(from
http://www.nature.com/nsu/021111/021111-13.html)
Smallpox strategies simulated
Models call for vaccinations before bioterror attack.
15 November 2002
KENDALL POWELL
Immunity levels of people vaccinnated in the 1970s are uncertain.
© SPL
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.
I wonder if you can take the vaccine while pregnant or breastfeeding? There are plenty of people with compromised immmune systems that might have problems with it, but what about pregnant women?