The B.1.1.529 (Omicron) variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has previously been reported as more transmissible, but less severe than other SARS-CoV-2 variants. To test this assumption, we linked state-level vaccination data with quality-controlled electronic health records from a large healthcare system, including 13 hospitals, in Massachusetts, USA. We then performed a weighted case-control study to compare risks of hospital admission and mortality across the SARS-CoV-2 waves in over 130,000 COVID patients. Although the unadjusted rates of hospital admission and mortality appeared to be higher in previous waves compared to the Omicron period, after adjusting for confounders including various demographics, Charlson comorbidity index scores, and vaccination status (and holding the healthcare utilization constant), we found that the risks of hospitalization and mortality were nearly identical between periods. Our analysis suggests that the intrinsic severity of the Omicron variant may be as severe as previous variants.