Targeted Smallpox Vaccination Could Be Effective

By Amy Norton

NEW YORK (Reuters Health) - Small-scale smallpox vaccination begun after a bioterror attack with the virus could contain the threat nearly as effectively as vaccinating the US public at large, researchers said Thursday.

Under the right conditions, they report in the November 15th issue of Science, such "targeted" smallpox vaccination could protect Americans about as well as a mass, post-attack vaccination campaign would.

"Targeted vaccination would be nearly as good as mass vaccination in terms of the number of (smallpox) cases and deaths prevented," study co-author Dr. Ira M. Longini, Jr., told Reuters Health.

This estimate, though, assumes that Americans who got the smallpox vaccine before 1972, when it was discontinued in the US, have some leftover immunity to the virus, explained Longini, a researcher at Emory University in Atlanta, Georgia.

Whether long-ago vaccinated adults have such immunity is unclear, although there is evidence suggesting they may still harbor some smallpox protection. The eradication of smallpox was officially declared in 1980, and vaccination programs had stopped in all countries by the early 1980s, according to the World Health Organization.

Officially, the only stores of smallpox are in laboratories in the US and Russia, but several other countries -- including Iraq and North Korea -- are suspected of having samples of the deadly virus. And the theoretical risk of a bioterrorist attack with smallpox has officials in the US and elsewhere mulling over the best strategy for protecting the public.

There is no treatment for smallpox, which kills about one third of those infected. The disease begins with flu-like symptoms, followed by a rash, then pus-filled lesions on the face and body. Vaccination within a few days of exposure to the virus can prevent disease or reduce symptoms.

However, the smallpox vaccine itself -- which contains a live virus related to smallpox called vaccinia -- carries risks. When smallpox vaccination was routine, about 1,000 people per million had significant side effects such as an allergic reaction at the site of vaccination or spread of the vaccinia virus to other parts of the body. And for every million people vaccinated, one or two may die, according to the US Centers for Disease Control and Prevention.

It is, in part, due to concerns over the vaccine's safety that a limited vaccination response to a bioterror attack would be preferable -- if its effectiveness rivals that of a mass vaccination campaign, Dr. M. Elizabeth Halloran, the new study's lead author, pointed out in an interview.

Halloran, Longini and their colleagues used a computer model to simulate how intentionally-released smallpox might spread under several scenarios. They looked at the potential effectiveness of both targeted vaccination -- in which health workers locate and vaccinate only the close contacts of the first known or suspected smallpox patients -- and mass vaccination, in which any American who wanted the vaccine would get it.

The researchers found that, assuming some leftover smallpox immunity in the population, targeted vaccination could prevent almost as many infections and deaths as post-attack mass vaccination.

They point out that the effectiveness of targeted or mass vaccination might be enhanced by boosting the current smallpox immunity in the population. A possible way this could be done, according to the researchers, is to vaccinate the emergency workers who would respond to a smallpox attack and to offer voluntary, pre-emptive vaccination to others. Last month, US health officials issued recommendations to vaccinate more than half a million healthcare professionals.

The estimates from the new study differ from those of another recent report, in which investigators concluded that post-attack mass vaccination would be the best way to stop a smallpox epidemic. But Halloran and her colleagues used a different approach to simulate the spread of smallpox -- a model that assumes any one person usually interacts with a limited group of people, as opposed to the earlier study's "continuous population" model, which is based on "random mixing" of the general population.

Smallpox is thought to spread mainly through close and fairly prolonged contact, rather than the casual contact of sitting near someone on a bus, for example.

Halloran's team did find that "rapid" mass vaccination could be more effective than targeted vaccination, but its advantage over small-scale vaccination was much smaller than in the previous study, she noted.

So the question of how much leftover immunity is out there is key, Halloran said.

But, Longini added, "even though there's uncertainty, our results suggest that putting the vaccine where it's needed" -- the targeted approach -- "could be effective."

Still, neither this study nor the earlier one in which mass vaccination showed a distinct advantage will settle the questions surrounding smallpox, according to an accompanying editorial.

"Neither study will put to rest the current debate over the best policy for protecting the US population against a bioterrorist attack," writes Jim Koopman of the University of Michigan in Ann Arbor.

But, he adds, both studies should help guide the research policymakers will rely on to make their decisions.