This study will examine how people's immune systems respond to inoculation with vaccinia virus -the standard vaccine used to protect against smallpox-and how these responses correlate with symptoms they develop after receiving the vaccine.

People 18 years of age and older who are scheduled to receive smallpox vaccination as a routine part of their employment (e.g., laboratory worker, health care worker, or emergency response worker) may be eligible for this study. They may or may not have been vaccinated previously. In addition, individuals who were vaccinated against smallpox at least 6 months before starting the study may participate as control subjects. All candidates will be screened with a brief medical history and physical examination.

Participants in the following vaccination categories will undergo the procedures described for their group:

Vaccine Recipient - Frequent Follow-up

Participants will come to the NIH Clinical Center every 2 to 3 days for a total of 7 visits over a 2-week period. At each visit, starting the day of vaccination, they will have the following procedures:

• Brief skin examination, possibly with photographs of skin lesions;
• Throat and skin swabs for vaccinia virus culture;
• Blood draw (about 8 teaspoonfuls).

Additional blood samples will be collected 1 month after vaccination and again within a year after vaccination. The blood will be analyzed for the immune response to the vaccine, genetic differences that might influence differences in immune response, and the presence of vaccinia virus.

Participants will fill out a diary card every day for 3 weeks after vaccination to record any symptoms. Individuals who develop symptoms lasting more than 2 weeks, such as persistent or new skin lesions, will return to the clinic for additional skin exams and blood tests. Individuals who develop vaccine side effects may have a urine culture for vaccinia virus.

Vaccine Recipient - Infrequent Follow-up

Participants will come to the NIH Clinical Center for blood tests on the day of vaccination, 4 weeks after vaccination, and once again within a year after vaccination. At each visit, 6 teaspoonfuls of blood will be drawn. This group will also include individuals who have been vaccinated within 8 months of entering the study and are not currently receiving the vaccine, but for whom blood samples are not available.

Control Group - Vaccinated at Least 6 Months Before Entering the Study

Participants will come to the NIH Clinical Center for blood tests every 2 to 3 days for 2 weeks, then at 1 month after the first blood draw, and again within a year of the first blood draw. About 8 teaspoonfuls of blood will be drawn at each visit.