WASHINGTON (AP) _ 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 bioterror 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 a benefit in the event of a bioterror 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.
This shows 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 Sunday 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 U.S. 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 pointed out.
Mark L. Buller, an immunologist at Saint Louis University School of Medicine, said Slifka's findings seem ``very reasonable.''
Buller, author of a paper on the outbreak in Virology earlier this year, said he still believes the monkeypox virus that circulated in the United States was a mild version.
But Buller, who was not part of Slifka's team, noted that studies have shown residual protection for people who had received smallpox vaccinations.
``Although you get the disease, it protects you from the lethal effect,'' he said in a telephone interview.
That, combined with the weaker virus, resulted in less illness and no deaths in this country, Buller said.
Slifka's team also noted that some people came down with the disease, which was spread by prairie dogs that had been infected by an imported African rodent, despite not having been in contact with the prairie dogs.
``In one case a subject contracted monkeypox after an infected prairie dog was carried into her home when she was not present,'' they reported. The animal was not placed on the floor or furniture and yet the woman, who had no contact with the animal or infected people, came down with the disease.
Noting there were also other cases where people got ill but had no contact with the animals, they suggested it was spread by aerosols or by cloth or other materials.
This was the first time potential transmission through the air has been discussed, Buller said, ``they really tried to look at the potential for transmission.''
In their study, Slifka and colleagues used a test seeking antibodies to monkeypox that the immune system produces to fight the infection. They said it is the first test that can distinguish 100 percent of monkeypox infected people both from those without an infection and from people who have antibodies to smallpox from vaccination.
The study was funded by the Public Health Service and the Oregon National Primate Research Center.