Senior author Dr. Gregory Poland, director of the Mayo Clinic Vaccine Research Group, said worldwide vaccination is believed to have eradicated smallpox, but the highly contagious and sometimes fatal illness remains a bioterrorism concern.
The research studied 44 participants from Mayo Clinic and the Naval Health Research Center who had received the smallpox vaccine -- members of the military -- in the previous 48 months.
Two samples were prepared from each of the 44, one uninfected and one that was infected with vaccinia, a smallpox-like virus. RNA -- ribonucleic acid, molecules that represent the DNA makeup -- from the samples was then tested in the high-speed sequencing facilities at Mayo Clinic's Center for Individualized Medicine.
Genetic differences were found between people with robust protective antibodies and those with lower immunity from smallpox, Poland said.
"We were looking into the intercellular reactions that occur when vaccinated and unvaccinated persons are exposed to and infected with smallpox virus. We were able to use blood samples taken directly from vaccinated patients," Poland said in a statement.
The findings were published online in the journal Genes and Immunity.
Notable deaths of 2014 [PHOTOS]