Trend of influenza mortality during the period 1920-51.

During the period 1920-51, influenza mortality-rates-both during epidemic and non-epidemic years-have, in most countries for which data are available, shown a downward trend, the rate of fall in mortality being, in general, accelerated after the middle 1930's. The author discusses the various factors which may have contributed to this decline, but emphasizes that the reason for it cannot at present be fully explained, nor-owing to the comparatively short period during which the accelerated fall has been apparent-can a conclusion yet be drawn as to whether influenza mortality has already reached its lowest level or whether it will continue its downward trend in the future.