N95 masks may be more effective at preventing influenza transmission than surgical masks

Efficacy rates of N95 masks were between 55% and 70% against several barometers of respiratory illness and infection, according to findings presented at the 49th Annual Interscience Conference on Antimicrobial Agents and Chemotherapy. The findings indicated that N95 masks may be superior to surgical masks in providing protection from respiratory illness.

Data were presented by Raina MacIntyre, PhD, of the University of New South Wales in Sydney. “Surgical masks may place health care workers at unacceptable risks,” she said. “I have not seen a shred of evidence supporting use of surgical masks. We might as well tell health care workers to wear nothing.”

The goal of the current study was to compare the clinical efficacy of fit-tested N95 masks and non-fit-tested surgical masks with controls among front line health care workers in Beijing.

“During a pandemic, there could be shortages of antivirals and delays in vaccine production, so it is necessary to take as many precautions as possible to reduce the incidence of transmission,” MacIntyre said.

The researchers conducted a randomized, controlled trial of 24 hospitals involving 1,936 front-line hospital health care workers. Participants in the first arm wore fitted N95 masks. Arm two wore non-fitted N95 masks; arm three wore surgical masks; arm four was the control group.

Participants wore masks for a four-week period during winter and were observed for a five-week period in total to determine whether any illness developed. Participants who exhibited symptoms were tested for respiratory viruses, including influenza.

Primary outcomes included incidence of clinical respiratory illness, influenza-like illness, any respiratory virus infection confirmed by a laboratory and any influenza confirmed by a laboratory.

Intention to treat analysis indicated that surgical masks had no efficacy for any of the studied outcomes.

Compared with controls, N95 masks were significantly more protective. Efficacy rates for N95 masks compared with control were as follows:

60% against clinical respiratory illness
75% against influenza-like illness
56% against laboratory confirmed respiratory viral infection
75% against confirmed influenza

There was no improvement in the efficacy of fit-tested masks compared with non-fit-tested masks.

The researchers wrote that previous evidence about N95 masks was limited to experimental studies. They also said that although fit-testing did not improve the efficacy of N95 masks, even non-fit-tested masks were superior to surgical masks.

“N95 should be the standard for healthcare worker protection, based on all evidence,” MacIntyre said. “When the masks are worn, fit-testing should be standard practice, because there is variability in the quality of fit between different N95s.”

MacIntyre C. #K-1918b. Presented at: the 49th Annual Interscience Conference on Antimicrobial Agents and Chemotherapy; San Francisco; Sept. 11-15, 2009.

There is much about which to be concerned in this report. Many critical details of when and how are not provided. No information about health care worker compliance is provided, nor are we told under what circumstances respiratory protection was used. In addition, the nature of the “surveillance,” if any, on the health care workers is not specified, nor were the symptoms that merited testing. I would urge readers to await a published report to evaluate this study more thoroughly and before accepting any of these conclusions.

– Theodore C. Eickhoff, MD

Infectious Disease News Chief Medical Editor