This study aimed to evaluate the effectiveness (VE) of mix-and-match vaccination against SARS-CoV-2 Omicron variant infection and severe outcomes. An SARS-CoV-2 PCR-confirmed retrospective cohort from Chang Gung Medical System in Taiwan was constructed. Vaccination records were tracked from the National Immunization Information System and categorized by different regimens or unvaccinated status. The main outcomes are VE against PCR-confirmed infection and COVID-19-associated moderate to severe disease. Participants were observed during the Omicron wave from March to August 2022. Of 298,737 PCR testing results available, 162,219 were eligible for analysis. VE against infection was modest, ranging from 38.3% to 49.0%, while mRNA-based vaccine regimens revealed better protection against moderate to severe disease, ranging from 80.8% to 90.3%. Subgroup analysis revealed lower VE among persons with major illness in preventing moderate to severe disease. For young adults, the VE of protein-based vaccine regimens showed a comparable protection with other mixed vaccine regimens. The mix-and-match vaccination strategy provided modest clinical effectiveness in preventing Omicron variant infection. mRNA vaccine-based regimens were superior to other regimens against moderate to severe disease especially in older adults. The mix-and-match vaccination strategy could be an alternative to prevent COVID-19 in unstable vaccine supply regions.