Vaccine for All? Math model supports mass smallpox inoculation
By Nathan Seppa
Widespread vaccinations after a terrorist attack with the smallpox virus would save thousands more lives than the response plan currently being considered by the U.S. government, a new study finds.
The government plan, devised by the Centers for Disease Control and Prevention (CDC) in Atlanta, would combat a smallpox outbreak by quarantining sick people and vaccinating anyone who had come in contact with an infected person. The plan calls for vaccinations of entire communities only if the more targeted approach doesn’t stop the outbreak.
Using a mathematical model, scientists at the Massachusetts Institute of Technology and Yale University applied the CDC approach to a hypothetical smallpox attack in which 1,000 people suddenly become infected in a city of 10 million. Such an outbreak would result in 367,000 cases of smallpox and 110,000 deaths–and would take nearly a year to quell–the scientists report in an upcoming issue of the Proceedings of the National Academy of Sciences.
In contrast, blanket vaccination of the whole city shortly after the initial cases would limit the damage to 1,830 cases and 560 deaths, and the outbreak would last only 115 days, the model indicates. Vaccinating 40 percent of the city’s population ahead of time would protect even more people, the researchers find.
Study coauthor Edward H. Kaplan of Yale University’s School of Management says that CDC’s plan is outdated because it’s based on lessons from natural epidemics. “We need a policy that withstands a worst-case assault by a really smart terrorist,” he says.
Theoretically, a single infected terrorist could spread the virus by coughing on people in subways, airports, and hospitals during the period of high contagion 7 to 17 days after his or her exposure, says William J. Bicknell of Boston University School of Public Health. “To find these contacts and vaccinate them is completely impossible,” he says. The CDC plan appears not to account for that limitation, he says.
Reports from past epidemics suggest that vaccination prevents the disease in an individual only if given within a few days of that person’s exposure, a time when he or she may feel no symptoms.
The new study makes an “extremely compelling” case for mass vaccinations before or shortly after an attack with the virus, Bicknell concludes.
CDC officials declined comment on the study. Bill Pierce, a spokesperson for the Department of Health and Human Services, says the government has included the new study among the information it’s gathering as it formulates a policy on smallpox terrorism.
About 100 million doses of smallpox vaccine are currently available, and there may be enough for the entire U.S. population by the end of this year.
Margaret Rennels of the University of Maryland School of Medicine in Baltimore serves on the committee that advises CDC on vaccination strategy. The panel, she says, was told to assume that the risk of a smallpox attack was low. After weighing the fact that many people would have side effects after receiving skin-prick smallpox vaccinations, and one person per million would die, the committee rejected mass vaccinations, she says.
“If the committee had been told that there will be an attack,” she says, “it may well have made a different recommendation.”
CDC has published its response strategy, which it calls an interim plan, online at http://www.cdc.gov/nip/smallpox/.
