"In general, people with neutralizing antibodies are believed to carry a low risk of reinfection," Yamanaka told a news conference. "This gives some hope" in relation to the vaccines set to be released, with the study showing that immunity obtained through natural infection can last at least six months, according to Yamanaka. He cautioned, however, that immunity acquired through natural infection is not necessarily the same as that obtained from a vaccine. The study was conducted on individuals who expressed an interest in participating, mostly from Tokyo, Kanagawa and Osaka prefectures. The average age of the participants, with equal numbers of men and women, stood at 49. Of the participants, 71% had mild symptoms, 19% moderate symptoms and 6% severe symptoms, while 4% showed no symptoms. The neutralizing antibodies were detected in 97% of the participants who were asymptomatic or had mild symptoms and 100% of those who experienced moderate or severe symptoms. Antibodies tended to be stronger in those whose symptoms had been more severe.