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suffice to protect the community from an epidemic. Tn the 643 households
attacked l>y small-pox, however, the corresponding proportions who had ever
been vaccinated were only 12, 29 and 13%.
3. The liability of home contacts unprotected by vaccination to be attacked
readied a maximal value in August and September, 1929, by which time most
of tlie Borough was involved. From that point onwards the liability steadily
declined, and a straight line fitted to either the declining apparent infectiousness
or to the declining attack-rate in unprotected contacts, up to the end of
19:50, reached the zero point in July, 1932, Since the epidemic actually died
out at the end of the second quarter in 1932, this may provide a means of
predicting the duration of an epidemic.
4. The steady decline in attack-rate amongst unprotected home contacts
after the epidemic had become diffused indicates that either the virus became
progressively less infective or that latent immunity was being acquired by the
population. Although no evidence in favour of the latter explanation is found
from subsequent attack rates of contacts who escaped immediate infection, it is
a curious fact, that the immediate attack-rate in unvaccinated children under
10 years of age who were exposed to infection was no lower in 1930 than in
1929, whereas in adults it was much lower. This would be difficult to account
for on the first hypothesis, but could be explained if the bulk of the propagation
of small-pox and latent immunisation associated with it, takes place in workplaces,
factories and places where adults arc crowded together.
5. In the first year of the epidemic the mean number of persons per house
allected by small-pox was 9.0, of whom 6.6 were unprotected by vaccination;
in the second year the corresponding figures were 8.2 and 5.0. In the first
3 months of the epidemic two-thirds of the previously unvaccinated home
contacts accepted vaccination, but after 6 months this proportion fell to 30%,
and with recrudescence of the epidemic gradually rose again to 45%. At the
start of the epidemic 55% of previously vaccinated home contacts accepted
revaccination, but this proportion rapidly fell to about 17% and did not rise
again.
6. I he incubation period of variola minor seems to lie outside the limits of
12-16 days in at least one-eighth of all cases, but it can only very rarely be less
than 10 or more than 18 days. The mean interval from onset of symptoms to
appearance of the rash was 2.7 days. When a small-pox case developed the
rash 11-21 days after a previous case in the same house, it was regarded as
being secondary to, that is to say infected by, the latter. When the interval
was less than 11 or more than 24 days, some other source of infection was
postulated. I he justification for selecting these limits is explained in Section
(4).
7. When a small-pox case was removed to hospital on the day of appearance
of the rash, the subsequent attack-rate amongst unprotected home contacts