an assessment of the evidence - Council of Canadian Academies

Influenza Transmission and the Role of PPRE: An Assessment of the Evidence 13via inhalation of influenza virus can be examined in a manner that does not confuseissues such as the size, behaviour and travel distances of particles.Infection control measures for the prevention of droplet transmission have beenmade with the assumption that droplets do not remain suspended in the air forsignificant periods of time, are affected primarily by gravity, follow a ballistictrajectory and travel no further than one to two metres from the infected person(Papineni, 1997). These measures are based primarily on epidemiologic evidencesuggesting that close contact is required for transmission of most diseases of therespiratory tract, and on studies demonstrating that spacing beds in barracks orfield hospitals or desks in schools at least three feet apart resulted in a substantialreduction in transmission of infection (Glover, 1920; Feigin, 1982). There is, however,accumulating evidence that, while the risk of acquisition of respiratory pathogensdecreases with increasing distance, transmission of infection across distances ofgreater than one metre may occur (Xie, 2007; Wannamaker, 1954; Aintablian,1998; Wong, 2004; Scales, 2003). Thus, the Centers for Disease Control andPrevention (CDC) have recently reconsidered this “short-range” distance benchmark(often referred to as the “three-foot rule”) and expanded it to two metres(CDC, 2007).In this report, short-range transmission is defined as infection occurring within abouttwo metres of the source and long-range transmission is defined as infection at distancesgreater than about two metres.It is clear that the risk of transmission of respiratory pathogens decreases as thedistance from an infected person increases. However, there is no empirical evidencethat increasing the recommended distance for control measures to two metres willresult in lower influenza attack rates, and there are no data that permit thequantification of differences in risk as a function of distance from the infectedperson. The panel has nevertheless adopted the “two metre” definition ofshort-range as a precautionary decision based on the revised estimates of particletrajectories as described by Xie (2007).