The use of N95 respirators vs medical masks among outpatient healthcare personnel (HCP) resulted in no differences in incidence of laboratory-confirmed influenza, according to results of a trial published in JAMA.

The effect of N95 respirators for preventing influenza and other viral respiratory infections was investigated using a cluster randomized pragmatic effectiveness study ( identifier: NCT01249625). The trial was conducted at 137 outpatient sites at 7 medical centers in the United States between September 2011 and May 2015, with a final follow-up until June 2016.

During the 12-week period of peak cases of viral respiratory illness each year for 4 years, pairs of sites within each center were matched and randomly assigned to the N95 respirator or medical mask groups. These pairs formed the clusters in this study; in total 1993 participants in 189 clusters were assigned to wear N95 respirators and 2058 in 191 clusters were given masks to wear. Over the study period, 2371 participants completed the study and accounted for 5180 HCP seasons.

Among the N95 group, there were 207 laboratory-confirmed influenza infections, 8.2% of HCP seasons. In the mask group, laboratory-confirmed influenza occurred in 193 participants and 7.2% of HCP seasons for a difference of 1.0% (95% CI, −0.5% to 2.5%; P =.18; adjusted odds ratio 1.18 [95% CI, 0.95-1.45]). Acute respiratory illness events were 1556 in the respirator group vs 1711 in the mask group for a difference of −21.9 per 1000 HCP-seasons (95% CI, −48.2 to 4.4; P =.10). Laboratory-detected respiratory infections for each group were 679 and 754 in the respirator and mask groups, respectively, for a difference of −8.9 per 1000 HCP-seasons, (95% CI, −33.3 to 15.4; P =.47). Laboratory-confirmed respiratory illness events were 371 and 417 in the respirator and mask groups, respectively, for a difference of −8.6 per 1000 HCP-seasons (95% CI, −28.2 to 10.9; P =.39). Influenza-like illness events among the groups were 128 in the respirator group vs 166 in the mask group for a difference of −11.3 per 1000 HCP-seasons (95% CI, −23.8 to 1.3; P =.08). The percentage of participants who reported always or sometimes wearing the assigned device were 89.4% in the respirator group vs 90.2% in the mask group. 

It is possible that the criteria for polymerase chain reaction testing may have missed participants who were asymptomatic but infected, which may have increased the probability of finding no difference between the interventions. Incomplete participant adherence to wearing the assigned device might have increased this probability also, but nonadherence was similar for both groups. The self-reporting of symptoms likely underestimated illness as well. Participants did not wear the devices outside of work, which may have biased results toward no difference; further, two N95 respirator and medical mask models were studied. Finally, the sample size of 10,000 needed to definitively determine which device is more effective was unfeasible with the available funding and resources.

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According to investigators, there was no significant difference in incidence of laboratory-confirmed influenza in this trial of the 2 devices. However, investigators also believe, “the morbidity and mortality associated with a wide range of viral respiratory infections, including novel and emerging pathogens, renders a secondary outcome in this study, laboratory-confirmed respiratory illness, important.”


Radonovich LJ Jr, Simberkoff MS, Bessesen MT, et al. N95 respirators vs medical masks for preventing influenza among health care personnel: a randomized clinical trial. JAMA. 2019;322:824-833.