Smallpox Vaccine Is Oldest, Least Safe
Mon Jul 8, 4:08 PM ET

By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) - When Edward Jenner tested his experimental vaccine against smallpox in 1796, he scraped the oozing pustule of a calf infected with cowpox and used a knife to press it into the skin of a small boy.

Today's vaccine is not much more advanced, although the boy escaped smallpox infection and the modern vaccine eradicated the disease.

But a vaccine that uses a live virus is harder to control, and this is the problem that U.S. health officials are struggling with in trying to decide who to vaccinate against smallpox.

Smallpox was declared eradicated in 1980, but the United States is worried that the virus may be used in a biological attack. The U.S. government is working to "stretch out" existing supplies of vaccine by diluting it and has ordered millions of new doses.

Routine vaccination stopped in the 1970s, so there has been no incentive to update the old tried-and-true vaccine to modern standards.

It is made from the lymph of heifers infected with cowpox -- a virus so closely related to smallpox that the body uses the same defenses for both. A person who has been infected with cowpox, a usually benign virus, is immune to smallpox.

The lymph in the vaccine is literally scraped from the pustules that cowpox causes in a calf.

FILTERING THE HAIR OUT

Dr. Donald Henderson, who helped lead the global campaign that wiped out smallpox as a naturally occurring disease, describes it as being made by "scraping the blisters off an infected calf and filtering the hair out."

Because of this crude manufacturing method, and because the vaccine uses a live virus, it had a relatively high rate of side-effects, compared to more modern vaccines such as those against measles and hepatitis.

About three of every 1 million people who got the old DryVax smallpox vaccine developed encephalitis, an inflammation of the brain, and two in 1 million died. Anywhere between seven and nine people, mostly infants, died every year in the United States from the smallpox vaccine.

During the last U.S. smallpox outbreak, in New York in 1947, 11 people got smallpox and two died. In the mass vaccination campaign that followed, 6 million people were immunized in a month, and nine died from vaccine-related complications.

A newer vaccine is being formulated from cells grown in the laboratory but experts expect that about one in 10,000 people who are inoculated will have side-effects severe enough to send them to a doctor.

In comparison, many fewer children given the combined measles, mumps and rubella vaccine develop severe side effects and none has died as a result of the vaccination since 1990.

HOW MANY TO VACCINATE?

The other issue is deciding how many people must be vaccinated to control an outbreak.

Edward Kaplan, who makes computer models of epidemics at Yale University, questions the current interim policy of "ring vaccination." This calls for isolating anyone who catches smallpox and vaccinating everyone they may have had contact with.

Ring vaccination worked, Kaplan said, when most of the population either was vaccinated or had immunity by virtue of having survived an infection. It also worked in small, isolated villages.

In a modern, mobile society, he says the best approach would be to vaccinate everyone in the affected region.

A population that never had been exposed to smallpox would be at much higher risk -- just as the American Indians were when European settlers brought smallpox with them.

University of California at Los Angeles physiologist Jared Diamond in his book "Guns, Germs and Steel" proposes that Old World diseases such as measles and smallpox wiped out 95 percent of the Indians who once populated the Americas.

"Suppose I am an unfortunate person who has been infected with smallpox. I am feeling lousy, but not lousy enough to stay away from work," Kaplan proposes.

When he finally is diagnosed with smallpox, two days later, he then must remember everyone he spoke or to sneezed on in the office or on the way to work.

During these two days, anyone he infected would be incubating and passing on the virus themselves.

But other experts point out that smallpox, which has an incubation period of seven to 17 days, cannot be passed on until the characteristic pustules form.

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