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Mask Wearing in Hospitals Had Little Impact on COVID-19 Omicron Transmission

During the first 10 months of Omicron activity the real-world benefit of mask-wearing in isolation was likely to be, at best, modest, said UK researchers.

Mask wearing has been part of a package of infection control measures employed to reduce COVID-19 in hospitals throughout the pandemic, highlighted the researchers behind a new study to be presented as an abstract at the 2023European Congress of Clinical Microbiology & Infectious Diseases (ECCMID 2023), in Copenhagen from 15-18 April.

In the NHS, a mask mandate for staff was introduced in June 2020, specifying the use of type IIR, fluid-resistant surgical face masks by all staff and visitors. The NHS mask mandate remained in place in clinical areas in hospitals until June 2022 – after which, decisions about mask policy were entrusted to individual hospitals.

The low-tech, low-cost intervention "without well-established benefit" was reasonable early in the pandemic, explained the authors from St George's Hospital, London. 

However, Dr Aodhan Breathnach, consultant medical microbiologist at St George's University Hospitals NHS Foundation Trust in London and senior author, described how many hospitals had "retained masking at significant financial and environment cost and despite the substantial barrier to communication".

The authors alluded that with a reduction in the severity of COVID-19 disease, in later variants, the risk-benefit balance of masks had been "called into question".

No Change in COVID Infection Rates

To investigate this further, the researchers analysed routinely collected infection control data over a 40-week period between 4 December 2021 (the first week when the Omicron variant became dominant) and 10 September 2022 (when universal COVID-19 screening by PCR on admission stopped) to examine hospital-acquired SARS-CoV-2 infections in relation to changes in mask-wearing policies over time. Throughout the study period, the Omicron variant was the dominant strain.

During the first phase of the study (4 December 2021 to 1 June 2022) all staff and visitors were required to wear masks in both clinical and non-clinical areas of the hospital. In phase two of the study, at week 26 (2 June 2022 to 10 September 2022) the surgical mask-wearing policy was removed in the majority of wards (study group), while a subset of high-risk wards (including renal, haematology, and oncology wards, the medical admissions unit, and intensive care units) retained the mask obligation policy for staff (control group).

The hospital SARS-CoV-2 infection rate was adjusted for the underlying community infection rate identified by routine admission screening.

During a general community surge in SARS-CoV-2 infection in June 2022, removal of the mask policy was not associated with a statistically significant change in the rate of hospital-acquired SARS-CoV-2 infection in the study group, "with SARS-CoV-2 infection no higher than the rate when masks were obligatory".

Moreover, the authors pointed out that they did "not observe a delayed effect", with no change in the SARS-CoV-2 infection rate during the time when the mask-wearing policy was removed. Similarly, they added, the control group who continued wearing masks, had no immediate or delayed change in infection rate.

"Requirements to wear surgical masks in a large London hospital during the first 10 months of Omicron activity (December 2021 to September 2022) made no discernible difference to reducing hospital-acquired SARS-CoV-2 infections," the authors concluded.

Masks Are Not Worthless

"Our study found no evidence that mandatory masking of staff impacts the rate of hospital SARS-CoV-2 infection with the Omicron variant," commented Dr Ben Patterson, from the St George's University Hospitals NHS Foundation Trust in London and lead author. "That doesn't mean masks are worthless against Omicron," he said, "but their real-world benefit in isolation appears to be, at best, modest in a healthcare setting".

The authors acknowledged limitations of the study, including that it was an observational study so can't prove causation, staff adherence to the mask-wearing policy was not assessed, and that the researchers were unable to determine staff infection rates.

Dr Breathnach expressed hope that the study findings would "help inform a rational and proportionate mask policy in health services".

The authors declared no external funding for the study, and they declared no conflicts off interest.