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Airborne flu viruses threaten health workers, expert says

Robert Roos * News Editor

Sep 29, 2006 (CIDRAP News) – A microbiologist who reviewed the evidence about how influenza viruses spread says that some official guidelines, including the US pandemic influenza plan, may not go far enough in protecting healthcare workers who take care of flu patients.

Writing in Emerging Infectious Diseases, Dr. Raymond Tellier of the University of Toronto says there is good evidence that flu viruses often spread via tiny airborne particles, despite a common belief that they travel mainly in large droplets that quickly fall to the ground after a flu patient coughs or sneezes.

Good protection from airborne particles requires the use of an N95 respirator. Yet the US, Canadian, and British pandemic flu plans advise healthcare workers to use simple surgical masks, which are much less effective, Tellier contends.

"Compelling evidence in the literature indicates that aerosol transmission of influenza is an important mode of transmission, which has obvious implications for pandemic influenza planning, and in particular for recommendations about the use of N95 respirators as part of personal protective equipment," he writes.

"Airborne particles" are usually defined as particles about 5 microns or less in diameter, Tellier says. Particles larger than about 10 to 20 microns fall quickly to the ground, while those smaller than 3 microns essentially do not settle. Coughing and sneezing generate particles in a range of sizes, many of them small enough to stay airborne for a long time. Airborne particles can penetrate into the lungs, whereas the larger particles and droplets are more likely to be trapped in the upper respiratory tract.

Experiments have shown that mice, monkeys, and human volunteers can be infected by exposure to aeorosol flu viruses, according to Tellier. In addition, various epidemiologic observations indicate that aerosol transmission is important. One example was a 1979 outbreak on an airliner with a defective ventilation system.

Tellier says many guidelines and review articles state that large droplets appear to be the main vehicle for flu virus transmission, but they offer little supporting evidence. "Despite extensive searches, I have not found a study that proves the notion that large-droplets transmission is predominant and that aerosol transmission is negligible (or nonexistent)," he writes.

Further, he says, infection control experts often argue that large-droplet precautions have proved adequate to stop flu outbreaks. But he contends that several factors cast doubt on the evidence for this view. For example, without laboratory diagnosis, what is believed to be a flu outbreak can be some other virus; serologic studies often are omitted, and asymptomatic flu infections in healthcare workers are probably missed; many people have partial immunity to seasonal flu viruses; and surgical masks provide some limited protection against aerosols.

Tellier goes on to say that evidence suggests that current strains of H5N1 avian influenza predominantly infect the lower respiratory tract, which in turn suggests that airborne particles are involved, since large droplets don't reach into the lungs.

"Given the strong evidence for aerosol transmission of influenza viruses in general, and the high lethality of the current strains of avian influenza A (H5N1), recommending the use of N95 respirators, not surgical masks, as part of the protective equipment seems rational," he states.

The current US pandemic influenza plan, according to Tellier, "acknowledges the contribution of aerosols in influenza but curiously recommends surgical masks for routine care; the use of N95 respirators is reserved for 'aerosolizing procedures.'"

In contrast, the Centers for Disease Control and Prevention's (CDC's) current infection control guidelines for healthcare facilities treating avian flu patients say that workers should use a fit-tested respirator at least as good as the N95 type when in a patient's room.

Bill Hall, a Department of Health and Human Services (HHS) spokesman in Washington, said today that the infection control guidance in the US pandemic flu plan is being updated. "We are conducting a rather in-depth process by which we'll be reviewing the science that is available on masks and the various positions out there," he told CIDDRAP News. "This journal article certainly adds one more data source to inform the decision-making."

"There's still a lot of debate on how flu is transmitted, whether it's aerosol or large droplets," Hall said. "The [pandemic] plan issued in November last year was based on current science and information at that point in time," but it is subject to change, he added.

Hall said HHS plans to cooperate with the Institute of Medicine to hold some workshops to discuss "community mitigation" in a pandemic, meaning measures such as closing schools, canceling public events, and recommending the use of masks. But no dates or other details have been decided.

Tellier R. Review of aerosol transmission of influenza A virus. Emerg Infect Dis 2006 Nov;12(11) (early online publication) [Full text]

See also:

CDC interim guidance on use of masks to control influenza
http://www.cdc.gov/flu/professionals/infectioncontrol/maskguidance.htm

CDC interim infection control guidelines for healthcare facilities treating people with avian flu
http://www.cdc.gov/flu/avian/professional/infect-control.htm