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34
The biennial swing in the incidence of measles observed
in pre-war years was not peculiar to London but was observed
in most towns in the northern hemisphere. Although measles
did not become notifiable in London until 1938, such information
as was available indicated that the two year fluctuation which
occurred was very similar to that occurring in New York.
Occasionally the epidemic rhythm "changed step". Since 1940
New York has maintained the pre-1940 rhythmic pattern, whereas
in London the post-1940 pattern is somewhat different in that
each year has contained a greater or lesser outbreak of
epidemic nature, but never attaining the proportions of a prewar
peak. Studies of the rise and fall of measles epidemics
in the counties of England and in London and the larger towns
during the period 1940 onwards, have disclosed a remarkably
wide variation in the timing of peaks in different areas. The
peaks were by no means all in the late winter and spring, the
scatter being wide (although not completely random) throughout
the year. One feature only was universal, i.e. low incidence
in autumn.
Whatever variations there may be in the duration of an
epidemic, the rhythm of the various phases generally
corresponds to a set pattern.
It hat. been shown that if the graphs of epidemics are
superimposed, it appears that each epidemic is comparable
within the two yearly period, so that the resemblances in
fact are not annual but bienniel, and that a repetitive form
emerges which, whatever departures from rigid conformity,
remains an unmistakable type. The rise in incidence begins
in each case at the beginning of the first year, rises to its
maximum in the first three months of the second year, falls
to the lowest level again in August and September of the
second year, and remains at this low level until the end of
the year. The ascending limb of the wave is marked by
a depression known as the "Autumnal notch" which, rightly or
wrongly, has been attributed to the influence of school
closure.
Attempts have been made from time to time to explain
the biennial periodicity of measles, and among others have
been theories that it is dependent on the proportion of
susceptibles to immunes, the topography and rate of spread
in an urban unit, the dispersal or concentration of the
population, the possibility of latent immunisation arising
from sub-clinical attacks, or the periodic fluctuation in
the infecting power of the virus. While no final agreement
has been reached, it seems likely that war-time dispersal of
the population and the wide variation in the timing of
epidemic peaks in different areas may account for the
irregularity of the epidemic curves which has occurred during
and since the war.
So far as Kensington is concerned, the position since
the introduction of notification is shown in the following
table of annual figures, and the attached graph showing the
four weekly incidence during this period:-