A study of immunity to smallpox in persons who have experienced a previous attack.

Most persons who recover from an attack of smallpox have lifelong immunity to a second attack but some persons lose their immunity more or less quickly and become susceptible to another attack. A study was made in Afghanistan in 1963-64 on 425 persons of all ages and both sexes who had previously suffered an attack of smallpox to determine their immunity to vaccinia virus. In all, 53.9% of those tested reacted positively to vaccination with vaccinia virus and there were no differences in response in males and females. It was demonstrated that the loss of immunity to vaccinia virus depends upon the time that has elapsed since the attack of smallpox and upon the severity of the attack. Loss of immunity to vaccinia virus rarely occurs among persons who have suffered an attack of smallpox less than 5 years previously. The authors point out that in some endemic areas, such as Afghanistan and Nepal, from 2.5% to more than 10% of the population has been affected by smallpox and should not be excluded from mass-vaccination campaigns.