All English stocks held at St Marys Hospital, London were transferred to more secure facilities at Porton Down and then to the U.S. at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia in 1982, and all South African stocks were destroyed in 1983. By 1984, the only known stocks were kept at the CDC in the U.S. and the State Research Center of Virology and Biotechnology (VECTOR) in Koltsovo, Russia. : 1273–76  These states report that their repositories are for possible anti-bioweaponry research and insurance if some obscure reservoir of natural smallpox is discovered in the future. Anti-terrorism preparation
Among more than 270,000 US military service members vaccinated with smallpox vaccine between December 2002, and March 2003, eighteen cases of probable myopericarditis were reported (all in first-time vaccinees who received the NYCBOH strain of vaccinia virus), an incidence of 7.8 per 100,000 during the 30 days they were observed. All cases were in young, otherwise healthy adult white men and all survived.In 2002, the United States government started a program to vaccinate 500,000 volunteer health care professionals throughout the country. Recipients were healthcare workers who would be first-line responders in the event of a bioterrorist attack. Many healthcare workers refused or did not pursue vaccination, worried about vaccine side effects, compensation and liability. Most did not see an immediate need for the vaccine. Some healthcare systems refused to participate, worried about becoming a destination for smallpox patients in the event of an epidemic.
Clinical trials have found that MVA-BN is safer and just as immunogenic as ACAM2000.LC16m8 is a replicating attenuated strain of vaccinia that is manufactured by Kaketsuken in Japan. Working at the Chiba Serum Institute in Japan, So Hashizume passaged the Lister strain 45 times in primary rabbit kidney cells, interrupting the process after passages 36, 42, and 45 to grow clones on chorioallantoic membrane and select for pock size. The resulting variant was designated LC16m8 (Lister clone 16, medium pocks, clone 8). Unlike the severely-damaged MVA, LC16m8 contains every gene that is present in the ancestral vaccinia. However, a single-nucleotide deletion truncates membrane protein B5R from a residue length of 317 to 92. Although the truncated protein decreases production of extracellular enveloped virus, animal models have shown that antibodies against other membrane proteins are sufficient for immunity. LC16m8 was approved in Japan in 1975 after testing in over 50,000 children. Vaccination with LC16m8 results in a vaccine "take," but safety is similar to MVA. Safety
Vaccinia is infectious, which improves its effectiveness, but causes serious complications for people with impaired immune systems (for example chemotherapy and AIDS patients) or history of eczema, and is not considered safe for pregnant women. A woman planning on conceiving should not receive smallpox immunization. Vaccines that only contain attenuated vaccinia viruses (an attenuated virus is one in which the pathogenicity has been decreased through serial passage) have been proposed, but some researchers have questioned the possible effectiveness of such a vaccine.