A "major reaction" is the internationally accepted term for a successful smallpox vaccination.
Major (i.e., primary) reaction is defined as a vesicular (blister) or pustular lesion or an area of definite palpable induration (hardness) or congestion surrounding a central lesion that might be a crust or an ulcer. The usual progression of the vaccination site after primary vaccination is as follows:
a. The inoculation site becomes reddened and itchy 3 to 4 days after vaccination.
b. A vesicle (blister) surrounded by a red areola then forms, which becomes umbilicated (sunken center) and then pustular (pus-filled) by days 7 to 11 after vaccination.
c. The pustule begins to dry, the redness subsides, and the lesion becomes crusted between the second and third week.
d. By the end of about the third or fourth week, the scab falls off, leaving a permanent scar that at first is pink in color, but eventually becomes flesh-colored.
Skin reactions after revaccination might be less pronounced with more rapid progression and healing than those after primary vaccinations. Revaccination is considered successful if a pustular lesion is present or an area of definite induration or congestion surrounding a central lesion (i.e., scab or ulcer) is visible upon examination 6 to 8 days after revaccination.
An individual (a) born before 1972, or (b) employed as a health care worker before 1977, or (c) who travelled internationally before 1983, or (d) who was on active duty before 1991 or after 2002, or (e) who has a Jennerian scar and who does not have a cutaneous response ('major reaction' or 'take') following Smallpox vaccination is presumed to have been previously vaccinated and therefore, in accordance with the ACAM2000 package insert, does not require a second vaccination attempt to try to elicit a cutaneous response. The patient is considered adequately protected against smallpox (immune) and is fit for all military-related assignments, including deployment. No further diagnostic evaluation is required.
A smallpox vaccinee not meeting the presumptive prior-vaccination criteria (above) who fails to demonstrate a cutaneous response ('major reaction' or 'take') after receiving 15 jabs with ACAM2000 requires a second vaccination attempt in accordance with current policy (reference?). If after a second attempt there is still no evidence of a cutaneous reaction the individual is considered adequately protected against smallpox (immune) for all military-related assignments, including deployment. No further diagnostic evaluation is required.