UK trial finds Hepa air filters do not reduce viral infections in care homes or schools

Summary

A UK NIHR trial tested Hepa air filters in communal areas of 91 care homes for the elderly, comparing residents' and staff members' respiratory infections and sickness absence with homes that did not use filters. Results showed no difference in infection rates or staff illness between the two groups. In an open, unblinded setting, participants knew whether filters were present, a factor that can bias reports toward perceived improvement, though this would more likely inflate benefits than explain a lack of effect.

Source: The Conversation

• Hepa filtration did not reduce respiratory infection rates or staff absenteeism in real-world care-home settings
• Open, unblinded design could bias results, but would more likely help treatments appear effective rather than show no benefit
• Broader evidence since 2022 also points to limited proof that air-treatment technologies reduce infections in practice
• Policy takeaway: continue with established infection-prevention measures rather than relying on filters as a primary control method

Overview

The article discusses a UK funded study led by the National Institute for Health and Care Research (NIHR) that evaluated the effectiveness of Hepa air filters in reducing viral respiratory infections in real-world settings. Involving 91 care homes for the elderly, the trial allocated 47 homes to receive Hepa filters in communal areas and 44 homes to continue without filters. Researchers tracked residents and staff for respiratory infections and staff sickness absence to determine whether filtration conferred any protective advantage. The key finding is that infection rates and staff absenteeism were essentially the same in both groups, suggesting that Hepa filtration does not reliably reduce viral transmission in such environments. The article places these findings in the context of prior reviews and subsequent studies, illustrating a broader pattern that while air-treatment technologies can lower viral load in the air, they do not consistently translate into reduced infections among people in real-world settings.

Author and publisher context: The original piece is published by The Conversation, a platform that curates science news with expert commentary. The article cites the NIHR-funded trial and situates its conclusions within a broader evidence base that includes other studies conducted since 2022. The aim is to inform policy and practice by weighing the practicality and effectiveness of air filtration as a public health intervention, rather than presenting filtration as a standalone infection-control solution.

Trial design and results

The UK study enrolled 91 care homes, with 47 receiving Hepa filters for communal spaces and 44 serving as controls. Researchers monitored residents and staff for incidents of viral respiratory infections and noted staff sickness absence. The results showed no statistically meaningful differences between the filtered and unfiltered groups in either infection rates or staff illness or absenteeism. The open-label nature of the trial—participants knew whether filters were present—raises concerns about reporting biases, as expectations can shape subjective reporting of symptoms or illness. However, the article notes that such bias would typically inflate perceived benefits, not create a spuriously positive outcome where none is observed, making the null finding more credible.

Context within the broader evidence base

The piece references a 2023 review by the same outlet that acknowledged while air-treatment technologies can reduce virus levels in the air, there is no solid proof that these technologies lower respiratory or gastrointestinal infections in real-world settings. Since 2022, three additional studies are cited as reinforcing this pattern: a randomized Australian trial found no significant reduction in infection rates in care homes using Hepa filters; two non-randomized studies—one in German kindergarten classrooms and another Swiss study—showed mixed results, with the German study suggesting higher illness in filtered classrooms and the Swiss study showing no reduction in COVID transmission risk. A large US study, designed to test whether air filters could reduce asthma risk in schools, also found no difference in the total amount of virus in classroom air between real and sham filters. The converging message across these studies is that filtration lowers airborne viral concentrations but does not consistently translate into fewer infections in real-world settings.

Why filtration may not reduce infection risk

The article explains that the dominant transmission mechanism for many respiratory viruses is close-range spread, often happening within a meter of an infected person. A filter placed in a corner cannot act quickly enough on the air moving directly between two people standing near each other, and people constantly move between spaces, which diminishes the impact of localized filtration. Filtration may have a role in specific contexts, such as preventing invasive aspergillosis in immunocompromised individuals, but it does not appear to reduce viral respiratory infection risk in communal settings. The article argues that given current evidence and the scale of the new UK study, it is unlikely future research will overturn this conclusion.

Practical implications and recommendations

Based on the accumulating evidence, installing Hepa filters should not be treated as an effective strategy to prevent viral infections in care homes or schools. The authors advocate sticking with prevention measures that are already recommended and rely on established infection-control practices rather than filtration as a primary intervention for reducing viral respiratory infections.