The assumption that better air filtration straightforwardly translates into fewer respiratory infections in communal living settings deserves scrutiny — and this process evaluation of a real-world cluster randomised trial provides a rare, rigorous look at why a theoretically sound intervention can still produce a null result in practice.
The AFRI-c trial deployed high-efficiency particulate air (HEPA) filtration units across 22 UK care homes and paired the effectiveness data with a mixed-methods process evaluation spanning 25 staff, 20 residents, and 12 relatives, supplemented by questionnaires from over 1,158 residents and up to 351 staff. The key finding is that self-reported fidelity was high — filters were used as intended, became normalised into daily routines, and did not measurably alter staff satisfaction or existing infection-control behaviours. Despite this strong implementation signal, the primary trial outcome showed no meaningful reduction in respiratory infection rates. A minority of residents reported discomfort from air movement, but this did not undermine overall acceptability.
This matters beyond care homes. The null effectiveness finding cannot be dismissed as a compliance failure — which is the most common confound in negative trials of environmental interventions. Fidelity was demonstrably maintained, meaning the filtration itself is what failed to move the needle in this setting. Several plausible explanations exist from prior research: HEPA devices capture airborne particulates efficiently, but care-home transmission chains are heavily contact- and droplet-mediated rather than purely aerosol-driven, potentially limiting the ceiling on benefit any filtration strategy could achieve. Room size, ventilation architecture, and the immunological vulnerability of elderly residents also interact in ways that bench-based filtration models do not fully capture. This is an important, incrementally confirmatory finding for infection-prevention science — it signals that air filtration is not a universal solution for communal elderly care, and that transmission route profiling should precede environmental intervention decisions.