Wayland eNews :: Pandemic Flu Taskforce

The Nov 06 CDC Journal- Emerging Infectious Diseases reviews the spread of influenza A virus by airborne transmission. Below are some of the articles findings, conclusions and recommendations in an abridged form, more appropriate for lay individuals, with less medical-speak.

Background

In theory, influenza viruses can be transmitted through aerosols ( very small micro-size droplets which stay airborne), large droplets ( which drop onto surfaces and do not stay airborne), or direct contact with secretions or contaminated clothing/bed items (called “ fomites”). What the researchers studied was the role of small micro size droplets (aerosols) vs. large droplets in the transmission of Influence A, which is the same type as the Avian flu virus.

The concerns were that, if large-droplet transmission was the main mechanism by which influenza virus infection is transmitted, then simple surgical masks ( not protective against micro size particles) together with hand washing might be sufficient to prevent infections, and that special N95 respirator masks may not be needed for protection. The authors reviewed the literature and concluded the following. First the Review and then the Recommendations.

Influenza Virus Aerosols

By definition, aerosols are suspensions in air (or in a gas) of solid or liquid particles, small enough that they remain airborne for prolonged periods because of their low settling velocity. Coughing or sneezing generates a substantial quantity of particles, a large number of which are micro size. Aerosols, though heavily diluted, remain airborne and thus can be carried over large distances thru ventilation systemsà potential for long-range infections. The occurrence of long-range infections is affected by several other factors. These include the infectious dose, the amount of infectious particles produced, the duration of shedding of the infectious agent, and the persistence of the agent in the environment.

Lab Observations

Humans, acutely infected with influenza A virus, have a good deal of virus in their respiratory secretions, which makes generation of virus aerosols possible. If these viruses do not dry out (good relative humidity exists), then viral aerosols persist (float in the air). Indeed, infectious influenza viruses in an aerosol can exist for up to 24 h. Of interest, for the flu virus, both too little humidity or too much humidity decreases infectivity.

Researchers, using mice, squirrel and human volunteers, have clinically demonstrated, in multiple experiments, that actual influenza infection via the aerosol mechanism clearly occurs.

In one experiment, researchers found that, while nose drops with zanamivir were protective in preventing flu when flu virus was artificially delivered to the nasal passage to the volunteers under laboratory conditions, the nasal zanamivir was not as effective in preventing natural flu illness. This suggested to the researchers that the lower respiratory tract (which is most efficiently reached by the aerosol route) is the preferred site of infection by Flu and, indeed, the second half of the study showed that inhaled zanamivir was highly protective in preventing “natural flu”.

These lab experiments and observations strongly support the view that many, possibly most, natural influenza infections, occur by the aerosol route and that the lower respiratory tract may be the preferred site of initiation of the infection.

Epidemiologic Observations

When researchers looked at actual influenza A outbreaks, further support for aerosol transmission occurred.

In one study, which analyzed a flu outbreak in an airplane, where there was defective ventilation, the aerosol mechanism seemed responsible for the spread throughout the plane. Passengers seated far away from the source patient became infected, not just those seated nearby.

Another study cited a VA experience whereby a group of isolated Tb patients protected by UV ceiling lights (highly effective in inactivating viruses floating in the air((aerosols)), but not effective for surface or droplet contamination) had much lower rates of flu illness compared to other isolated Tb patients in rooms without protective UV lights.

Conclusion /Recommendations

The bottom line is that flu virus transmission occurs by all three mechanismsà aerosols, large droplets, and direct contact with secretions or fomites. To reduce flu infections, frequent handwashing is crucial. While simple surgical masks afford some protection, using a N95 mask respirator provides more optimal protection. It would be wise to have a small home stockpile of N95 respiratory masks available.

Arnold Soslow, MD