Influenza and airborne transmission

There has been a long-standing gap in our understanding of the mechanisms behind the transmission of human influenza. Yep, that’s right, we do not know exactly what the major route of transmission is. That’s a strange reality given we regularly send men and women into space and bring them back, yet we cannot definitively state how the majority of flu infections are acquired.

Do we catch “the flu” from people coughing and sneezing on us? What about collecting our change from the cashier harboring an active infection? Or, do we risk infection by simply breathing the same air as someone with influenza? Are all three routes of infection? Which one poses the most risk to the uninfected? Are there degrees of danger of infection, or does each route carry equal risk of transmission?

A recently published paper, Measurement of Airborne Influenza Virus in a Hospital Emergency Department, Francoise M. Blachere, et al.¹ informs us of findings that will help us close our knowledge gap on influenza transmissions.

…we confirmed the presence of airborne influenza virus and found that 53% of detectable influenza virus particles were within the respirable aerosol fraction. Our results provide evidence that influenza virus may spread through the airborne route.

“Respirable aerosol fraction“: particles small enough to remain suspended in the air for an extended period, our “exhaled respiratory secretions”, and small enough for us to take them into our own respiratory tracts.

These findings are important because we are informed that if we remain outside of the “close personal contact” measure, three feet or more away, we are at negligible risk of infection. In fact, “social distancing”, maintaining three or more feet of space between people, is touted as one of the main non-pharmaceutical interventions during an influenza pandemic.

This minimum distance recommendation in the “social distancing” recommendations is meant to encourage and enable as many workers as possible to carry on their work, some of which is indispensible to a well-running society. This minimum is the minimum recommended; there are also recommendations for those who are able to do so to remain at home, limiting as much social contact as possible.

Many of the pandemic recommendations, planning, and preparations are based on “best available information” at the time of their drafting and/or acquisitions. As time has passed studies have been carried out and our base of knowledge has expanded, albeit, not fast enough or broadly enough, but it has expanded.

What we don’t see enough of is policy advisors and pandemic planners adjusting their recommendations, plans, and provisions, to keep up with even the glacial pace of our expanding and broadening influenza knowledge base. This study clearly, or clearly to me, informs us that if we want certain folk to report to work during a time of a severe influenza pandemic we damned well better supply them with nothing less than N-95 masks.

Should that “severe influenza pandemic” happen, folk will flock to the internet and search out information on the spread of influenza and how to avoid it. The results of this paper are going to filter through the internet, this post being an example, will be read, and the information passed along. Workers will come to know the latest “best available information”, if plans and provisions do not account for or provide for that information workers will be all the more reluctant to report to work.

All of the above said: The experiment did not account for various temperatures and rates of humidity. Samples were collected in one hospital emergency department over the course of several days. During the sample collections the paper reports the temperature at the various collection points was a mean of 23.5⁰ Celsius, +/- 1.4^o Celsius, [74.3⁰ Fahrenheit] and a mean relative humidity of 30%, +/- 3.3%. It has been shown that the influenza virus has variable viability dependent on temperature and humidity.

Generally, the cooler the temperature and the lower the humidity, the longer the virus will remain viable (able to infect a receptive cell). Interestingly, Pope Clement VI is reported to have locked himself up in Avignon in front of a raging fire (or between two) during the plague outbreak of 1347-50, to avoid infection. Pope Clement VI was successful in his avoidance tactic. Heat kills viruses.

It would be interesting to see the experiment repeated at higher temperatures, passive prevention – perhaps. But as it is, most artificially controlled environments are fairly friendly to the survival of the influenza virus, so we would do well to remember that we do run the risk of casual contact infection when someone in a room has an active infection of influenza.

SZ