Review
Transmission of influenza A in human beings

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Summary

Planning for the next influenza pandemic is occurring at many levels throughout the world, spurred on by the recent spread of H5N1 avian influenza in Asia, Europe, and Africa. Central to these planning efforts in the health-care sector are strategies to minimise the transmission of influenza to health-care workers and patients. The infection control precautions necessary to prevent airborne, droplet, and contact transmission are quite different and will need to be decided on and planned before a pandemic occurs. Despite vast clinical experience in human beings, there continues to be much debate about how influenza is transmitted. We have done a systematic review of the English language experimental and epidemiological literature on this subject to better inform infection control planning efforts. We have found that the existing data are limited with respect to the identification of specific modes of transmission in the natural setting. However, we are able to conclude that transmission occurs at close range rather than over long distances, suggesting that airborne transmission, as traditionally defined, is unlikely to be of significance in most clinical settings. Further research is required to better define conditions under which the influenza virus may transmit via the airborne route.

Introduction

The current avian influenza epidemic, which is affecting birds in Eurasia and Africa, has heightened world awareness about the possibility of a human influenza pandemic resulting from either antigenic drift or avian viral reassortment with a human strain.1 Pandemic planning has been accelerated and is occurring at many levels, from primary care to national governments worldwide.2, 3, 4

Despite 70 years of research since the influenza virus A was discovered,5 there continues to be vocal debate about the modes of influenza transmission, specifically whether influenza is transmitted via the airborne route, via the droplet or contact route, or a combination of these routes. Establishing how influenza is transmitted under different circumstances, and whether transmission requires close contact, is of great importance because the results will have a major influence on the choice of infection control measures in health-care settings.

Airborne precautions involve special ventilation controls, and to a lesser extent specialised masks (respirators), to prevent transmission. According to standard definitions, close contact is not required for airborne transmission to occur.6, 7 Necessitating the use of airborne precautions during a pandemic would require extraordinary resources and substantial advance planning; acknowledgment of a significant contribution of airborne transmission will affect where patients should be treated, how they would be triaged, the use of antiviral agents, and the choice of personal protective equipment. However, preventing droplet and contact transmission would require quite different control measures, since close contact with an infected case would typically be required for transmission to occur.

Some resolution to this debate is urgently needed. To better inform current infection control guidelines and continuing pandemic planning efforts, we did a systematic review of the English language literature in an attempt to assess the evidence about the routes of influenza transmission. Previous investigators have identified categories of evidence that should be considered when attempting to determine the modes of transmission of a respiratory pathogen, including: survival of the pathogen in the environment; experimental infections in laboratory animals and in human beings; and epidemiological studies of naturally occurring and laboratory-acquired infections.8 We have applied this framework specifically to influenza transmission in this review.

Section snippets

Methods

Articles of interest included only those that provided evidence for the route of influenza virus transmission in mammals. We specifically concentrated on whether a study provided evidence that a particular mode of transmission of the virus could be definitively ruled out of or into the exclusion of other modes of transmission. The standard US Centers for Disease Control and Prevention and Public Health Agency of Canada definitions used for the modes of transmission are shown in the panel.6, 7

Survival of the influenza virus in the environment

Six experimental studies examining the survival of influenza as an aerosol were reviewed.10, 11, 12, 13, 14, 15 These studies showed that, in general, different strains of influenza virus remain viable if artificially aerosolised from a liquid suspension, and retain their infectivity to several different host cell types (Table 1, Table 2). Specifically, erythrocytes, chick embryos, mice, and ferrets were all shown to be capable of acquiring infection via this route. Influenza was detected in

Discussion

Communicable viral respiratory diseases may be transmitted through various routes, including airborne, direct and indirect contact, and droplet transmission (panel). Droplet and contact transmission are traditionally defined as requiring close contact to occur, whereas airborne transmission may occur over much larger distances. As such, transmission of natural infection is seen over long (greater than 1 m between source and susceptible individual) and shorter (less than 1 m between source and

Conclusion

We conclude that natural influenza transmission in human beings occurs over short distances rather than over long distances. In turn, because it is well documented that airborne pathogens result in infection over long distances (in addition to close range), we conclude that natural influenza transmission occurs primarily via the droplet and contact routes. Although none of the reviewed studies could specifically rule out airborne transmission, we believe that the airborne route is neither the

Search strategy and selection criteria

These are described in detail in the Methods section on page 2.

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