Routine smallpox vaccinations were halted in the United States in 1972. Given the 10-year limit of protection, it was assumed that immunity had long ago worn off and everybody was susceptible to the virus.
The new study, which involved lab workers who conduct research with smallpox, found that their immune systems show signs of significant levels of protection up to 35 years after being vaccinated.
The finding could change the strategy for vaccinating against smallpox in the event of a bioterrorist attack, Jeffrey A. Frelinger, chairman of microbiology and immunology in the University of North Carolina School of Medicine and lead author on the study, told United Press International.
The federal government is trying to ramp up production of the smallpox vaccine so there is enough available by the end of the year to vaccinate every American in the event of a bioterrorist attack.
The Centers for Disease Control and Prevention reportedly is considering a strategy to begin vaccinating medical and emergency personnel who would be the first to respond to a smallpox outbreak, but no final decision has yet been made.
Smallpox is one of the most feared biological weapons because it is fatal in up to 30 percent of those infected and is highly contagious, meaning it rapidly could spread to infect thousands or hundreds of thousands of people.
Frelinger said he would vaccinate all first responders, whether they previously had been vaccinated or not, because it will be critical to ensure they do not become infected and can continue to treat victims.
If an attack occurs before a full stockpile of vaccine is ready, the new findings suggest it would be prudent to first begin vaccinating those born after routine smallpox vaccination was halted. This group of people in the United States would have no protection at all, whereas those born before 1972 likely still retain some immunity.
Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told UPI: "That is a moot point because we will have enough vaccine."
He noted 155 million doses are available now, more than enough to protect the number of potentially exposed people in the event of a bioterrorist attack.
In the study, Frelinger and a colleague drew blood samples from four laboratory workers who had been vaccinated against the virus within the past 5 years and nine other lab workers who had been vaccinated between 5 and more than 35 years ago.
After exposing the blood samples to the smallpox virus, the researchers found the body's immune system cells exhibited a response indicative of the ability to suppress the virus whether the person had been vaccinated 5 years ago or 35 years ago, Frelinger said.
Those vaccinated 35 years ago showed some drop-off in activity but it was only a small decline, he noted. They still had two-thirds the response of people vaccinated 5 years ago.
"We don't know if that is enough to protect people because it is unethical to expose these people to smallpox," he said. "But I would guess based on mouse studies that it probably is sufficient."
He noted studies of other viruses in mice had shown two-thirds immunity was sufficient for incurring resistance to infection.
Rafi Ahmed, director of the vaccine research center at Emory University in Atlanta, agreed. "We don't know if these people vaccinated long ago retain protective immunity, but we can be pretty sure that these individuals will do better than unvaccinated people," he told UPI.
They may not be completely protected from infection, but the severity of disease should be less, he added.
Ahmed noted the findings are supported by a previous study conducted in the early 90s that found people had active immune responses to the smallpox virus for up to 50 years after vaccination.
Frelinger also pointed out that during an outbreak of smallpox in Liverpool, England, in 1902, people who had been vaccinated 50 years previously were protected from death and serious disease. The vaccine being used at that time was essentially the same as the one in use today, so this suggests people may be protected for up to 50 years, he said.
If it holds true, it would mean people born as far back as 1942 may retain some protection against the smallpox virus, he said.
The findings appear in Wednesday's The New England Journal of Medicine.
(Reported by UPI medical correspondent Steve Mitchell in Washington.)