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SAN FRANCISCO -- Used consistently, N95 respirator masks prevented 75% of respiratory infections for high-risk healthcare workers, but regular surgical masks didn't appear to be effective against respiratory infections at all, researchers reported here.

In the first randomized trial of their efficacy, the N95 masks worked regardless of whether seal against the skin (a "fit test") was confirmed, according to investigators at the Interscience Conference on Antimicrobial Agents and Chemotherapy.

The findings supported the Institute of Medicine's recommendation for N95 respirators to prevent spread of H1N1 in healthcare settings.

Although surgical masks were the face of the 2003 SARS epidemic in Asia, they don't protect against pandemic H1N1 (swine flu) or any other respiratory infection, C. Raina MacIntyre, MBBS, PhD, of the University of New South Wales in Sydney, Australia, and colleagues warned.

Last month, MacIntyre presented a preliminary version of these findings to an Institute of Medicine panel deliberating guidelines for personal protective equipment standards for healthcare workers. Aside from her trial, there was little but anecdotal testimony.

"There are many guidelines -- quite sweeping guidelines -- about the use of masks without really a shred of high-level evidence to support them," said MacIntyre. "Until recently, the only data supporting mask use are retrospective, observational data from outbreaks such as SARS."

Confirmation of surgical masks' failure came as no surprise to Frank Lowy, MD, of Columbia University, who was not involved in the study.

He noted that unlike surgical masks, N95 respirators are made of material certified to block 95% of particles 0.3 microns or larger in diameter and can be fit tested to seal around the nose and mouth.

However, he said he was surprised by the similarity of outcomes whether or not N95 respirators were fit tested, as recommended.

If anything, the non-fit tested respirators appeared slightly more effective against clinical respiratory illness compared with controls (RR 0.36, P<0.001) and surgical masks (RR 0.49, P=0.014) than were fit tested respirators (RR 0.5, P=0.005, and RR 0.67, P=0.151, respectively).

"It's very time consuming and labor intensive to get hospital employees fit tested," Lowy said. "So if you don't have to do that, that's a considerable saving in time, effort, and expense."

The researchers still recommended fit testing as the standard for practice.

Moreover, American hospitals could face problems if they don't follow that guidance, since the Occupational Safety and Health Administration (OSHA) still requires testing for air leakage into the respirators, John S. Adams, MD, of Knoxville (Tenn.) Infectious Disease Consultants noted during a question-and-answer session after the late-breaking presentation.

The trial included 1,936 emergency and respiratory ward nurses and physicians at 24 hospitals in Beijing during the winter cold and flu season. They were cluster-randomized to wear surgical masks (3M brand), fit-tested N95 respirators (3M brand), or non-fit tested N95 respirators during all work hours for four consecutive weeks. They were followed for an additional week off randomization.

Because the culture of mask use was so prevalent in China, the control group consisted of participants following usual practice at nine hospitals, though the researchers selected out centers with relatively low mask use for further analysis.

Consistent surgical mask use was no better than controls for prevention of clinical respiratory illness (6.7% versus 9.2%, P=0.159) or of influenza-like illness (0.6% versus 1.3%, P=0.336).

But compared with controls, N95 respirators together reduced the rate of clinical respiratory illness 60% (3.9% versus 9.2%, P<0.001) and the rate of influenza-like illness by 75% (0.36% versus 1.3%, P=0.035).

The advantage of N95 respirators was substantial compared with surgical masks (RR 0.58 for clinical respiratory illness, P=0.019).

Adjustment for differences between hospitals in the level of handwashing, vaccination, and other factors that would impact infection risk only increased the apparent effectiveness of the N95 in staving off influenza to 96% (OR 0.04, 95% CI 0.01 to 0.15).

Although the study could not rule out a small degree of efficacy for surgical masks that could be enough for the low-exposure general public, MacIntyre said masks can't be recommended to protect healthcare workers.

"Given the importance of protecting frontline workers and the need to maintain essential public services during a pandemic, I feel it places healthcare workers at unacceptable risk to recommend they wear a surgical mask," she said. "You might as well tell them to wear nothing."

Her group noted that the study was limited by use of convenience controls, though use of cloth masks by some controls would only have biased the results toward no effect.

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