A side-effect of vaccines has always been that, in some people, the treatment actually induces illness rather than immunity. After all, the idea behind vaccines, ever since Edward Jenner, is that exposure to a teeny bit of a pathogen is enough to kick in the body's antibodies and develop resistance, but not enough to actually foster disease. There's always been some imprecision in this, though, given that vaccines are deployed across huge populations - some number of which will always have lowered immunity or other risks that will mean even a small exposure is enough to germinate disease. That's all backstory to this week's MMWR report from the CDC, which includes a case of a woman who developed a strange infection after having sex with a Army soldier who'd recently had a smallpox vaccine. The genital infection wasn't herpes or any other common sexually transmitted disease. This was a second-hand infection, meaning she developed a disease even though she hadn't herself gotten vaccinated (thus making for a particularly tricky outbreak investigation).
At first glance, this is terrifying: If the woman got infected from a smallpox vaccine, does that mean she has smallpox? And since that diseaes was famously eradicated, are we at risk of spreading than bane of mankind all over again?
Thankfully, no on both counts. The virus used in smallpox vaccines isn't actually attenuated smallpox - it's closer to a strain of cowpox - vaccinia - which is similar enough to smallpox to foster immunity, but not nearly so lethal as the smallpox virus proper. Interestingly, this is the second case of secondary infections from smallpox vaccines that I've seen in recent months. In March, a child developed a dire case of eczema vaccinatum, a rare type of vaccinia virus skin infection, after contact with his father, who had recently been vaccinated against smallpox.
Oh, and you may be wondering: If smallpox has been eradicated from the face of the earth, why are we still vaccinating military personnel for the virus? Good question. It seems that even though the virus officially exists only in two, highly guarded places - at the CDC in Atlanta and at the Russian Vector State Research Center of Virology and Biotechnology in Siberia â€“ there's reason to believe the Soviets weren't content to just let it sit under lock and key, and instead developed smallpox as a possible bioweapon (there's a terrifying book by Ken Alibek that describes the Soviet effort to weaponize smallpox and other deadly pathogens - Biohazard).
Chance enough that as of 2002, all personnel are now vaccinated. So while there's no sign of smallpox out there (so far), there will continue to be these little eruptions of vaccinia, because of the vaccinations.