Tests of an experimental smallpox vaccine have produced encouraging results in monkeys and mice. The discovery may lead to an alternative for humans unable to receive current smallpox vaccines. Monkeys treated with modified vaccinia virus Ankara (MVA) were immune to monkeypox, which is similar to smallpox in humans, report Bernard Moss and colleagues in this Nature (Vol. 428, No. 6979, pp. 182-185). Monkeys that did not receive the treatment became severely ill after being exposed to monkeypox.
Monkeys were most resistant to the disease when treated with MVA and then with the existing smallpox vaccine Dryvax, the authors report. This suggests that MVA could be used as a "prevaccine", to which Dryvax could be added in the event of a specific smallpox threat, or as an alternative for those unable to take the existing vaccine for medical reasons.
In a separate study published in Proceedings of the National Academy of Sciences, Moss's team administered MVA and Dryvax to immunocompromised mice. Dryvax inoculations led to severe weight loss, skin sores, and eventual death. However, immunocompromised mice remained healthy even when treated with 1,000 times as much MVA.
The authors report that both MVA and Dryvax vaccinations led to similar levels of poxvirus immunity in healthy mice. Furthermore, MVA protected mice with specific immune deficiencies. These results suggest that MVA could be a suitable replacement for Dryvax in immunocompromised humans.
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