The live vaccinia virus, contained in the vaccine, is similar to smallpox but doesn’t actually cause the disease. In the US, the disease was eradicated in 1972, and so members of the public were no longer vaccinated against smallpox. However, smallpox vaccination was resumed in 2002 by the Department of Defence, for its military personnel and civilian employees.
If you touch the vaccination site on a person who has been vaccinated, or you come into contact with clothing that has been contaminated with the virus, you can become infected with the vaccinia virus, and experience symptoms such as a rash, a fever, and head and body aches. According to a review published in 2011, there have been 115 reports of vaccinia virus transmission between vaccinated and unvaccinated people since 2002, mostly occuring through intimate contact, such as sexual relations, or contact between a mother and child. Transmission at gyms has also been reported.
However, the researchers said that the new case is the first to report tertiary transmission through sex. In this case, the recently vaccinated man did not keep his vaccination site properly covered, and so his male sexual partner became infected. This caused the infected man to experience a rash in his anal area and on his lip, as well as fever, fatigue and nausea. He later had sex with a different man, who, in turn, also caught the disease, and developed a rash on his penis and forearm.
Both men were treated and recovered within a few weeks, and study researcher Rachael Joseph, of the CDC’s Epidemic Intelligence Service, said that they are now immune to smallpox, because they were infected with the vaccinia virus. However, the researchers added that the study, which appeared in the CDC’s Morbidity and Mortality Weekly Report, underscores the importance of covering the vaccination site, as vaccinated people are instructed to do.