An experimental smallpox vaccine believed to be safer than the traditional vaccination has been shown in laboratory tests to effectively protect monkeys and mice against a pox virus, researchers say.
A preparation called modified vaccinia Ankara (MVA) was found to prompt the immune system to produce antibodies and protective lymphocytes — white blood cells — about as effectively as the traditional smallpox vaccine, called Dryvax, when tested on monkeys.
Dr. Bernard Moss of the National Institute of Allergy and Infectious Diseases, one of the National Institutes of Health, said the test proved that MVA was safer because monkeys injected only with MVA did not develop the sores and other side effects common in traditional smallpox vaccinations.
A report on the monkey study is published this week in the journal Nature. A second study, in which mice were the test animals, is being published by the Proceedings of the National Academy of Sciences.
Milder side effects
Moss said when MVA was injected first into the monkeys and then followed with the traditional smallpox vaccine, some animals developed sores, but the lesions were much milder than with Dryvax alone.
The researchers used 24 cynomologus monkeys in the experiment and divided them into four groups of six, with three groups receiving vaccines and one receiving none.
One group received two shots of MVA, two months apart. A second group got one MVA shot followed in two months by the traditional smallpox vaccine. The third group received only the traditional vaccine.
Two months after the last injection, all the monkeys were exposed to monkeypox, a close cousin to smallpox.
The immunized monkeys remained healthy after the monkeypox exposure, although some of the animals that received only the MVA shots developed mild sores. The unvaccinated group of monkeys developed extensive sores and became seriously ill. One animal died and another had to be euthanized.
Moss said the study proves that MVA provides some pox protection, but that it has milder side effects than the Dryvax, or traditional smallpox vaccine.
Better for weak immune systems?
“We don’t believe the MVA will be better than the standard vaccine which is excellent, but we do believe it will be safer,” said Moss. “If it approaches the effectiveness of the standard vaccine, then it would be very useful for people who are unable to take the standard vaccine.”
In the mouse study, Moss and his co-workers found that MVA injections protected the lab animals against a lethal dose of vaccinia virus, a close relative of smallpox. Even mice that had compromised immune systems were able to survive the killing dose of vaccinia, said Moss.
This suggests, he said, that MVA may be a protective alternative to Dryvax for people with weak or crippled immune systems — such as cancer patients on chemotherapy or with HIV.
Researchers have sought a new smallpox shot because the traditional vaccine causes serious side effects in a small percentage of people.
The government recently offered to vaccinate medical workers, police and firefighters with the traditional vaccine to prepare the nation for a possible biological attack using smallpox. But about 2 percent of the more than 38,000 emergency response personnel who received the shots experienced some sort of serious side effect.
After the start of the war on terror, Bush administration officials were uneasy that stocks of smallpox virus could be used to make biological weapons. The government purchased doses of the traditional vaccine and intensified efforts to develop a safer smallpox shot.
The traditional vaccine uses a live vaccinia virus to produce a mild infection, which primes the immune system to protect against exposure to smallpox.
Moss said MVA is also based on vaccinia virus, but it is highly diluted and is unable to produce an infection.
