People With Smallpox Vaccine Can Transmit Infection to Some Bystanders

Contact After Vaccination Can Transmit Infection in High-Risk People

Oct. 15, 2002 -- If large numbers of people opt for smallpox vaccination, some people will die -- even if they don't get the vaccine. And many more people will risk disfiguring infections with the live vaccine virus.

How many? Nobody knows, but it's sure to be more than in the years of universal smallpox vaccination. That's the conclusion of an article in the Oct. 16 issue of The Journal of the American Medical Association. John M. Neff, MD, of the University of Washington in Seattle, and colleagues considered the implications of the CDC's plan for large-scale, voluntary smallpox vaccination to counter the threat of a bioterror attack with smallpox virus.

The vaccine uses a live virus called vaccinia -- cowpox virus -- instead of smallpox virus itself. But vaccinia can cause horrible infections in two types of people: those with poor immune systems and those with eczema. Because the vaccine is a living virus, infection can spread from contact between someone who is vaccinated and someone who isn't.

"The frequency of possible contact spread of vaccinia and the likelihood of adverse events cannot be predicted," Neff and colleagues write. Careful planning can keep these events to a minimum. But any large-scale smallpox vaccination plan will result in unintended cases of serious vaccinia infection.

Those most at risk are people with eczema. Accidental infection can also happen to people who get vaccinia virus in their eyes or mouths. Sensitive people can get an infection that can leave them horribly scarred. It can also kill. In the past, most deaths have been among infants who caught the virus from recently vaccinated brothers or sisters.

Huge CDC studies conducted after the 1963 and 1968 smallpox vaccination campaigns found fewer than 11 eczema-related vaccinia infections per million people vaccinated. With nearly 12 million people vaccinated in those two years, there were only three deaths. Nearly all of the cases were spread from one vaccinated family member to another member of the family. There were rare cases of hospital patients getting the virus from a recently vaccinated healthcare worker.

However, the authors note that more vaccinia infections would be expected today. In the 1960s, many people already were vaccinated. Routine vaccination in the U.S. ended in 1972, making the population highly vulnerable to vaccinia spread. Eczema-related vaccinia wasn't well defined in the 1960s, so many cases probably went unreported. Most importantly, there are many, many more people undergoing chemotherapy or infected with HIV who can't fight off vaccinia infection.

Another factor is that in the 1960s, people with vaccinia infection could be treated with immune serum from people who had recovered from vaccinia infection. Such immune serum is today in very low supply.

"An orderly, systematic approach along with careful screening to identify potential vaccinia-susceptible individuals and household contacts and close monitoring for adverse effects are essential to reduce the risk of transmission of vaccinia following smallpox vaccination," Neff and colleagues conclude.

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