According to the results of a randomised controlled trial involving 8 tertiary care hospitals in Ontario, the use of a surgical mask by nurses resulted in non-inferior rates of laboratory-confirmed influenza compared to use of an N95 respirator. 

The authors of an accompanying editorial note that this is (to their knowledge) the first published RCT evaluating respiratory protection for preventing influenza transmission.  The investigators sought to determine the effectiveness of the surgical mask in this situation due to the likelihood that N95 respirators will be in short supply during a pandemic.  A total of 446 nurses enrolled from a total of 22 units (9 acute medical units, 7 emergency departments, and 6 paediatric units) were randomised (stratified by centre) to either a N95 respirator (n=221) or a surgical mask (n=225) when providing care to patients with febrile respiratory illness during the 2008-2009 influenza season.  Although the study was initially due to terminate at the end of influenza season, it was stopped early due to the current 2009 influenza A(H1N1) pandemic (the Ontario Ministry of Health and Long-Term Care recommended N95 respirators for all health care workers taking care of patients with febrile respiratory illness).

The primary outcome was laboratory-confirmed influenza measured by polymerase chain reaction (PCR) or a 4-fold rise in serum antibodies to circulating influenza strain antigens (laboratory personnel were blinded to allocation). Effectiveness of the surgical mask was assessed as non-inferiority of the surgical mask compared with the N95 respirator. The criterion for non-inferiority was met if the lower limit of the 95% CI for the reduction in incidence (N95 respirator minus surgical group) was greater than –9%.

The two study groups were well balanced with similar risk factors for influenza vaccination, including for example vaccination and febrile respiratory illness among household members.  Only 68 participants (30.2%) in the surgical mask group and 62 (28.1%) in the N95 respirator group had received the 2008-2009 trivalent inactivated influenza vaccine.  In the study period, influenza infection occurred in 50 nurses (23.6%) in the surgical mask group and in 48 (22.9%) in the N95 respirator group (absolute risk difference, –0.73%; 95% CI, –8.8% to 7.3%; P = 0.86).  The lower confidence limit was inside the non-inferiority limit of –9% and therefore the definition of non-inferiority was met.  

The authors of a related editorial note the strengths and limitations of the research, and comment that “a single study will not end the debate over influenza respiratory protection for HCP. Unfortunately, this intense discussion over respiratory protection has distracted attention from the critical importance of implementing other strategies known to prevent the transmission of influenza in health care settings (e.g. annual vaccination; effective isolation).”  They suggest that the use of personal protective equipment should be considered the ‘last line of defence’ in a hierarchy of infection control measures.