Transmission of influenza A in human beings
Authors: Julian W Tang, Yuguo Li
Journal: The Lancet Infectious Diseases (2007)

We read with interest the Review by Gabrielle Brankston and colleagues1Brankston G Gitterman L Hirji Z Lemieux C Gardam M Transmission of influenza A in human beings.Lancet Infect Dis. 2007; 7: 257-265Summary Full Text Full Text PDF PubMed Scopus (612) Google Scholar on the transmission of influenza A, but were disappointed to find it very biased against any evidence presented in favour of the airborne transmission of influenza. One surprising example of this was the authors' discussion of the classic study by Moser and co-workers,2Moser MR Bender TR Margolis HS Noble GR Kendal AP Ritter DG An outbreak of influenza aboard a commercial airliner.Am J Epidemiol. 1979; 110: 1-6Crossref PubMed Scopus (492) Google Scholar which they dismiss in a single sentence: "because of the free movement of passengers throughout the aircraft, close range transmission of influenza through droplet or direct contact could not be ruled out".1Brankston G Gitterman L Hirji Z Lemieux C Gardam M Transmission of influenza A in human beings.Lancet Infect Dis. 2007; 7: 257-265Summary Full Text Full Text PDF PubMed Scopus (612) Google Scholar Although we do not dispute the relevance and presence of this phrase in the original study, many other papers (including the reply by Tellier3Tellier R Questioning aerosol transmission of influenza—author's reply.Emerg Infect Dis. 2007; 13: 174Google Scholar to recent criticism of his earlier Review4Tellier R Review of aerosol transmission of influenza A virus.Emerg Infect Dis. 2006; 12: 1657-1662Crossref PubMed Scopus (640) Google Scholar) have cited this particular study as more supportive than not, of the airborne transmission of influenza.5Goldmann DA Epidemiology and prevention of pediatric viral respiratory infections in health-care institutions.Emerg Infect Dis. 2001; 7: 249-253Crossref PubMed Scopus (69) Google Scholar What makes the interpretation of this study by these other authors any less accurate than that of Brankston and colleagues?1Brankston G Gitterman L Hirji Z Lemieux C Gardam M Transmission of influenza A in human beings.Lancet Infect Dis. 2007; 7: 257-265Summary Full Text Full Text PDF PubMed Scopus (612) Google Scholar Another example of bias against evidence of airborne transmission in Brankston and co-workers' Review is their discussion about whether the ferret is a good model for human influenza. In fact, the ferret is now one of the preferred small animal models for studying human influenza in terms of pathogenesis and transmission.6Maines TR Chen LM Matsuoka Y et al.Lack of transmission of H5N1 avian-human reassortant influenza viruses in a ferret model.Proc Natl Acad Sci USA. 2006; 103: 12121-12126Crossref PubMed Scopus (304) Google Scholar Are the authors now saying that these other researchers are using an inappropriate model for studying human influenza? Regarding droplet dynamics, it is likely that the use of respiratory-assist devices, such as high-flow (up to 10–15 L/min) oxygen masks and mechanical ventilation, is likely to increase the potential risk of naturally (as opposed to artificially) produced aerosols containing influenza, as was suggested during the severe acute respiratory syndrome (SARS) outbreaks of 2003.7Hui DS Ip M Tang JW et al.Airflows around oxygen masks: a potential source of infection?.Chest. 2006; 130: 822-826Crossref PubMed Scopus (73) Google Scholar, 8Hui DS Hall SD Chan MT et al.Noninvasive positive-pressure ventilation: an experimental model to assess air and particle dispersion.Chest. 2006; 130: 730-740Crossref PubMed Scopus (132) Google Scholar Also, at one point, Brankston and colleagues seem to underinterpret one of their own references,9Yassi A Bryce E Moore D et al.Protecting the faces of health care workers: knowledge gaps and research priorities for effective protection against occupationally-acquired respiratory infectious diseases. Report to the Change Foundation 2004.http://www.changefoundation.com/tcf/TCFBul.nsf/(S001)/787D44CC13A1825C85256EA900452A6FGoogle Scholar in which particles of 6–10 μm diameter are listed as being able to remain suspended for "several hours" while falling a height of 3 m during which "deposition in nasal passages" is possible. Even accepting their statement that coughing mostly produces particles greater than 8 μm, this does not preclude coughed particles of sizes 8–10 μm being able to remain suspended and transmit infection over long distances. They themselves admit that "there is no exact particle size cut-off at which pathogen transmission changes from exclusively droplet to airborne, or vice versa". The exact proportion of different sized droplets produced in a cough will differ between individuals in different situations. After the droplets have left the mouth, their size will also be affected by the ambient temperature and relative humidity. For these reasons at least, Brankston and colleagues' generalisation that coughed particles are too large to sustain airborne influenza transmission is unacceptable. We realise that the practical and economical consequences of accepting influenza as an airborne infection are significant. However, this should not make us deliberately downplay or underinterpret any data that are supportive of this route of transmission. Hence, we would offer the counterpoint that this issue is not closed and echo Oshitani10Oshitani H Potential benefits and limitations of various strategies to mitigate the impact of an influenza pandemic.J Infect Chemother. 2006; 12: 167-171Summary Full Text PDF PubMed Google Scholar who stated that "the proportion of influenza infections that can be acquired by the airborne transmission is largely unknown". The potential role for airborne influenza transmission, therefore, still remains an important issue in pandemic influenza preparedness. We declare that we have no conflicts of interest. Transmission of influenza A in human beings – Authors' replyWe are pleased to respond to the correspondence regarding our systematic review. The passion evident in these letters clearly shows that the transmission of influenza has become a topic of debate beyond that of scientific interest. Over the past 10 years, several countries have engaged in planning for the next influenza pandemic, undoubtedly spurred on by concerns about the current H5N1 influenza epidemic in birds. These plans must include strategies that will effectively decrease the burden of disease, especially in health-care workers who will be essential to control efforts and at high risk of contracting influenza. Full-Text PDF