"Having the HLA DR15 risk gene did not appear to be affected by smoking or not," Simon said. But higher antibody levels of EBV did affect risk in those who had ever smoked, compared to those who had never smoked.
"The increasing risk of MS associated with higher EBV antibody [levels] was stronger among ever-smokers than never-smokers," Simon said. Among the participants with higher levels of the EBV antibody, smokers were twice as likely to have MS as those who had never smoked.
The association was not seen in those with lower antibody levels, however.
Exactly how the smoking enhances the link between the high antibody levels and MS risk isn't known, the researchers added.
Previous research has found those already diagnosed with MS who smoke are at higher risk for getting the brain lesions associated with the disease, and for brain shrinkage.
Overall, a person's lifetime risk of getting MS is one in 200 for women and one in 600 for men in the United States. Those with the higher antibody levels in the study had up to a twofold increase in risk if they smoked, compared to nonsmokers, the Harvard researchers found.
The new study provides more clues about who gets MS, said Patricia O'Looney, vice president of biomedical research for the National Multiple Sclerosis Society, who reviewed the findings. "What's unique and novel is, the study looks at three populations," she said, referring to the three large-scale studies from different parts of the world. "They looked at three geographically distinct populations and found the same thing."
While the Harvard team has found a link between EBV levels and MS, O'Looney said that "the association of EBV and MS is still under investigation." But the new research, she added, "looks at two risk factors that have been very prominent."
While the new findings don't yield many practical fi
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