The science of Smallpox vaccination

All Hail the CDC, the ‘science’ of smallpox vaccination clearly explained.

(Emphasis mine)

Vaccinia (Smallpox) Vaccine

Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2001

Vaccine Efficacy

Neutralizing antibodies induced by vaccinia vaccine are genus-specific and cross-protective for other Orthopoxviruses (e.g., monkeypox, cowpox, and variola viruses) (16–18). Although the efficacy of vaccinia vaccine has never been measured precisely during controlled trials, epidemiologic studies demonstrate that an increased level of protection against smallpox persists for 10 years (19,20). Antibody levels after revaccination can remain high longer, conferring a greater period of immunity than occurs after primary vaccination alone (3,19). Administration of vaccinia vaccine within the first days after initial exposure to smallpox virus can reduce symptoms or prevent smallpox disease (2–4).

Although the level of antibody that protects against smallpox infection is unknown, after percutaneous administration of a standard dose of vaccinia vaccine, >95% of primary vaccinees (i.e., persons receiving their first dose of vaccine) will experience neutralizing or hemagglutination inhibition antibody at a titer of >1:10 (21). Neutralizing antibody titers of >1:10 persist among 75% of persons for 10 years after receiving second doses and The level of antibody required for protection against vaccinia virus infection is unknown also. However, when lack of local skin response to revaccination with an appropriately administered and potent vaccine dose is used as an indication of immunity, <10% of persons with neutralizing titers of >1:10 exhibit a primary-type response at revaccination, compared with >30% of persons with titers although it can also indicate unsuccessful vaccination because of improper administration or less potent vaccine.