WASHINGTON, July 14 (Reuters) - Two weakened versions of the smallpox vaccine seem to work safely and are just as effective as the existing vaccine, considered the most dangerous vaccine in current use, researchers said on Monday.

And the study, done in mice, suggests the vaccines could be given through the nose, instead of the current, messy immunization that involves up to 15 little scratches on the arm, the researchers said.

The researchers, led by Igor Belyakov of the National Cancer Institute, did the first head-to-head comparison of the old Wyeth (nyse: WYE - news - people) DryVax vaccine with another, called Modified Vaccine Ankara or MVA and a third, genetically weakened vaccine called NYVAC made by Aventis-Pasteur <AVEP.PA>.

Smallpox was eradicated in 1979 and general vaccination stopped in the United States in 1972.

But the U.S. government believes a smallpox biological attack is possible, and is vaccinating 500,000 troops and tens of thousands of health care workers just in case.

They are using the old DryVax vaccine, and are not finding an especially high rate of side-effects in a young, healthy population being monitored carefully.

But doctors fear the vaccine could cause severe sickness and even death if it had to be used in the general population.

When widely used in the 1960s, DryVax killed between one and two in every million people immunized and caused severe reactions in up to 52 per million.

DryVax uses a live relative of the smallpox virus, called vaccinia. MVA and ALVAC -- designed to be the basis of an AIDS vaccine -- use a weakened form of the virus.

Writing in the Proceedings of the National Academy of Sciences, Belyakov's team said the vaccine seemed safer than DryVax and also worked when given to the mice via their noses.

The oozing lesion caused by smallpox vaccine is a big concern, because it can spread virus to people who have not been vaccinated and who should not be -- people with the AIDS virus for example, cancer patients and those with eczema.

When exposed to vaccinia virus, the vaccinated mice did not get sick, the researchers said.

Copyright 2003, Reuters News Service