Modes
of Transmission
Transmission
Factors (22:V2:20-27)
Studies showed
that vaccine can provide full protection against smallpox but that
the degree of protection decreases over time. Because more was
learned about the incubation period of smallpox and the immune
response to vaccine during the Eradication, we learned that vaccination
even a few days after exposure may still prevent disease or can
at least decrease the severity of the illness. Transmission of
smallpox did not occur during the incubation period, before the
onset of symptoms, like it can for other transmissible diseases.
Good surveillance for cases with targeted vaccination of those
around the cases significantly decreased the transmission of disease,
even in areas with low herd immunity rates from vaccination.
Several
factors can influence how well smallpox spreads. The viability
of variola and vaccinia viruses was less prolonged at higher temperatures
and higher humidity. This accounts for smallpox’s seasonal
pattern, prior to eradication it was more prevalent during the
winter and spring in those countries that experienced low temperatures
and humidity during those two seasons. The intensity and duration
of contact and the severity of the illness in the case also contributed
to higher rates of secondary transmission. Highest attack rates
occurred in close family contacts of overt cases of smallpox that
had constant exposure as opposed to those whose exposure was only
daily. Residents of the same house or people who lived in the same
compound also had higher attack rates. Those who lived in the same
compound would often have visited the house of the patient, increasing
their risk of contracting the disease. Those who had contact over
7 or greater day period had a much higher attack rate than those
who had some degree of contact over less than 7 days. The average
attack rate for previously vaccinated household contacts was 3.8%,
demonstrating the efficacy of vaccination.
Although a person
is contagious until the rash of smallpox was completely resolved,
which could be several weeks, they were much more contagious in
the early part of their illness than they were during the later
part. In outbreaks where airborne transmission over greater distances
occurred, the initial case exhibited a greater degree of coughing
or sneezing which contributed to the creation and distribution
of airborne particles. Other outbreaks of smallpox in close, defined
populations demonstrated that the disease didn’t spread as
rapidly through the population as might be expected if smallpox
were as easily transmitted as previously believed.
The multiplier,
or number of people that will be infected by a single smallpox
case, is a debated statistic. Estimates range from two to twenty.
Studies during the eradication period demonstrated that smallpox
may not be as easily transmitted as originally believed as the
overall attack rate was less than 60% in unvaccinated household
members. Three studies had unvaccinated household transmission
rates of greater than 70%, which may have been related to more
favorable environmental conditions (low temperature, low humidity)
that existed during the time periods for these studies. The case
studies of outbreaks in Meschede, Germany in 1970 (5:3) and
Yugoslavia in 1972 (15:213), exemplified
the scenarios of highly contagious strains where the multiplier
was high.
A Case Study
in Epidemiology of Transmission: Germany 1970
The potential
of aerosolized smallpox to spread over a considerable distance
and to infect at low doses was vividly demonstrated in an outbreak
in Germany in 1970. That year, a German electrician returning from
Pakistan became ill with high fever and diarrhea. On January 11,
he was admitted to a local hospital and was isolated in a separate
room on the ground floor because it was feared he might have typhoid
fever. He had contact with only two nurses over the next 3 days.
On January 14 a rash developed, and on January 16 the diagnosis
of smallpox was confirmed. He was immediately transported to one
of Germany’s special isolation hospitals, and more than 100,000
persons were promptly vaccinated. The hospital had been closed
to visitors because of an influenza outbreak for several days before
the patient was admitted. After the diagnosis of smallpox, other
hospital patients and staff were quarantined for 4 weeks and were
vaccinated; very ill patients received vaccinia-immune globulin
first. However, the smallpox patient had had a cough, a symptom
seldom seen with smallpox; coughing can produce a large-volume,
small-particle aerosol like what might occur after its use as a
terrorist weapon. Subsequently, 19 cases occurred in the hospital,
including four in other rooms on the patient’s floor, eight
on the floor above, and nine on the third floor. Two were contact
cases. One of the cases was in a visitor who had spent fewer than
15 minutes in the hospital and had only briefly opened a corridor
door, easily 30 feet from the patient’s room, to ask directions.
Three of the patients were nurses, one of whom died. This outbreak
occurred in a well-vaccinated population. (5:3)
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