in the mortality at all ages, is still unexplained. The fact that this change became very manifest only after the War of 1914-18 has naturally caused the two events to be linked in our minds. From study of the figures over a long series of years, the Registrar- General, who is usually cautious in offering expla- nations, says in relation to the mortality from all forms of tuberculosis, "The rise or arrested fall of mortality at ages between 15 and 25 from 1913 to 1923 can be attributed to the immediate effects of food shortage in some of the intervening years on young adults." Again, in discussing similar varia- tions in the mortality from respiratory tuberculosis, he notes that "stationary periods of arrested fall

~ 200

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100,

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8

i 100-

C~'IOB~ 1872

/ O0~OR~ 1912

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FIG. 3.--England and Wales: Male death-rates from Tuberculosis (all forms) by age, in successive 10-year cohorts.

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COHO

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FIG. 4.--England and Wales : Female death-rates from Tuberculosis (all forms) by age, in successive 10-year cohorts.

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occurred both for male and female rates at ages 15 to 20 between the years 1926 and 1931, and it may be significant that the persons concerned had been children between the ages of 0 and 12 during the period of food shortage in 1916-18. Similar stationary or rising periods occurred in the rates at ages 20 to 25 between the years 1930 and 1933, the persons comprising these groups of the popula- tion having been children of ages 3 to 12 during the 1916-18 period. ''* The association seems striking when stated in this way, and here again is the suggestion that a state of affairs is to be explained by something experienced by the affected persons some 10 to 20 years earlier. But it is now evident that the arrest of the decline of mortality among young females had begun well before the War and the food shortage. Indeed if the suggestions of Kermack, et al., and of Frost are read into Charts 3 and 4, it would appear reasonable to seek influences at the end of the 19th century, or early in the 20th, to account for the change in the behaviour of tuberculosis among both males and females.

It is not the object of this article to discuss these possible causes. So far as this country is concerned they have been exhaustively considered by Brad- ford Hill ~' and Hart and Wright, 5 but neither of these elaborate statistical investigations appears to me to offer a readily acceptable explanation of so curious a phenomenon as this selective interrup- tion in the otherwise continuous fall in the mortality from tuberculosis. Sir Arthur Mac- Nalty 6 mentions the possibility that the resumed and accelerated decline of mortality among young adults since 1933 may mean that the check we have been discussing is now passing away. It is perhaps worth noting, however, that the curve of female age-mortality for 1937 (the latest year for which figures are available) shows general con- formity in shape with those for all the post-war years. The death-rate in each age-group is falling but there is no sign of reversion to the peak formerly observed at ages 35 to 44, or still earlier at ages 25 to 34. On the other hand there is some evidence that the male curve is now resuming its pre-war form. The influences, therefore, which brought about the change in the relative mortali- ties at different ages from tuberculosis seem not only to have begun to affect females earlier than males, but also to be continuing longer to do so.

Summary 1. The curve of age-mortality from tuberculosis

(all forms) in England and Wales, when traced throughout a generation or "cohort," differs from the corresponding curve for a cross-section of the population at any one time.

2. The behaviour of this curve for successive generations resembles that traced by F ros t for Massachusetts, up to the generations born about the end of last century.

3. The curves for later generations, both male and female, in England and Wales are markedly affected by the special incidence on young adults which began to be felt before the War of 1914-18, but especially during and after it.

4. None of the explanations offered are very satisfactory as to this phenomenon, which appears to be persisting among females but gradually dis- appearing among males.

REFERENCES. 'Frost, W.H. (1939). Amer. J. Hyg., 80, 91. ~Hill, A. Bradford. (1936). 9~. Roy. Stat. Sot., 99, 247. 8Kermack, W. O., McKendrick, A. G., and McKin]ay,

P.L. (1934). Lancet, 1,698. 4Registrar-General for England and Wales, Annual

Statistical Review, 1936. Text, 72-75. 5Hart, P. D'Arcy, and Wright, G. Payling (1939).

Tuberculosis and Social Conditions in England. Nat. Assoc. Prev. Tub., London.

eMinistry of Health, Annual Rep. of the Chief M;O., 1938, 133.

THE NATIONAL HOSPITAL BILL OF THE U.S.A.

If the American Congress passes a Bill which is now before it, extensive new powers will be given to the Surgeon General in relation to hospitals. It pro- vides for the erection of hospitals in the less opulent rural areas by the Federal Government, to be leased to the local community on condition that they are operated and maintained according to standards laid down by the Surgeon-General.

He is to be advised by a council of six leading medical or scientific authorities appointed by himself with the approval of the Federal Security Adminis- trator. The council will not only help in formulating standards, rules and regulations, but also conduct and foster studies in the need for hospitals and in hospital operation.

For the coming financial year an appropriation of ten million dollars is provided for the purpose of the Bill. The term "hospital" is defined as including "the physical facilities for the prevention, diagnosis and treatment of disease, and for the protection of the public health," so that a wide variety of institutions ,nay be established by the operation of this measure.

The Royal Sanitary Institute invite the attendance of members of the Society at a sessional meeting to be held at 90, Buckingham Palace Road, S.W.1, on Tuesday, April 16th, at 5 p.m., when the discussion on "Food in War-Time" will be opened by Dr. S. J. Cowell, Professor of Dietetics in.the University of London. The chair will be taken by Professor Sir Wilson Jameson.

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