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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