background: The primary aim of using vaccines in public health responses to SARS-CoV-2 variants of concern is to reduce incidence of severe disease, for which T-cell responses are essential. There is a paucity of data on vaccine-induced T-cell immunity to omicron (B.1.1.529). We aimed to compare SARS-CoV-2 omicron BA.1-specific T-cell responses in adults vaccinated with CoronaVac or BNT162b2. methods: For this observational cohort, we recruited adults (aged ≥18 years) from three vaccination centres in Hong Kong. We included participants from four cohorts (cohort 1: participants who received two doses of either BNT162b2 or CoronaVac, cohort 2: participants who received two doses and a booster, cohort 3: participants who received two doses and a booster and had a breakthrough omicron infection, and cohort 4: participants who had a previous non-omicron infection and subsequently received one dose of vaccine). People with confirmed history of COVID-19 at recruitment were excluded from cohort 1 and cohort 2. We collected blood samples before vaccination (for cohort 1 and 2), 1-month following vaccination (for all cohorts), and during convalescence for cohort 3 and 4) and determined the proportion of IFNγCD4 and IFNγCD8 T cells in peripheral blood against SARS-CoV-2 using flow cytometry with peptide pools of SARS-CoV-2 wild type or omicron BA.1. The primary outcome was proportion of CD4 and CD8 T cells against SARS-CoV-2 1 month after exposure (ie, vaccination or breakthrough infection). findings: Overall, between May 21, 2020, and Aug 31, 2021, we recruited 659 participants (231 [35%] men and 428 [65%] women). Of these participants, 428 were included in cohort 1 (214 [50%] received BNT162b2 and 214 [50%] received CoronaVac); 127 in cohort 2 (48 [38%] received all BNT162b2, 40 [31%] received all CoronaVac, and 39 [31%] received two CoronaVac and a booster with BNT162b2); 58 in cohort 3, and 46 in cohort 4 (16 [35%] received CoronaVac and 30 [65%] received BNT162b2). Vaccine-induced T-cell responses to the wild-type and omicron BA.1 variants were generally similar in adults receiving two doses of either CoronaVac (CD4 cells p=0·33; CD8 cells p=0·70) or BNT162b2 (CD4 cells p=0·28; CD8 cells p=1·0). Using a peptide pool of all structural proteins for stimulation, BNT162b2 induced a higher median frequency of omicron-specific CD4 T cells in adults younger than 60 years (CD4 cells 0·012% vs 0·010%, p=0·031; CD8 cells 0·003% vs 0·000%, p=0·055) and omicron-specific CD8 T cells in people aged 60 years or older (CD4 cells 0·015% vs 0·006%, p=0·0070; CD8 cells 0·007% vs 0·000%, p=0·035). A booster dose of either BNT162b2 or CoronaVac after two doses of CoronaVac boosted waning T-cell responses, but T-cell responses did not exceed those at 1 month after the second dose (CoronaVac CD4 p=0·41, CD8 p=0·79; BNT162b2 CD4 p=0·70 CD8 p=0·80). interpretation: The evidence that mRNA and inactivated vaccines based on the ancestral SARS-CoV-2 virus elicited T-cell responses to SARS-CoV-2 omicron variants might explain the high observed vaccine effectiveness against severe COVID-19 shown by both types of vaccine, despite great differences in neutralising antibody responses. The use of either vaccine can be considered if the primary aim is to reduce severity and death caused by the new omicron subvariants; however, BNT162b2 is preferable for adults older than 60 years. funding: The Health and Medical Research Fund Commissioned Research on the Novel Coronavirus Disease and S H Ho Foundation.