Antibody titers against SARS-CoV-2 decline, but do not disappear for several months by Yamayoshi, S, Yasuhara, A, Ito, M, Akasaka, O, Nakamura, M, Nakachi, I, Koga, M, Mitamura, K, Yagi, K, Maeda, K, Kato, H, Nojima, M, Pattinson, D, Ogura, T, Baba, R, Fujita, K, Nagai, H, Yamamoto, S, Saito, M, Adachi, E, Ochi, J, Hattori, S-I, Suzuki, T, Miyazato, Y, Chiba, S, Okuda, M, Murakami, J, Hamabata, T, Iwatsuki-Horimoto, K, Nakajima, H, Mitsuya, H, Omagari, N, Sugaya, N, Yotsuyanagi, H, Kawaoka, Y