Ethical and policy considerations for COVID-19 vaccination modalities: delayed second dose, fractional dose, mixed vaccines by Wolff, J, Atuire, C, Bhan, A, Emanuel, E, Faden, R, Ghimire, P, Greco, D, Ho, CWL, Kochhar, S, Moon, S, Schaefer, OG, Shamsi-Gooshki, E, Singh, JA, Smith, MJ, Thomé, B, Touré, A, Upshar, R