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re-vaccinated cases reach the stage of areola two or three days sooner than in those persons
vaccinated for the first time." (Reynold's System of Medicine.)
The Lambeth figures show that re-vaccination after exposure of previously vaccinated
persons was able to reduce the proportion of attacks more largely than the vaccination after
exposure of previously unvaccinated persons. The difference is probably due to the fact that a
re-vaccination, as Marson points out, runs a more rapid course than a primary vaccination.
Reports of other medical officers of health supply further information as to the importance
of limiting the interval of time elapsing between infection and subsequent vaccination. Thus
Dr. Caldwell Smith writes that in Wandsworth "no case occurred in a re-vaccinated person, and
only four among persons vaccinated or re-vaccinated in the incubation period. One of these was
re-vaccinated within three days, but the re-vaccination was unsuccessful. Another was revaccinated
on the fifth day, a third on the sixth, and a fourth, an unvaccinated infant, was not
vaccinated until nine days after exposure to infection." Dr. Jackson states that in Fulham "no
case occurred among those who were vaccinated within four days of their exposure to infection,
while eleven cases occurred among those who refused to be at once vaccinated." Dr. Bryett
mentions the attack of a disinfecting officer in Shoreditch who was not re-vaccinated until 48 hours
after beginning his work. He suffered, however, from a very slight attack of smallpox.
Dr. Allan writes that in Westminster "twelve persons [who subsequently suffered from
smallpox] had been re-vaccinated within the infected period (twelve days); of these, three were
less than four days before the patient began to feel ill, and probably, therefore, eight days after
receiving the smallpox infection; six were between four and six days; and three were between
eight and ten days. With one exception, all these were modified cases; the exception was a
female with one mark, who developed confluent smallpox, caught from her husband who had
the hemorrhagic type, in which possibly the incubation period is shorter than in ordinary
smallpox."
Through the courtesy of the Metropolitan Asylums Board I am able to show in the following
table the interval which occurred between the vaccination after exposure to infection and
the appearance of the eruption of smallpox of those persons in London who subsequently suffered
from that disease.

The incubation period of smallpox is usually stated to be twelve days and the eruption
appears on the third day of the illness, i.e., on the fourteenth or fifteenth day after infection. In
all infectious diseases there are variations in the actual period of incubation, as well as variations
in the period which elapses between the first symptoms and the appearance of the eruption.
Assuming, however, that the periods above mentioned are in the case of smallpox fairly constant,
it will be seen that the behaviour of the disease was in very few cases different from that observed
by Marson, i.e., very few cases of smallpox occurred in which the sufferer had been vaccinated or
re-vaccinated within 48 hours after infection, or, in other words, who had been vaccinated or
re-vaccinated more than 12 days before the appearance of the eruption.
The further point will be noticed that the average interval between a re-vaccination and
the appearance of the eruption of smallpox was less than the average interval between a primary
vaccination and the eruption, in the former (if cases with a longer interval than 14 days be
excluded) the interval was 5.8 days, in the latter 6.8 days; i.e., assuming the interval between
infection and the appearance of the eruption to be fourteen days, the average interval between the
infection and the re-vaccination was 8.2 days, and between the infection and the primary
vaccination was 7.2 days, or, in other words, the interval during which the re-vaccination afforded
protection was a day longer than the interval during which primary vaccination protected.
As already stated, the early vaccination of persons known to have been exposed to
smallpox must have largely limited the spread of the disease. Necessarily, however, in numerous
cases the sufferers did not know that they had been exposed to infection. Indeed, averaging the
results obtained by several medical officers of health, it may be stated that it was impossible to
discover the source of infection in more than one-third of the cases which occurred, and no doubt
in many cases the source of infection, even when discovered, was unknown until after the attack
of the persons whose illness led to the inquiry. In many cases, moreover, a person attacked by
smallpox was not recognised to be suffering from this disease until the period had elapsed in which
the vaccination or re-vaccination of other persons he was infecting would have protected them
from smallpox.
Some estimate can be formed of the period of illness at which the nature of the disease was
recognised, by the information contained in several reports as to the day of illness on which the
patient was removed to hospital, for so prompt was the administration of the Metropolitan