By Gary Evans, Medical Writer

In findings that have implications for the next influenza pandemic, the authors of a decade-long study of real-world use of respiratory protection by healthcare workers found no difference between N95 respirators and standard surgical masks.

The study, which included clinicians but was led by the National Institute for Occupational Safety and Health (NIOSH), was conducted at 137 outpatient sites at seven U.S. medical centers between September 2011 and May 2015, with final follow-up in June 2016.

“Each year for four years, during the 12-week period of peak viral respiratory illness, pairs of outpatient sites within each center were matched and randomly assigned to the N95 respirator or medical mask groups,” the authors reported.1 Overall, 1,993 healthcare workers wore N95 respirators, and 2,058 wore medical masks when near patients with respiratory illness.

“There was no significant difference in the incidence of laboratory-confirmed influenza among healthcare personnel with the use of N95 respirators (8.2%) vs. medical masks (7.2%),” the researchers concluded. “As worn by healthcare personnel in this trial, use of N95 respirators, compared with medical masks, in the outpatient setting resulted in no significant difference in the rates of laboratory-confirmed influenza.”

The calm rationality of that conclusion belies the considerable chaos that sparked the study a decade ago. In 2009, a novel H1N1 A influenza strain appeared suddenly in the U.S., with the virus having made the classic antigenic shift that rendered available flu vaccine useless. An influenza pandemic had begun, the first since 1968 and the fourth since the 1918 Spanish flu killed millions of people.

Generally, pandemics are known to strike younger, healthier people than seasonal flu, in part of because of an inflammatory immune response. Given the lack of an immediate vaccine, the CDC viewed the novel virus with an abundance of caution and recommended N95 respirators for healthcare workers, a step beyond surgical masks long used for seasonal flu.

Several nurses died of H1N1, although it remains unclear in all cases if the virus was acquired in the community or the hospital, and whether respiratory protection or the lack thereof was a contributing factor. Although infectious disease experts said that H1N1 was spreading like normal flu virus — meaning droplet precautions with masks were sufficient to stop transmission — OSHA stepped in to require the N95 respirators for healthcare workers. Further complicating the pandemic response, there were not enough respirators at some facilities to allow compliance with the mandate. Infection preventionists at the time said the respirator issue undermined the medical response, partly because it delayed transfer of H1N1 patients to facilities that did not have enough gear.

Thus, the NIOSH Respiratory Protection Effectiveness Clinical Trial (ResPECT) was undertaken to try to resolve the issue before the next flu pandemic.

A Recurrent Question

“Anytime there is a discussion about risks of the next pandemic, this question about whether healthcare workers would be better protected by N95 respirators or masks always seems to come back up,” says Hilary Babcock, MD, who co-wrote a commentary on the study.2 “This study was an attempt to try to more clearly define the safety of surgical or isolation masks in routine use during a range of flu seasons.”

While it may seem obvious that tight-fitting N95 respirators would be more protective, their proven efficacy against small particles may not be as critical for influenza droplets.

“Influenza traditionally has been understood to be primarily transmitted through larger droplets that don’t hang in the air a long time, so they are able to be largely caught in surgical isolation masks,” she says. “The respirators really provide benefit for very small-particle aerosols.”

In experimental models and in occasional anecdotal reports, it appears that there may be some capacity for flu to be transmitted this way, similar to an airborne pathogen like measles. Thus, respirators still are recommended for potential aerosol-generating procedures like bronchoscopy on infected patients. The ResPECT study authors did not look at comparative respiratory protection for potentially aerosol-generating procedures.

“The majority of the time, influenza is still transmitted through larger droplets,” Babcock says. “Those larger droplets are caught by the surgical masks, so they provide equivalent protection for the primary mode of transmission for influenza.”

Although an equivalency was established in the study in terms of respiratory protection, the trial was conducted in outpatient facilities because of cost and feasibility.

“[S]ome differences may potentially affect transmission in inpatient settings, including higher viral burden among more symptomatic patient populations, a higher proportion of immunocompromised patients, and longer and closer interactions between healthcare personnel and patients while providing care (e.g., bathing patients),” Babcock wrote in the commentary.

While seasonal flu viruses mutate and reassemble through a process of antigenic drift that warrants a new flu vaccine every year, it is thought that the basic modes of transmission do not change in seasonal or pandemic strains.

