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The War Against Smallpox David W. Tschanz, MSPH, PhD

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Pushing the Enemy Back

Slowly, inexorably, smallpox was beaten back as a result of vaccination and the effects of herd immunity. New cases and mortality decreased throughout the 1800s and into this century. As a result of systematic vaccination smallpox vanished from England by 1940, from the United States by 1950, from China by 1965. By the mid-1960s the World Health Organization, announced an ambitious plan to fulfill Jefferson's promise to Jenner that "future generations will know by history only that loathsome smallpox has existed."

In theory smallpox eradication was practical. The symptoms of smallpox were obvious and needed no confirmatory laboratory tests. Furthermore, anyone contracting smallpox developed the disease. There simply was no carrier state, and there were no asymptomatic infections. Transmission was from person to person so there were no vectors or intermediate hosts to contend with. Man was the only known reservoir, hence the virus had no animal reservoir to retreat to. Patients were only infectious in the four days prior to onset, allowing for rapid tracking. Long term, if not lifetime, immunity followed vaccination or natural infection. The virus could not survive outside the human body. And finally, alone among the anti-viral vaccines, smallpox did not use the actual variola virus or any of its components -- there was not the slightest chance of a vaccine related case.

Materiel

The first problem confronting Henderson and the eradicators was the vaccine itself. A great deal of progress had been made since Jenner's first experiment. Arm to arm vaccination was no longer the approved method. The vaccine in use at the start of the campaign was fairly stable. Though ideally it should be refrigerated, it could survive for one month if kept cool. Since it could be freeze-dried, transport was no problem. Once shaken in diluent of sterile water or saline, it was ready.

The problem lay in its manufacture. There was no standardization among the various sources with regard to potency, stability and purity. Some samples of vaccine revealed no virus whatsoever. Under Henderson's prodding, cajoling and outright demanding, all the participants in the program agreed to submit samples of their vaccine to either a laboratory in the Netherlands or one in England for quality control.

The device to deliver it was equally effective -- the traditional scratching fork was displaced by a rapid-fire jet- gun, which in turn gave way to the bifurcated needle that delivered just one drop and needed neither repair nor maintenance.

Strategy

When local and regional smallpox eradication programs had begun in 1959, the strategy had called for mass vaccination campaigns designed to reach 80% of the population. This proved unsatisfactory and in 1964 WHO called for 100% coverage (Henderson, 1980). The one bit of good news was that the eradication effort was proving that immunity provided by high quality vaccine was surprising durable, and that boosters, originally thought essential, were unnecessary. However even with this discovery, 100% vaccination seemed unachievable. There simply was neither enough vaccine nor enough field workers to blanket the planet.

Tactics

Then in 1968, William Foege, director of the Western and Central Africa eradication effort, ran short of vaccine. He abandoned the concept of 100% coverage in favor of another strategy based on rapid surveillance. When word of a case or an outbreak was received, Foege targeted the outbreak village and those surrounding it for intensive containment vaccination. The new strategy essentially isolated the existing smallpox cases behind a wall of immune individuals. Transmission was interrupted and the outbreak died out within the containment area. Smallpox could be eradicated -- even when the overall population was less than 50%.

Foege's discovery meant that detection and containment of outbreaks was not of equal, but of greater priority, than mass vaccination. Henderson ordered a shift in emphasis. The primary strategy was the rapid detection and containment of each outbreak. Since smallpox must spread from person to person in a continuing chain of infection, the objective of the strategy was to isolate each of the chains by isolating the case and vaccinating the contacts. Military theoreticians call this approach either "divide and conquer" or "defeating the enemy in detail." The elegance of the strategy was that if surveillance was maintained at a high level, smallpox could be defeated with much less effort than initially thought necessary.

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