A new study suggests that some people infected by the monkeypox outbreak in 2003 were protected by previous smallpox vaccinations. This finding could prove beneficial in the event of a bioterrorism attack, researchers say.

The monkeypox outbreak made 72 people ill in several Midwestern states, but did not cause any fatalities.

Using a new test, researchers found three people who were infected with the illness did not develop symptoms and five who had some symptoms but did not become severely ill. All had been previously vaccinated against smallpox, The Associated Press reports.

Mark K. Slifka, who lead the team of scientists, says the results show the outbreak was larger than realized and that protection against monkeypox can continue for decades after smallpox vaccination.

Some 100 million Americans have at least some protection from smallpox vaccines they received as children, even if it was decades ago, Slifka says to the AP.

The results of this study are published in the journal Nature Medicine