Massachusetts General Brigham hospitals-led research has reported a significant increase in hospital-onset respiratory viral infections following the cessation of universal masking and SARS-CoV-2 testing policies. Researchers found fewer infections when masking policies for healthcare workers were reinstated during a winter viral surge.

Universal masking and admission testing were phased out in May 2023. Masking was reinstated for healthcare workers in January 2024 due to rising viral infections during the winter season.

An observational study, "Infectious Diseases Testing and Masking Policies and Hospital-Onset Respiratory Viral Infections," published in JAMA Network Open, conducted across ten hospitals in a Massachusetts health care system, examined the effects of masking and testing protocols on rates of hospital-onset infections caused by SARS-CoV-2, influenza, and respiratory syncytial virus (RSV).

Researchers reviewed data from 641,483 admissions at two tertiary hospitals, seven community hospitals, and one speciality eye and ear hospital between November 2020 and March 2024.

Using a Poisson interrupted time-series design, researchers modelled hospital-onset infections, defined as positive PCR tests more than four days after admission, relative to community-onset infections, defined as positive tests within the first four days of admission.

Periods were categorised into four phases: pre-omicron universal testing and masking, omicron universal testing and masking, omicron without universal testing and masking, and omicron with health care worker masking reinstated.

Adjustments for seasonality and other confounding variables were incorporated using system-wide testing data and bootstrapped confidence intervals. Misclassification of hospital-onset cases was assessed in 100 randomly selected cases based on clinical symptoms, known exposure, and PCR cycle thresholds.

The study recorded 30,071 community-onset and 2,075 hospital-onset respiratory viral infections. During the pre-omicron phase with universal precautions, the mean weekly ratio of hospital-onset to community-onset infections was 2.9%. This ratio increased to 7.6% during omicron dominance with precautions, rose to 15.5% after precautions ended, and decreased to 8.0% after healthcare worker masking resumed.

After the cessation of universal masking and testing, hospital-onset infections rose by 25% compared to the prior period of omicron dominance with precautions. The resumption of masking among health care workers was associated with a 33% reduction in hospital-onset infections.

Findings indicate a measurable relationship between masking and testing policies and the incidence of hospital-onset infections. The study demonstrates that masking effectively mitigates viral infections, particularly during periods of elevated community viral activity.