Why not cowpox? If it worked for Jenner, shouldn't it work now? It isn't fatal or even terribly serious.
Background: More than 200 years ago, in one of the first demonstrations of vaccination, Edward Jenner inoculated a young English boy with cowpox material from a dairymaid and showed that the boy became resistant to smallpox. Today, cowpox is a rare disease, largely confined to small mammals on the European continent and in Great Britain, with occasional transmission to humans. Most cases present with a small number of vesicopustular lesions on the hands or face that subsequently ulcerate and develop a black eschar before spontaneously resolving. Rarely, cutaneous dissemination and even death may occur.
Pathophysiology: Cowpox is caused by the cowpox or catpox virus, a member of the orthopoxvirus family, which also includes smallpox and vaccinia. The virus is thought to be acquired by direct contact with an infected animal, most often a cat in the case of humans, with lesions occurring where the virus gains access through broken skin. Infection generally remains localized at the initial site of inoculation, though lymphatic spread in a sporotrichoid pattern and generalized skin infection have been reported.
Frequency:
In the US: Cowpox has never been reported in the United States.
Internationally: Cowpox is a very rare infection of humans, with only approximately 60 human cases ever reported. The majority of cases have been reported in Great Britain, with a small number from Germany, Belgium, France, Sweden, and Norway. Most cases occur in the late summer and fall.
Mortality/Morbidity: Human cowpox is normally a self-limited disease. The host immune response is usually sufficient to control the viral infection and the only sequelae are scars at the site of the pox lesions. Of the 3 cases of severe generalized skin infection that have been reported, 2 of the patients had atopic dermatitis, and the third patient had hay fever.
Yes cowpox too and there might be others that provide cross-immunity against smallpox. All these are natural diseases that can be beneficial by preventing a far worse disease and they aren't attenuated vaccines like some so there's nothing really that special about them. It makes me wonder on one hand why the government can come up with that line about it taking months and months to make it available to the public because viruses don't take that long to replicate, but then on the other hand it wouldn't be difficult to get around the government either if it became necessary.