The smallpox vaccine can spread virus through sexual contact - watch out!

A man in San Diego who was recently received a smallpox vaccine passed a milder form of the vaccinia disease virus to someone he had sex with - and that man passed it on to someone else through sexual contact.

The surprising news was revealed today in the weekly Morbidity and Mortality Report (MMWR) from the Center for Disease Control and Prevention (CDC).

The 24-year-old man in San Diego who sexually contracted the vaccinia virus received it from another man who was given a smallpox vaccine as part of a Department of Defense vaccination program. The vaccination program began in 2002 in response to the bioterrorism threat, and was required for all military personnel and civilian employees.

CDC researchers said this is the first report of tertiary sexual transmission of the vaccinia virus from the smallpox vaccine - that is, transmission from a vaccinated person to an unvaccinated one, who then transmits it to a third person.

The United States eradicated smallpox in the general population in 1972 after an aggressive vaccination campaign, and the national smallpox vaccine program was discontinued after the worldwide eradication of smallpox.

Smallpox vaccines contain a weakened form of the live vaccinia virus, which is similar to smallpox but cannot cause smallpox itself. However, the live vaccinia virus can cause symptoms in people who have it, and can be contagious.

"The smallpox vaccine is a live-virus vaccine, and it's not news that it can infect people, but it cannot convert to smallpox," Dr. Marc Siegel, a clinical associate professor of medicine at NYU Langone Medical Center, told Medical Xpress. "It's a different virus; it's a kissing cousin of smallpox. It can be transmitted if you are not careful."

Vaccinia is related to the cowpox virus, which was used to create the first smallpox vaccine.

The vaccinia virus is not necessarily sexually transmitted. It can spread by skin-to-skin contact, when someone touches the smallpox vaccination site on another person or comes into contact with clothing contaminated by the virus.

Vaccinia symptoms are much milder than smallpox, generally consisting of rash, fever, and head and body aches.

The case was diagnosed when the 24-year-old man with secondary vaccinia virus transmission sought treatment at a private hospital in San Diego County for a rash in June 2012. The patient had lesions on his anus and lips, and developed fever, fatigue, and nausea.

The doctors diagnosed him with the vaccinia virus after realizing he had sex a week earlier with a man who received a smallpox vaccine, who had not kept his vaccination site adequately covered. The infected man had also had sex with a third man two days before he sought treatment. The third man sought treatment a week later after developing lesions on his forearm, penis, and scrotum, and experiencing malaise, sore throat, and nasal congestion.

Both infected men were then given a treatment of vaccinia antibodies to fight the infection, and recovered in several weeks with no complications. The CDC reported that neither patient had any sexual contact with others in the weeks after their treatment, so there was no additional transmission of the smallpox vaccine virus.

On the plus side, the two men's exposure to vaccinia means they are now immune to smallpox.

According a 2011 review, 115 cases of vaccinia virus transmission have been reported from vaccinated to unvaccinated people since 2002. Vaccinia is typically transmitted from smallpox vaccine recipients through sexual contact, though there have been reports of transmission at gyms, or from mother to child.

The CDC report stresses prevention recommendations for people who receive the smallpox vaccine.

Covering the smallpox vaccine injection site is most important in order to avoid spreading vaccinia to others, and people who are vaccinated should remember that they can spread the vaccinia virus through sexual contact.