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69
In the Annual Report for 1934, a history of Measles in Acton
was given and a table inserted to show the regularity of its biennial
visitations. If the table had been drawn up for separate years,
and ending, say, on July 1st each year, the regular periodicity
of the disease would be still more marked, because the outbreak
usually occurs in the winter months; sometimes the epidemic
started in the last quarter of the year, at other times it originated in
the first quarter of the year.
Recently the interepidemic period was slightly more than
104 weeks. In the epidemic of 1934, the first cases were reported
in January, but the cases in the school did not become numerous
until February and March. In the previous epidemic of 1932,
the first cases were reported in December 1931, and several cases
were notified from the schools when they re-opened in January
1932, after the Christmas holidays. Although it could almost
certainly be predicted that 1936 would be an epidemic year, we did
not expect the epidemic until February or March. Besides, in
recent epidemics, our Measles outbreaks have followed the rise of
deaths in London ; we usually have felt the full force of it a month
or so after the epidemic has reached its peak in London. Last
year's epidemic behaved in a slightly different manner. The height
of the epidemic was not reached until the early part of 1936 ; there
were only 12 deaths in November and December 1935, reaching 71
in January' and February 1936. The first case here was reported
in November 1935, and as will be seen from the School Report the
outbreak had a firm hold before the end of January 1936. The
epidemic lasted until the summer of 1936, but of the 9 deaths, 6
occurred in the first quarter of the year.
An intensive study has for years been made into the bacteriology
and the epidemiology of measles ; probably no disease
has recently been more intensively studied, and we have learnt
a good deal at any rate of the behaviour of an epidemic of Measles.
Although the germ or virus which is the probable cause of the illness
has not been isolated, and is therefore unknown, we do know
how Measles in the mass behaves. A Measles epidemic does not
arise in a community of children until the percentage of susceptible
children rises beyond 25, and seldom subsides until the percentage
of the susceptible population is reduced a mark below 20.
The question of immunity is a complex one, and usually a
person becomes immune or non-susceptible to Measles by contracting
the disease and suffering from the symptoms of Measles.
The immunity which is conferred by a clinical attack of Measles
is usually a permanent one, and lasts a lifetime. But it is now held