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was significantly less than in cases where removal was delayed. The attackrate
also increased from 8% when 4 days elapsed from onset of symptoms to
removal, to 25% when, 8, 9 10 or 11 days elapsed.
8. The cases of smallpox included 157 persons who had been vaccinated
less than 24 days before attack, but in only five of these did the interval from
vaccination to onset exceed 12 days, and only in one case was it greater than
16 days. Owing to the time required for complete protection to develop, which
is presumably variable like the incubation period of small-pox itself, it is
evident that immunity from attack cannot be guaranteed even if vaccination is
performed immediately after exposure to infection. Of 145 previously unvaccinated
home contacts who were vaccinated on the day of, or day following, the
appearance of the rash in the first case in the house, six were attacked, which
indicates that vaccination after exposure cannot reduce the risk to the previously
unvaccinated below about 4%. Comparing the actual attacks with
those expected in the same age distribution of unvaccinated persons, vaccination
of unprotected contacts within 2 days of appearance of the rash in the first
case reduced the risk of immediate attack to one-seventh, vaccination
3 to 4 days after reduced the risk to one quarter, vaccination 5, G or 7
days after reduced it to two-thirds, but with longer intervals here is no
evidence that it was effectual. For statistical purposes "protection " has been
regarded as adequate if vaccination was done within 4 days. There is definite
advantage to be gained, however, by not postponing vaccination of contacts
beyond the second day after appearance of the rash.
9. The primary cases of small-pox showed a relative excess of young
adults in comparison with the secondary cases, which suggests that small-pox
is spread in work-places more than in schools. The proportion of young
adults in affected houses who had been previously vaccinated increased during
the epidemic, owing no doubt to vaccination in factories, but this was not the
case with children. The ages of maximal attack-rate in the whole population
were 5-10, both for primary and secondary cases, but the change of attack-rate
with age was greatly influenced by the differing proportions protected by
vaccination at different ages, this proportion being highest at 20-25. The total
effect of vaccination before and after exposure was to protect adequately about
30% of home contacts at ages 0-5, 40% at 5-.10, 50% at 10-15, 60% at .15-30,
45% at 30-45, 40% at 45-50, 30% at 50-60, and 8% after 60.
10. The susceptibility to attack in unprotected home contacts was highest
at ages 15-20 and 30-50; at 20-30 it was significantly lower and after 50 it
rapidly declined. The aggregate risk of immediate attack to the unprotected
contacts of all ages was 1 in 4. At ages under 20 the risk to those vaccinated
in infancy only was one-twentieth of that in the unvaccinated, at ages 20-40
about one-tenth, and at ages over 40 about one-half. This was also true when
the vaccinated and unvaccinated living in the same houses were compared.
Comparing the actual attacks with those expected in the same age distributions
of unvaccinated contacts, vaccination less than 20 years before reduced the risk