Smallpox strategies simulated
Models call for vaccinations before
bioterror attack.
15 November 2002
KENDALL POWELL
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| Immunity levels of people
vaccinnated in the 1970s are uncertain. |
| © SPL |
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Pre-emptive vaccination of healthcare workers is the key to
thwarting a deadly smallpox epidemic, according to the latest
simulation1.
A previous study concluded that the best strategy would be to
vaccinate the entire US population after an attack2.
But vaccinating only those who are likely to be exposed
initially, such as healthcare workers, could be almost as
effective, say statistician Elizabeth Halloran of Emory
University in Atlanta, Georgia, and her colleagues.
The existing immunity of people vaccinated before 1972 - when
immunization of US children stopped - might reduce the need for
mass vaccination. "With residual immunity, the two strategies
become close," says Halloran.
The current US plan for a smallpox outbreak calls for
targeted vaccination, followed by mass vaccination if necessary.
But public-health and national-security agencies continue to
seek the best protection strategy.
Halloran says that targeted vaccination might be the best
strategy only if vaccine supplies are limited, or to reduce the
rare, but sometimes fatal, reactions to vaccination. The United
States stockpiles enough vaccine for all its residents.
Stopping the spread
Halloran's model charts the fates of 2,000 citizens in a
neighbourhood containing either one or five infected people. The
previous model, developed by a team led by Edward Kaplan of Yale
University, began with 1,000 infected people in a city of 10
million.
Kaplan's model assumes that people mix freely, resulting in
millions of interactions. In fact, says Halloran, people usually
make fewer than 10 contacts close enough to be infectious per
day.
"We were searching for a robust policy that could contain
worst-case scenario outbreaks," Kaplan counters. "That is common
sense - any intervention that can control the worst case can
also control anything less severe."
Kaplan is reassured that such different studies give similar
recommendations. Both show that any amount of pre-attack
vaccination limits the outbreak, and that the quicker the
response, the better.
Targeted vaccination was used to eradicate smallpox, but it
would not be as effective now, says Alan Zelicoff, a biowarfare
expert at Sandia National Laboratories in Albuquerque, New
Mexico. "Nailing down each case wouldn't work in New York City
in 2002," he says.
As more studies are done, notes Zelicoff, the number of
people recommended for pre-emptive vaccination has gone up and
up.
The federal government has called for about 500,000
healthcare 'first responders' to be vaccinated. Kaplan has
calculated that roughly 2 million people, including ambulance
drivers, police officers and vaccinators would be needed for a
mass vaccination effort. |
