WASHINGTON -- Some people infected in the monkeypox outbreak in 2003 were protected by previous smallpox vaccinations, a finding that could be of a benefit in the event of a bioterrorist attack, a new study suggests.
As many as 100 million Americans have at least some residual protection from smallpox vaccines they received as children, even if it was many decades ago, said Mark K. Slifka, who led a team that conducted the study.
This potentially could be helpful in the event of a bioterrorist attack, said Slifka of Oregon Health & Science University.
The 2003 outbreak of monkeypox, a related illness, sickened 72 people in several Midwestern states, but there were no deaths. While monkeypox kills about 10 percent of victims in Africa, the lack of fatalities here has been attributed to better medical care and the possibility that the strain of the disease was weaker than that in Africa.
Using a new test, researchers have found three people who were infected with monkeypox but who developed no symptoms of the illness and five who had some symptoms but did not become severely ill, who previously tested negative or equivocal for the virus. All had been previously vaccinated against smallpox.
The result indicates that the outbreak was larger than was realized and, more importantly, that protection against monkeypox can continue for decades after smallpox vaccination, the team led by Slifka reported yesterday in the online edition of the journal Nature Medicine.
''Unvaccinated people were the most susceptible," Slifka said in a telephone interview. ''There were some serious cases including two children, one of whom was in a coma for 12 days."
He noted that in Africa people who have had smallpox vaccinations rarely succumb to monkeypox, an indication they may have at least partial protection.
Slifka said he doesn't think the strain of monkeypox in this country was less virulent than that in Africa.
Routine smallpox vaccination was halted in 1972 in the United States, more than 20 years after the last case of smallpox in this country.
The US outbreak of monkeypox provided a rare opportunity to test whether there was residual protection, Slifka said.
Such a study could not be done in Africa, where monkeypox occurs, or Europe where cowpox is common, because any residual protection might be a result of exposure to those viruses, he noted.