Smallpox outbreak response: Targeted vaccination may be almost as effective as mass vaccination

November 14, 2002

This news release is also available in French.

Vaccinating those people in close contact with smallpox victims may be almost as effective as vaccinating the entire population, in the event of an outbreak, scientists have found. The study appears in the journal Science, published by the American Association for the Advancement of Science.

Using a computer model to investigate how smallpox might spread after being introduced by a few infected individuals, Elizabeth Halloran of the Rollins School of Public Health at Emory University and her colleagues also found that the success of a vaccination effort hinged, to a large degree, on the level of "pre-existing immunity" in the general population.

Researchers have no way of determining how much leftover protection people over thirty still have as a result of the stippled vaccines they received as children, up until 1972, according to Halloran.

One way to increase the level of existing immunity before a smallpox outbreak may be to vaccinate volunteers and "first responders"--hospital workers and others most likely to encounter the virus, according to the researchers.

If people simply stayed home during their illness, that significantly reduced the spread of the disease and increased the effectiveness of vaccination, the researchers also found.

"The last thing you want is thousands of people running out to get vaccinated [after smallpox has been introduced]," said Halloran. "The idea is that people should just stay calm and stay home."

Based on information from the U.S. Census, the researchers' computer model simulated the possible ways that smallpox could spread through a community of 2,000 people, and the impacts of various types of vaccination efforts.

Whether to use mass or targeted vaccination in response to intentionally spread smallpox has been a matter of debate among researchers. The vaccine, which is a live cowpox virus, poses a small risk of serious side effects.

Compared to the targeted approach, mass vaccination before any smallpox introduction or immediately thereafter was more effective at preventing and containing epidemics--when no prior immunity existed. (The researchers assumed that any leftover immunity would be half as protective as immunity from a fresh vaccination.)

With such leftover immunity, however, the effectiveness of both types of vaccination efforts after smallpox introduction increased, the authors report. Targeted vaccination became the more effective approach if vaccination was delayed until multiple cases had been detected.

Under all the scenarios, targeted vaccination prevented more cases per dose of vaccine.

"Our findings suggest that increasing the level of immunity in the population, possibly by vaccinating first responders or allowing people to be vaccinated if they so choose, would make a post-attack effort more effective," Halloran said. "Also, at increased levels of immunity in the population, targeting close contacts of smallpox cases may become competitive with mass vaccination as the strategy of choice"

"In this model people interact in known contact groups such as households, schools, and neighborhoods. In addition, transmission occurs more probably in closer contact, such as in households than in neighborhoods," said co-author Ira Longini of the Rollins School of Public Health at Emory University. "This aspect captures the general belief that smallpox is spread more effectively by close contact and allows the targeted strategy to have a better chance of success."

Historically, smallpox infection rates are higher in children than in adults, according to Halloran. Thus, the model includes more opportunities for children to spread the virus.

If 80 percent of the entire population were vaccinated (mass vaccination) once the 15th smallpox case occurred, and no immunity from earlier vaccinations remained, there would be 9.4 deaths for every 1,000 people, according to the model results. If some immunity did remain, the death rate would drop to 2.4 per thousand.

If 80 percent of the people in close contact with identified smallpox cases were vaccinated (targeted vaccination), there would be 19.6 deaths out of every thousand without preexisting immunity. With such immunity, however, the death rates dropped to 1.8 per thousand, the authors report.

Each death rate was lowest in scenarios where the vaccinations took place right after the first smallpox case occurred, and increased the longer the vaccinations were delayed.

"For all strategies, rapid response can make the difference between preventing versus merely containing an epidemic," the authors write in their Science study.

A related commentary by Jim Koopman of the University of Michigan at Ann Arbor accompanies the Science report.

The other authors of the study are Azhar Nizam, and Yang Yang of the Rollins School of Public Health, Emory University. The research was partially funded by the Fogarty International Center and the National Institutes of Health.

American Association for the Advancement of Science

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