“For most influenza, there is not any evidence, as new strains have emerged over the years, [showing] that they have picked up completely new transmission pathways,” she says. “They still appear to be transmitted primarily through large droplets. I think the surgical masks probably are still adequate.”

However, with emerging respiratory infections like SARS and MERS, it is reasonable to start with respirators until transmission is better understood, she adds.

“We should be actively looking for data to help guide evidence-based recommendations so we can say, ‘For most of the time, these [pathogens] are adequately protected by surgical masks, but for this virus we clearly need to stick to respirators,’” Babcock says. “With new viruses, as they arise, it is important to try to get that right.”

Although the study has limitations, the fact that it reflected real clinical use of the equipment is informative, even if the findings were affected by lack of compliance or inappropriate use.

“Having worn both N95s and face masks, they are both kind of irritating and uncomfortable,” she says. “I think people adjust them and touch them when they take them off and try to keep them from fogging up their glasses. The helpful thing is it’s a real-life study — if this is the way people wear N95s, then that’s the way they wear them. We need to understand the risks of transmission associated with the way these masks are really worn and used by healthcare personnel.”

According to the NIOSH paper, compliance with respiratory protection varies widely in historical studies, with reported ranges from 10% to 84%. In the recently published trial, healthcare personnel “kept diaries that included signs and symptoms of respiratory illness, annual influenza vaccination status, and exposure to household and community members with respiratory illness,” the NIOSH researchers stated.

NIOSH Perspective

Maryann D’Alessandro, PhD, director of the NIOSH National Personal Protective Technology Laboratory (NPPTL), Christopher Coffey, PhD, NPPTL associate director for science, and David Weissman, MD, director of the NIOSH Respiratory Health Division, replied to the following questions from Hospital Employee Health.

HEH: Why was the study undertaken?

NIOSH: The study was commissioned by several public health agencies 10 years ago, just after the 2009 H1N1 pandemic. At that time, there was significant controversy about what personal protective equipment should be used to prevent flu transmission to healthcare workers. The controversy centered around whether healthcare personnel should wear surgical face masks, which block large droplets, or respirators such as N95 respirators, which also can prevent inhalation of small airborne particles. A major problem was a lack of high-quality scientific data to answer this question.

HEH: Among outpatient healthcare personnel (HCP), there was no significant difference in lab-confirmed influenza in those who wore N95 respirators vs. medical masks. Are the findings strong enough to recommend that HCP can wear surgical masks instead of respirators during flu season, and even during the next pandemic?

NIOSH: Every flu season and pandemic is different. There are many factors that might affect recommendations for a given season, such as effectiveness of the influenza vaccine and severity of illness caused by the circulating strain. These might affect the intensity of prevention recommendations for that year, regardless of the results of this particular study. A very important take-away from the study is that whether they wear face masks or respirators, healthcare workers can get the flu. They should receive annual influenza vaccinations. We need to continue to conduct research to find better ways to protect them.

HEH: It seems intuitive that N95s, with tighter fit and greater particulate filtration, would be more protective than face masks. Why do you think researchers found no difference in protection and efficacy?

NIOSH: The study had some limitations, including self-reporting of symptoms in daily diaries [that] likely underestimated illness among HCP who often work while ill, or incomplete participant adherence to assigned protective devices that could have contributed to more unprotected exposures. However, there was no difference between study groups on this. Also, we cannot be sure what proportion of flu transmission occurred at work or in the community. Finally, we still do not know for sure the relative importance of contact transmission, droplet spray transmission, and aerosol transmission in spreading flu infection.

HEH: What do you think are the major implications of this study?

NIOSH: We need to continue efforts to better protect healthcare workers though multiple interventions, including engineering and work practice controls, influenza vaccination, and personal protective equipment. There still is a need to better understand influenza transmission and to continue to address the recommendations provided in the 2009 National Academies Report.3 It is too early to know what impact the current study may have on clinical practice. Health systems need to consider the broader body of scientific literature, including the current study, when making operational decisions.


  1. Radonovich LJ, Simberkoff MS, Bessesen MT. N95 respirators vs. medical masks for preventing influenza among health care personnel: A randomized clinical trial. JAMA 2019;322:824-833.
  2. Talbot TR, Babcock HM. Respiratory protection of health care personnel to prevent respiratory viral transmission. JAMA 2019;322:817-819.
  3. Institute of Medicine. Respiratory protection for healthcare workers in the workplace against novel H1N1 influenza A: A letter report. Sept. 1, 2009. Available at: Accessed Oct. 24, 2019.