'Target Zero' is the name given to what was thought to be the final phase of the smallpox eradication program at the eginning of Sep 1972. It was thought that smallpox incidence could be reduced to zero within 18 months. Although that proved not to be the case, 'Target Zero'

Before the 1967 Smallpox Eradication Programme, the "success" of an eradication programme was measured as the proportion of the population apparently vaccinated successfully. The plan drafted in 1966 for WHO's Director-General by Henderson and Dr Charles Cockburn states unequivocally that Further, "each case that occurs suggests the possibility of flaws in the programme. An outbreak, however small, demands a full critical review with appropriate revisions of the program."

Under "Target Zero", every case or suspected case of smallpox represented a potential failure. Every suspect case of smallpox would be dealt with as a public health emergency. Containment measures would be promptly and effectively carried out. In addition, an active search for cases would be required in many areas.

"Target Zero" became the programme's mantra. "Target Zero" T-shirts and bumper stickers were produced. The symbol of participation in the SEP, a small pin called the "Order of the Bifurcated Needle", was a bifurcated needle bent into a "zero" shape.

Target Zero — Progress Reports were issued approximately every 4 weeks from December 1972 through March 1976. The interruptions during 1975 reflect a crisis in the Bangladesh programme. Henderson "commuted" to Bangladesh from Geneva, spending 2-3 weeks there every month.