Age and frailty in COVID-19 vaccine development

Immune responses were measured using assays of anti-spike protein IgG and neutralising antibody titres for humoral immunity and IFN-γ enzyme-linked immunospot (ELISpot) for cell-mediated immunity. In IFN-γ ELISpot assays enumerating antigen-specific T cells done for those in the prime-boost standard-dose group, T-cell responses peaked at 14 days after a single standard dose and did not increase significantly after a boost dose (18–55 years, median 1187 spot forming cells [SFCs] per million peripheral blood mononuclear cells [IQR 841–2428], n=24; 56–69 years, 797 SFCs [383–1817], n=29; and ≥70 years 977 SFCs [458–1914], n=48; p=0·46). The inclusion of measures of cell-mediated immunity is important given the limitations of relying solely on antibody titres in older adults.4,5 The main study limitations were its single-blind design, the inclusion of few participants older than 80 years, and exclusion of people with substantial underlying chronic illnesses and frailty.