Air Purifier Intervention for Respiratory Viral Exposure in Elementary Schools: A Secondary Analysis of a Randomized Clinical Trial.

Importance: The magnitude of school respiratory virus exposure and the effectiveness of environmental mitigation measures remain unclear. Objective: To evaluate whether portable high-efficiency particulate air (HEPA) purifiers are associated with reduced respiratory virus exposure in elementary school classrooms. Design, Setting, and Participants: This ad hoc secondary analysis was performed between July 2023 and September 2024 and used data from a cluster-randomized, placebo-controlled trial (School Inner-City Asthma Intervention Study) of HEPA purifiers conducted from September 2015 to June 2020. A total of 200 classrooms from 39 public schools in Northeastern US were enrolled and randomized. Interventions: Classrooms were randomized 1:1 to receive either active HEPA purifiers or visually identical sham HEPA units (4 per classroom). School staff and investigators were blinded to intervention assignments. Main Outcomes and Measures: The primary outcome was high viral exposure, identified by K-means clustering of individual viral concentrations. Secondary outcomes included viral diversity (defined as number of detected virus types) and individual viral concentrations. Week-long bioaerosol samples were collected 3 times during 1 school year, and concentrations of 19 respiratory viruses were quantified by digital droplet polymerase chain reaction. Results: Of the 200 enrolled classrooms (91 in the sham purifier group and 109 in the HEPA purifier group) analyzed, the median (IQR) class size was 19 (18-20) and the median (IQR) grade was 3 (2-5). A total of 532 bioaerosol samples were collected; viruses were detected in 524 samples (98.5%), with a median (IQR) of 3 (2-5) viruses per classroom. Rhinovirus was most prevalent (476 [89.5%]), while respiratory syncytial virus A and B (66 [12.4%] and 127 [23.9%]) as well as influenza A and B (94 [17.7%] and 76 [14.3%]) were also detected. High viral exposure was present in 118 samples (22.2%). The HEPA purifier intervention was not associated with lower odds of high viral exposure (odds ratio [OR], 0.50; 95% CI, 0.08-3.25; P = .46) but did correspond to a modest reduction in viral diversity (β = -1.02; 95% CI, -1.68 to -0.35; P = .003). Elastic net regression identified relative humidity, grade, winter season, and coarse particulate matter as the environmental risk factors for viral exposure. Conclusions and Relevance: In this secondary analysis, HEPA purifiers were not associated with a reduction in high viral exposure but were associated with a modest decrease in viral diversity. Multicomponent interventions may be needed to mitigate respiratory viral exposures in schools.