Background: Severe patients hospitalized with COVID-19 suffered secondary infections which greatly increased the length of hospital stay and the mortality. We aimed to explore risk factors of secondary infections that can help clinicians early implement preventive measures to dispose of severe and critical inpatients with COVID-19.Methods: A case-control study enrolled 238 severe and critical patients with COVID-19. Characteristics of cases and controls were compared.Results: Severity of illness on admission, ICU admission, ventilator, central venous catheterization were common in the cases, however almost none of these factors was observed in the controls. Multivariable regression showed risk factors of secondary infections included male (OR 4.08; 95% CI 1.58-10.50), age 65 or older (OR 3.11; 95% CI 1.25-7.76), heart diseases (OR 3.96; 95% CI 1.40-11.27), hypoproteinemia on admission (OR 6.41; 95% CI 1.65-24.92) and corticosteroids (OR 19.83; 95% CI 7.3-53.55) and proton-pump inhibitors (OR 3.96; 95% CI 1.51-10.37).Conclusions: male, older age, heart diseases, hypoproteinemia, corticosteroid and proton-pump inhibitors were independent risk factors of secondary infections. Inpatients needing ICU admission and invasive devices still need to be given optimal cares and to be minimized the duration.

2-[N-(2-Hydroxyethyl)-carbamoyl]-3-methyl-oxoquinoxaline-1,4-oxides (OLA) was first mono-esterificated by succinic acid anhydride, and was then turned into a derivative with carboxyl. The synthetic product was purified by recrystallization with a yield of 52.47%. Its chemical structure was determined by NMR, IR and MS spectra. The determined structure was used to synthese OLA hapten, which had the molecular structural characteristics of OLA. The OLA hapten and bovine serum albumin (BSA) were conjugated using the activated ester method, while the hapten and ovalbumin (OVA) were coupled by the mixed anhydride method. The UV scanning and infrared spectrum results showed that the hapten and the carrier protein were successfully coupled to BSA and OVA, with the combined ratios of 3.8∶1 and 5∶1, respectively. OLA-BSA was used as the immunogen to immunize four New Zealand white rabbits, and the high titer anti-serum produced relatively. The titers were 1∶6400, 1∶1600, 1∶12800 and 1∶6400, respectively, determined by indirect ELISA OLA-OVA as the coating antigen. The intermediate inhibition concentration (IC 50 ) of OLA in the indirect ELISA test was 743.3 ng/mL. The lowest detection limit (IC 20 ) was 5.71 ng/mL. High-purity OLA polyclonal antibody has been obtained by saturated ammonium sulfate and ion-exchange chromatography.