(HealthDay)—A man recently vaccinated for smallpox under a U.S. Defense Department program passed a milder, related form of the disease on to a man he had sex with, and that man then passed it on to yet another man, federal health officials reported Thursday.
The virus, called the vaccinia virus, is the virus used in the smallpox vaccine. It is related to smallpox and helps the body develop immunity to smallpox. According to the U.S. Centers for Disease Control and Prevention, the smallpox vaccine does not contain the actual smallpox virus and cannot cause smallpox.
"The smallpox vaccine is a live-virus vaccine, and it's not news that it can infect people, but it cannot convert to smallpox," said Dr. Marc Siegel, a clinical associate professor of medicine at NYU Langone Medical Center in New York City.
"It's a different virus; it's a kissing cousin of smallpox," he said. "It can be transmitted if you are not careful."
Smallpox has been eradicated across the world, Siegel said, so most people are no longer vaccinated against it. The only reason someone is vaccinated today is because of fear of bioterriosm, he said.
The report was published in the March 1 issue of the CDC's Morbidity and Mortality Weekly Report.
In this case, a 24-year-old man in San Diego went to the hospital complaining of a rash, which he thought might have been caused by having sexual contact with a man who had received the smallpox vaccine.
In addition, the patient had sores on the anus and lips. The patient's condition cleared up without complications, according the report.
A second man, however, also developed similar sores after having sex with the first patient. Sores also developed on the patient's penis and scrotum. These, too, were caused by the vaccinia virus.
This patient was hospitalized and treated with immune globulin intravenous, which the CDC supplied. After treatment, the patient was discharged and his condition also cleared up with no further complications.
The vaccinated man was interviewed and confirmed that the first patient was his only sex partner in the two to 30 days after he had been vaccinated, according to the report.
Transmission of the vaccinia virus is not uncommon, especially if the wound left by the vaccine isn't kept covered as it heals, the CDC said. The open wound can easily transfer the virus to other parts of the body or other people.
This report, however, highlights the potential for the virus to be transmitted through sexual contact and reinforces the need to keep the vaccination wound covered, the agency said.
Explore further: Smallpox outbreak: How long would it take for vaccines to protect people? Would it work?
For more on smallpox vaccine, visit the U.S. Centers for Disease Control and Prevention.