What proportion of healthcare worker masks carry virus? A systematic review

Concerns have been raised by healthcare organisations in New Zealand that routine mask use by healthcare workers (HCW) may increase the risk of transmission of SARS-CoV-2 through increased face touching. Routine mask use by frontline HCW was not recommended when seeing 'low risk' patients. The aim of this review was to determine the carriage of respiratory viruses on facemasks used by HCW.

A systematic review was conducted with structured searches of medical and allied health databases. Two authors independently screened articles for inclusion, with substantial agreement (k = 0.66, 95% CI 0.54-0.79). Studies that at least one author recommended for full text review were reviewed in full for inclusion. Two authors independently extracted data from included studies including the setting, method of analysis and results. There was exact agreement on the proportion of virus detected on masks.

We retrieved 1233 titles, 47 underwent full text review and five studies reported in four articles were included. The studies were limited by small numbers and failure to test all eligible masks in some studies. The proportion in each study ranged from 0 (95% CI 0-10) to 25% (95% CI 8-54). No study reported clinical respiratory illness as a result of virus on the masks.

Although limited, current evidence suggests that viral carriage on the outer surface of surgical masks worn by HCW treating patients with clinical respiratory illness is low and there was not strong evidence to support the assumption that mask use may increase the risk of viral transmission.