Klompas Michael, Milton Donald K, Rhee Chanu, Baker Meghan A, Leekha Surbhi
Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, and Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts (M.K., C.R., M.A.B.).
Maryland Institute for Applied Environmental Health, School of Public Health, University of Maryland, College Park, Maryland (D.K.M.).
Ann Intern Med. 2021 Dec;174(12):1710-1718. doi: 10.7326/M21-2780. Epub 2021 Nov 9.
Policies to prevent respiratory virus transmission in health care settings have traditionally divided organisms into Droplet versus Airborne categories. Droplet organisms (for example, influenza) are said to be transmitted via large respiratory secretions that rapidly fall to the ground within 1 to 2 meters and are adequately blocked by surgical masks. Airborne pathogens (for example, measles), by contrast, are transmitted by aerosols that are small enough and light enough to carry beyond 2 meters and to penetrate the gaps between masks and faces; health care workers are advised to wear N95 respirators and to place these patients in negative-pressure rooms. Respirators and negative-pressure rooms are also recommended when caring for patients with influenza or SARS-CoV-2 who are undergoing "aerosol-generating procedures," such as intubation. An increasing body of evidence, however, questions this framework. People routinely emit respiratory particles in a range of sizes, but most are aerosols, and most procedures do not generate meaningfully more aerosols than ordinary breathing, and far fewer than coughing, exercise, or labored breathing. Most transmission nonetheless occurs at close range because virus-laden aerosols are most concentrated at the source; they then diffuse and dilute with distance, making long-distance transmission rare in well-ventilated spaces. The primary risk factors for nosocomial transmission are community incidence rates, viral load, symptoms, proximity, duration of exposure, and poor ventilation. Failure to appreciate these factors may lead to underappreciation of some risks (for example, overestimation of the protection provided by medical masks, insufficient attention to ventilation) or misallocation of limited resources (for example, reserving N95 respirators and negative-pressure rooms only for aerosol-generating procedures or requiring negative-pressure rooms for all patients with SARS-CoV-2 infection regardless of stage of illness). Enhanced understanding of the factors governing respiratory pathogen transmission may inform the development of more effective policies to prevent nosocomial transmission of respiratory pathogens.
医疗环境中预防呼吸道病毒传播的政策传统上是将病原体分为飞沫传播和空气传播两类。据说飞沫传播的病原体（如流感）是通过大的呼吸道分泌物传播的，这些分泌物会在1到2米内迅速落到地面，外科口罩就能有效阻挡。相比之下，空气传播的病原体（如麻疹）则通过气溶胶传播，这些气溶胶足够小且轻，能够传播到2米以外，并穿透口罩与面部之间的缝隙；建议医护人员佩戴N95口罩，并将这些患者安置在负压病房。在护理正在进行“产生气溶胶操作”（如插管）的流感或SARS-CoV-2患者时，也建议使用口罩和负压病房。然而，越来越多的证据对这一框架提出了质疑。人们日常会呼出各种大小的呼吸道颗粒，但大多数是气溶胶，而且大多数操作产生的气溶胶并不比普通呼吸产生的多很多，远少于咳嗽、运动或费力呼吸产生的气溶胶。不过，大多数传播仍发生在近距离，因为携带病毒的气溶胶在源头最为集中；然后它们会随着距离扩散和稀释，在通风良好的空间中远距离传播很少见。医院感染传播的主要风险因素包括社区发病率、病毒载量、症状、距离、接触时间和通风不良。未能认识到这些因素可能导致对某些风险认识不足（例如，高估医用口罩提供的防护，对通风关注不足）或有限资源分配不当（例如，仅为产生气溶胶的操作保留N95口罩和负压病房，或无论疾病阶段，要求为所有SARS-CoV-2感染患者提供负压病房）。加强对呼吸道病原体传播影响因素的理解，可能有助于制定更有效的政策来预防呼吸道病原体的医院感染传播。
