On the European continent there were similar recurrences in the first three months of 1920. In the large towns of England the recorded deaths from influenza made an increase in a steady curve from sixty-six in the last week of January until the week ending March 27th. After that date there was a gradual falling off. That the situation was in no way as serious as it was at the same time in the American cities and in certain other parts of Europe is indicated by an annotation in the Lancet of March 6th. According to this annotation, the weekly totals of deaths attributed to influenza in London and the 96 great towns had on the whole tended to increase in the early part of 1920, but the absolute increment was so small and the necessary uncertainty of classification so great that no unfavorable inferences could be drawn from these fluctuations alone. On the other hand, the notifications of cases of pneumonia increased appreciably, too much to be set down as a mere chance fluctuation. But notification for this disease had not been in force long enough to enable accurate comparison. There were no indications of epidemic influenza in any of the large factories situated throughout the country. But on the other hand there was proof of the existence of epidemic influenza of an infectious, but relatively non-fatal type in certain large schools situated in the South and Southwest of England.
The annotation concludes that influenza was epidemic in a few localized English and Welsh communities, and that the type was similar to, but less severe than that of 1918–19.
In the city of Paris between the 11th and 31st of January there was a very definite increase in the death rate from inflammation of the respiratory tract above the average for other years.
Renon and Mignot studied 141 cases of influenza (71 men and 70 women) during January and February, 1920, at L’Hopital Necker. Fifteen of the 141 died. According to these observers the grip of 1920 attacked all ages in contradistinction to the 1918 epidemic which affected especially the young and vigorous. One-third of their group were over forty years of age, while some were seventy and eighty years old. In spite of this the disease remained relatively mild. Sixty-four were cases of simple grip. Forty-three had associated bronchitis and pulmonary congestion and edema. Twenty-seven had pneumonia. One had acute pulmonary edema. There were cases of influenza in tuberculosis individuals. One developed an acute serofibrinous pleurisy. One had purulent pleurisy, and one meningitis.
In Copenhagen there occurred between the 18th and 24th of January, 1920, 1,204 cases of influenza with four deaths; in the following week, from the 25th to the 31st of January, 7,445 cases with forty-two deaths; from the 1st to the 7th of February, 11,038 cases with 207 deaths; from the 8th to the 14th of February, 8,308 cases and 327 deaths. This is to be contrasted with the month of December, 1919, in which there were 1,845 cases of influenza in Denmark, of which only 272 were at Copenhagen. In Christiania, Norway, during the week of January 25th to 31st, there occurred eleven deaths from influenza, whereas during the preceding two weeks there had not been a single death from this disease.
In December, 1919, there were reported in Switzerland only 511 cases of influenza. During the month of January, 1920, this increased to 13,162, and in February to 83,008, the estimated population being 4,000,000. From February the disease decreased in prevalence. In Zurich, with a population of 210,000, the epidemic resulted in 14,534 cases. The first increase began around January 4th. The total number of cases for January was 1,071. In February the records of the four weeks showed 2,721, 4,140, 3,341 and 1,899 cases, respectively; in March the decrease was rapid, 886, 442 and 45 cases being reported in the first three weeks. The total number of deaths, mostly due to pneumonia, was 229, a mortality of 1.5 per cent.
During 1920 epidemics were also observed in Valencia, Santander and other towns in Spain, and in Mexico City. In the latter city the number of deaths was reported as 1,649, as contrasted with 3,000 in 1918.
Incubation Period.
An accurate determination of the period of incubation in influenza presents great difficulties. The large number of cases with the consequent multiple opportunities for infection in the case of every individual add to the difficulty. Under any circumstances the period is very short. Parkes, many years before the 1889 epidemic, believed that an incubative period sometimes exists; that it was sometimes very short and sometimes of many days duration.
“In the Transactions of the College of Physicians it is stated that in the epidemic of 1782, seventeen persons came to London to an hotel, and on the following day three were attacked with influenza. Haygarth says that a gentleman came to Chester from London, on the 24th of May, 1782, ill of influenza; a lady, into whose family he came, was seized on the 26th, and was the first case in the town. Haygarth states, evidently with the view to point out the possibility of a direct contagion, that the gentleman was engaged to be, and was afterwards, married to this lady. In this case the longest possible incubative period was two days. In 1782 a family landed at Harwich, from Portugal, and came to London directly; the day after their arrival the lady, two servants and two children were all seized. Two men-of-war arrived at Gravesend from the West Indies; three Custom-house officers went on board; a few hours afterwards the crews of both vessels were attacked. Some other cases are on record where the incubative period, if it existed, could not have been more than a single day. On the other hand, some cases are on record in which the incubative period must have been two or three weeks.”