In March 2003, the French Ministry of Health implemented a program on preparedness and response to a biological attack using smallpox as weapon. This program included the establishment of a preoutbreak national team that could be revaccinated against smallpox.

To identify demographic and clinical factors associated with vaccination success defined as the presence of a pustule at the inoculation site at day 8 (days 7–9), with an undiluted vaccinia virus derived from a Lister strain among preimmunized volunteers.

From March 2003 to November 2006, we have studied prospectively 226 eligible volunteers. Demographic data were recorded for each volunteer (age, sex, number of previously smallpox vaccinations and date of the last vaccination). Smallpox vaccine adverse reactions were diagnosed on the basis of clinical examination performed at days 0, 7, 14, 21 and 28 after revaccination.

A total of 226 volunteers (sex ratio H/F = 2.7) were revaccinated. Median age was 45 years (range: 27–63 yrs). All volunteers completed follow-up. Median number of vaccinations before revaccination was 2 (range: 1–8). The median delay between time of the study and the last vaccination was 29 years (range; 18–60 yrs). Sixty-one volunteers (27%) experienced one (n = 40) or more (n = 21) minor side effects during the 2–14 days after revaccination. Successful vaccination was noted in 216/226 volunteers (95.6%) at day 8 and the median of the pustule diameter was 5 mm (range: 1–20 mm). Size of the pustule at day 8 was correlated with age (p = 0.03) and with the presence of axillary adenopathy after revaccination (p = 0.007). Sex, number of prior vaccinations, delay between the last vaccination and revaccination, and local or systemic side effects with the exception of axillary adenopathy, were not correlated with the size of the pustule at day 8.

Previously vaccinated volunteers can be successfully revaccinated with the Lister strain.