The minimum protective level of antibodies in smallpox.

Blood samples from 57 contacts of 6 smallpox cases were tested for haemagglutination-inhibiting (HI) and neutralizing antibodies. All 6 contacts who subsequently developed smallpox were unvaccinated and had neutralizing antibody titres of 10 or less. However, 6 unvaccinated contacts with similar antibody levels did not develop smallpox. None of the 41 vaccinated contacts, regardless of their antibody level, contracted the disease.