These programs became immense and were conducted at dozens of secret sites employing up to 65,000 people. Annualized production capacity for weaponized smallpox, for example, was 90 to 100 tons. In the 1980s and 1990s, many of these agents were genetically altered to resist heat, cold, and antibiotics. In the 1990s, Boris Yeltsin admitted to an offensive biological weapons program as well as to the true nature of the Sverdlovsk biological weapons accident of 1979, which had resulted in the deaths of at least 64 people. Defecting Soviet bioweaponeers such as Vladimir Pasechnik and Colonel Kanatjan Alibekov confirmed that the program had been massive and still existed. In 1992, a Trilateral Agreement was signed with the United States and the United Kingdom promising to end biological weapons programs and convert facilities to benevolent purposes, but compliance with the agreement—and the fate of the former Soviet bio-agents and facilities—is still mostly undocumented.