Abstract
Two independent studies by two separate research teams (from Hong Kong and Singapore) failed to detect any influenza RNA landing on, or inhaled by, a life-like, human manikin target, after exposure to naturally influenza-infected volunteers. For the Hong Kong experiments, 9 influenza-infected volunteers were recruited to breathe, talk/count and cough, from 0.1 m and 0.5 m distance, onto a mouth-breathing manikin. Aerosolised droplets exhaled from the volunteers and entering the manikin’s mouth were collected with PTFE filters and an aerosol sampler, in separate experiments. Virus detection was performed using an in-house influenza RNA reverse-transcription polymerase chain reaction (RT-PCR) assay. No influenza RNA was detected from any of the PTFE filters or air samples. For the Singapore experiments, 6 influenza-infected volunteers were asked to breathe (nasal/mouth breathing), talk (counting in English/second language), cough (from 1 m/0.1 m away) and laugh, onto a thermal, breathing manikin. The manikin’s face was swabbed at specific points (around both eyes, the nostrils and the mouth) before and after exposure to each of these respiratory activities, and was cleaned between each activity with medical grade alcohol swabs. Shadowgraph imaging was used to record the generation of these respiratory aerosols from the infected volunteers and their impact onto the target manikin. No influenza RNA was detected from any of these swabs with either team’s in-house diagnostic influenza assays. All the influenza-infected volunteers had diagnostic swabs taken at recruitment that confirmed influenza (A/H1, A/H3 or B) infection with high viral loads, ranging from 105-108 copies/mL (Hong Kong volunteers/assay) and 104–107 copies/mL influenza viral RNA (Singapore volunteers/assay). These findings suggest that influenza RNA may not be readily transmitted from naturally-infected human source to susceptible recipients via these natural respiratory activities, within these exposure time-frames. Various reasons are discussed in an attempt to explain these findings.
Highlights
In recent years, discussions over the most clinically significant routes of influenza transmission have been extensive [1,2].Confusion and disagreements surround the definitions of the various transmission routes including ‘close contact’ transmission, ‘airborne’ transmission and ‘droplet’ transmission [3,4,5,6].Traditionally in outbreak investigations, airborne transmission has been implicated in secondary cases where direct contact with the infected source has not been documented
In the close contact exposure scenario, small droplets generated by an infectious patient can be directly inhaled and deposit in both the upper and the lower airway, whereas large droplets can be be directly inhaled, but the majority of these will probably deposit in the upper airways only, or directly enter the recipient’s eyes or even the mouth as a direct droplet infection [1,9,10,11,12]
Ethics approval for the Singapore study was granted by the Domain Specific Review Board (DSRB) of the National Healthcare Group (2009/00341) Singapore, and informed verbal and written consent was obtained from each participant in the study
Summary
Discussions over the most clinically significant routes of influenza transmission have been extensive [1,2]. In outbreak investigations, airborne transmission has been implicated in secondary cases where direct contact with the infected source has not been documented. Close contact transmission has been used to explain secondary cases arising from documented close contact with the presumed index case [6]. It is important to note that even in close proximity, multiple transmission routes may all be responsible for disseminating the infection, i.e. person-to-person transmission in such situations can be potentially due to either airborne, droplet and/or direct physical contact transmission [7,8]. Long distance airborne transmission has been postulated to be introduced by small droplet nuclei being carried by ambient airflows, where the moisture from small droplets has mostly evaporated away [7]
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