Clinical characteristics of the first three waves of hospitalised patients with COVID-19 in Japan prior to the widespread use of vaccination: a nationwide observational study by Nobuaki Matsunaga, Kayoko Hayakawa, Yusuke Asai, Shinya Tsuzuki, Mari Terada, Setsuko Suzuki, Hiroshi Ohtsu, Koji Kitajima, Ako Toyoda, Kumiko Suzuki, Michiyo Suzuki, Sho Saito, Yukari Uemura, Taro Shibata, Masashi Kondo, Fukumi Nakamura-Uchiyama, Kazuhisa Yokota, Fumitake Saito, Kazuo Izumi, Wataru Sugiura, Norio Ohmagari

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