And Now, the Good News About Smallpox

In the event of a terrorist attack, we’re not all toast.

If you received a smallpox vaccine in infancy, as most everyone did in the United States before routine immunizations stopped in 1972, your immunity to this disfiguring and often lethal disease certainly has waned. Indeed, authoritative sources would have you believe that you have no immunity whatsoever. But if you dig out original scientific studies about the smallpox vaccine, a much different—and a much more optimistic—picture emerges.

According to U.S. Census Bureau data, about 40 percent of the U.S. population is 29 or younger, and having never received a smallpox immunization, up to 30 percent of that cohort would die if infected with the virus during a bioterrorist attack. But what of the remainder of the population, the 60 percent that got the vaccine at one point or another? What is their vulnerability?

The Centers for Disease Control and Prevention Web site offers this depressing answer in a smallpox FAQ: “Most estimates suggest immunity from vaccination lasts 3 to 5 years.” In 1999, leading experts offered similar estimates in a “ consensus statement ” on smallpox as a biological weapon that they published in the Journal of the American Medical Association. “Because comparatively few persons today have been successfully vaccinated on more than 1 occasion, it must be assumed that the population at large is highly susceptible to infection,” they concluded. “ Dark Winter,” a war game conducted in June at Andrews Air Force Base in which a smallpox “attack” was launched, proposed that 80 percent of the U.S. population is susceptible to the disease.

But data from a 1902-1903 smallpox outbreak in Liverpool, England, strongly suggest otherwise. A study analyzed the impact of the disease on 1,163 Liverpudlians, 943 who received the vaccine during infancy, and 220 who were never vaccinated. The study further separated people by age and by the severity of their disease. In the oldest age group, 50 and above, 93 percent of the vaccinated people escaped severe disease and death. In contrast, 50 percent of the unvaccinated in that age bracket died, and another 25 percent had severe disease. [Addendum Oct. 29 2001: I made a mistake about the extra 25 percent. Acctually 50 percent of the unvaccinated had severe disease and died. I misread one of the columns in a table that described the results of this study. My apologies—although it does not change the bottom line regarding the remarkable durability of vaccination.] To put it plainly, the vaccine offered remarkable protection after 50 years.

Frank Fenner, a virologist at Australia’s John Curtin School of Medicine who co-authored Smallpox and Its Eradication—a 1,400-page book that is the field’s bible—says the Liverpool study remains the best evidence that vaccine immunity lasts for decades. The Liverpool study, paradoxically, also helped create the common wisdom that vaccine immunity rapidly wanes. In the Liverpool study, Fenner notes, vaccinated kids who were 14 and younger had zero cases of severe disease or death. So out of “conservatism,” he explains, many smallpox experts began to advocate that anyone in an area where smallpox exists should be revaccinated every decade (Australia went one step further and said every five years). An added benefit of this aggressive vaccination policy was that it also slowed the spread of smallpox, because recently vaccinated people were less likely to transmit the virus than those who had received their immunizations decades before.

More recent data supports the Liverpool experience. In a 1996 study published in the Journal of Virology, a group led by Francis Ennis at the University of Massachusetts Medical Center pulled immune cells out of people who had received the smallpox vaccine decades before. When they tickled these cells to see whether they remembered the lesson the vaccine had taught them, they found that “immunity can persist for up to 50 years after immunization against smallpox.”

James Leduc, the CDC’s resident smallpox authority, concedes that the conventional wisdom posted on the CDC’s Web site might not tell the whole story. “The issues that you are raising are absolutely accurate and well founded,” he says. “What you see on the Web site is a first attempt to get a consistent message out,” he says, explaining that the public health quandaries—such as the need to produce more vaccine—sometimes overshadow the scientific ones.

Fenner, like several other smallpox experts queried, has no idea how much protective immunity exists now in the United States. “Oh, gosh, it is a guess,” he says. But as Bernard Moss, a researcher who works with the smallpox vaccine at the National Institute of Allergy and Infectious Diseases, stresses, a vaccine simply gives the immune system a head start in the race against a bug. In the case of smallpox, the bug is fairly slow to cause disease—symptoms typically don’t surface for a few weeks—and an infection in a vaccinated person can act like a booster shot, revving up an already primed immune system. “Everyone would agree that if you had a vaccination in your life,” says Moss, “you’re much better off than if you hadn’t.”

None of this good news argues against rebuilding the nation’s smallpox vaccine stockpile, which has dwindled to a mere 15.4 million doses. (The federal government has committed more than $500 million to produce 300 million doses.) Regardless of our country’s precise immune status against smallpox, widespread use of the vaccine during outbreaks repeatedly has worked: New York City dramatically aborted an epidemic in 1947 with a rapid and aggressive vaccination (and, importantly, isolation of victims) campaign that limited the spread to 12 cases and two deaths. And surely we have become more vulnerable to smallpox since routine immunizations stopped.

But the good news inspires the sort of confidence the country needs right now: The entire population isn’t at extreme risk in the event of a smallpox attack. As the CDC’s Leduc says, “This is not going to be a wildfire that overtakes the world.”

Addendum: Oct. 27, 2001:

Many people who are under 29 wrote me sharply worded notes after this article was posted. I thought it was implicit that if older people had some immunity, this should impact decisions about distribution of the vaccine, should a smallpox outbreak become a reality before the government builds a sufficient stockpile to immunize everyone. So as to avoid any further confusion, let me be even more explicit: If I had a say in it (and I do not), I would advocate that limited supplies of smallpox vaccine should first go to health-care workers, police, firefighters, and other first-responders in the area. After that, I think it makes sense to vaccinate the unvaccinated population before the vaccinated, which means the under-29-year-olds would be at the front of the line.