Heilbrigðisskýrslur - 01.12.1980, Síða 134

3). The cell around Akureyri is particularly well defined.
The written reports in Heilbrigðisskýrslur not only permit some
reconstruction of the internal spread process, but they also indicate
the country of origin of many of the measles epidemics which reached
Iceland. Figure 4 shows the recorded country of origin for eleven of
the sixteen waves.
Seasonal pattern of measles outbreaks
If study of Icelandic records helps towards an understanding of the
spread of measles epidemics over space, it can also throw light on
the timing of outbreaks. Standard epidemiological accounts of measles
frequently ascribe to it a strong seasonal pattern. Thus Ball (1977,
pp. 245-6) describes measles as having "a higher incidence in the
colder months, reaching a peak in January and February which is most
marked in an epidemic year". This generalisation is based on northern
temperate countries, and we should expect the corresponding months to
be July and August in southern temperate latitudes. Virologists do
not generally ascribe seasonal variations in the incidence of cases to
the direct effects of temperature and humidity upon the measles virus
itself. Instead, they focus upon the way in which seasonal variations
in the weather affect the extent to which those with the disease
(infectives) mix with those at risk (susceptibles). Thus, in the
United Kingdom, the January-February epidemic peak of measles cases
is often related to the re-opening of schools in early January after
the Christmas holiday (itself a period of high contact between members
of families), plus a two to three week lag while infectives are
incubating and spreading the disease.
Viewed against this background, the monthly distribution of measles
cases in Iceland is an interesting one. For the period since 1945,
Figure 5C shows a pattern much like that suggested by Ball. The period,
1900-44, was, however, very different, with a second distinct peak in
June (Figure 5B). Any explanation of this pattern must begin with the
reminder that, since measles are not endemic in Iceland, the seasonal
distribution will reflect the starting dates of the epidemics. Of those
waves which occurred before World War II, all but one commenced in the
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HEILBRIGÐISSKÝRSLUR 1980