Smallpox immunity may persist from childhood

Health care workers who were vaccinated as children may be protected against fatal smallpox infection even if they declined to participate in recent immunization efforts, according to a recent study. Researchers report that more than 90% of volunteers vaccinated 25-75 years ago still maintain substantial humoral and/or cellular immunity against vaccinia, the virus used to vaccinate against smallpox. Antiviral antibody responses remained stable between one to 75 years after vaccination, whereas antiviral T-cell responses declined slowly, with a half-life of eight to 15 years.

"If these levels of immunity are considered to be at least partially protective, then the morbidity and mortality associated with an intentional smallpox outbreak would be substantially reduced because of pre-existing immunity in a large number of previously vaccinated individuals," the researchers concluded. However, even if those previously immunized only were infected mildly during a smallpox attack, they still could spread the disease, so health care workers would need to be immunized to protect patients.