Smallpox vaccination confers long lasting immunityBMJ 2003; 326 doi: https://doi.org/10.1136/bmj.326.7400.1164-d (Published 29 May 2003) Cite this as: BMJ 2003;326:1164
Immunity to smallpox after vaccination can be long lasting, suggests a study conducted by Oregon Health Sciences University researchers and presented at the meeting of the American Society for Microbiology in Washington, DC.
The finding will reassure people in the United States who are worried about bioterrorism, because it means that any attempt to induce a smallpox epidemic is likely to be of limited effect, as 95% of US citizens over the age of 35 years have been vaccinated against the virus.
Moreover, the finding may spur unvaccinated people to be vaccinated and reassure people who have opted not to be revaccinated that they retain some level of protection against the disease from vaccinations received decades ago.
The Oregon scientists, led by Dr Mark Slifka and Dr Erika Hammarlund of the university's Departments of Molecular Microbiology and Immunology in Portland, collected blood samples from 306 volunteers who had received the smallpox vaccine. The vaccine is composed of a live attenuated strain of vaccinia, or cowpox, and provides protection against the variola or smallpox virus, to which it is genetically related.
The study volunteers were a mixed group and included people who were vaccinated as long as 75 years ago and others who had been vaccinated as recently as in the past year. Volunteers were recruited from 40 US states and 34 other countries.
Volunteers' blood cells were tested for activity against vaccinia in a Petri dish. Also, the researchers measured antibody titres directed against vaccinia and T cell responses. Unrestrained vaccinia virus typically destroys cultured cells.
Antibodies in serum from vaccinated volunteers were able to neutralise the virus, and the strength of that immunity could be determined by examination of how many cell colonies survived a vaccinia attack in the presence of antibody rich serum.
The researchers found that everyone who had been vaccinated had functional antibodies and that activity against the virus was relatively robust, regardless of time since vaccination. However, the T cell response did wane over time.
Participants who had had a booster vaccine had better immunity, but revaccination beyond two times did not increase protection considerably.
The results support other work that found long term immunity from smallpox vaccination. A recent small study from North Carolina found a durable T cell response to vaccinia in people immunised up to 35 years earlier and concluded that smallpox vaccine could give long lasting immunity (New England Journal of Medicine 2002;347:689-90).
In the United States, endogenous smallpox was eradicated in 1949 and routine smallpox vaccination was stopped in 1972. The current vaccination campaign has aimed to vaccinate 500 000 healthcare workers to serve as first responders to a possible smallpox bioterrorist attack.
However, the effort received a lukewarm response, with only 35 000 healthcare workers volunteering for vaccination. Although the vaccine is generally safe, complications can occur, including generalised vaccinia, auto-inoculation, and myopericarditis. Because the risk of smallpox attack could not be quantified, many people were reluctant to risk possible side effects and complications (BMJ 2003;326:179).