Development of a novel method to detect indoor SARS-CoV-2 from the dust collection filters of air purifiers

• A novel method was developed to detect indoor SARS-CoV-2 from air filters. • Ultrasonication and filtration methods had a higher detection ratio of SARS-CoV-2. • SARS-CoV-2 was detected in 100 % of the COVID-19 inpatient rooms. • 1.9–2.9 log copies/day of SARS-CoV-2 were detected in the COVID-19 inpatient rooms. • A positive correlation was observed between inpatient counts and daily viral load. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), can survive in the air for several hours, which poses a risk of airborne transmission and cluster infection, especially in enclosed settings. The recovery of viruses from filters associated with the air purifiers could be an effective means of detecting indoor SARS-CoV-2; however, no standard method has yet been established. This study developed a method to detect indoor SARS-CoV-2 from different types (Types A–C) of air purifiers by elution through ultrasonication, followed by virus concentration, RNA extraction, and reverse transcription quantitative polymerase chain reaction. Among the three virus concentration methods tested, the filtration method demonstrated superior effectiveness. Among 233 filters from three types of air purifiers collected 17 times from each of the 11 locations in a hospital, SARS-CoV-2 RNA was successfully detected from 64 % (11/17) to 100 % (17/17) in samples from all locations, and a higher viral load (mean ± standard deviation, 2.9 ± 0.8 log 10 copies/day) was recovered in the COVID-19 inpatient rooms. The recovered viral load from the Type A and B filters was not positively correlated with the COVID-19 inpatient cases (Pearson’s r = −0.07 and 0.35, respectively), while that from the Type C filter was positively correlated (Pearson’s r = 0.82). This method enables the visualization of SARS-CoV-2 load in each sampling location, thereby helping prevent nosocomial infections and contributing to the potential improvement of public health by facilitating the implementation of appropriate infection control measures.