BackgroundThe COVID-19 pandemic has been exacerbated by the highly contagious SARS-CoV-2 Omicron variant BA.1, which quickly became the predominant strain worldwide in early 2022. The COVID-19 pandemic has not only burdened healthcare systems but has also led to a new emerging medical question whether chronic underlying diseases (CUDs) are associated with clinical characteristics of populations infected with Omicron BA.1. The purpose of this study is to investigate the impact of CUDs on the clinical characteristics of adult patients (APs) with Omicron BA.1 infection.MethodsWe performed an analysis on 320 individuals who were admitted to Tianjin First Central Hospital for treatment in the initial largest outbreak of the Omicron variant BA.1 infection between January and February 2022 in Tianjin, China. Clinical data were collected during the treatment and recovery, including gender, age, vaccination, CUDs, clinical manifestations, and nucleic acid test. These data were statistically analyzed using SPSS software, version 22.0.ResultsOur findings suggest that 178 (56 %) out of 320 APs have CUDs at the time of COVID-19 diagnosis. APs with CUDs have a higher median age [55 (40-62)] compared to APs without CUDs [38 (30-47)] and lower vaccination rates [158 (89 %)] compared to APs without CUDs [138 (97 %)]. Multivariable logistic regression results indicate that CUDs and advanced age (≥60 years old) are unfavorable factors for the increase in the severity of Omicron BA.1 infection [age (≥60 years old), OR = 2.96, 95 % CI: 1.35-6.50, P = 0.01; CUDs, OR = 2.78, 95 % CI: 1.65-4.70, P < 0.001]. Meanwhile, we observe that APs with CUDs exhibit significantly elevated alanine aminotransferase (ALT), aspartate aminotransferase (AST), blood urea nitrogen (BUN), interleukin-6 (IL-6), and C-reactive protein (CRP) levels, and higher re-detectable SARS-CoV-2 RNA positive (RP) rates, while notably reduced lymphocyte levels compared with those without CUDs. Additionally, we propose that vaccination played a positive role in reducing IL-6 and CRP levels, and RP rates, as well as increasing SARS-CoV-2 specific antibody levels in APs with CUDs.ConclusionThis research not only enhances the understanding of the association between CUDs and the clinical manifestations of APs infected with Omicron BA.1, but also offers valuable insights for preventing and managing Omicron BA.1 infections in this demographic. Moreover, it supports the policy of prioritizing vaccination for adults with CUDs.