New research suggests that the smallpox vaccine's protection may last much longer than previously indicated. While it was previously thought that the vaccine's effects lasted three to five years, preliminary results from a new study reveal that partial protection could last decades after the shot is given. Researchers from Oregon Health & Science University's Vaccine and Gene Therapy Institute note that over 90 percent of Americans over the age of 35 years have been vaccinated against smallpox, resulting in about 150 million people who probably have some level of detectable immunity against the deadly disease.