an assessment of the evidence - Council of Canadian Academies

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Influenza Transmission and the Role of PPRE: An Assessment of the Evidence 31CONCLUSIONS ON MODES OF INFLUENZA TRANSMISSIONThe following summarizes the consensus opinion reached by the panel.1. Ballistic, nasopharyngeal, tracheobronchial and alveolar-sized particlesare all emitted from the human respiratory tract.2. Evidence about the relative contribution of the different modes oftransmission to the spread of influenza is sparse and inconclusive.3. There is evidence that influenza is transmitted primarily at short range.4. There is evidence that influenza can be transmitted via inhalation oftracheobronchial and alveolar-sized particles at short range.5. There is evidence that deposition of nasopharyngeal-sized particles in theupper respiratory tract can cause infection.6. There is evidence that contact transmission can occur. The current weightof evidence suggests that transmission of influenza by inhalation is moreprobable than by indirect contact.7. The evidence is lacking to determine whether long-range transmission ofinfluenza occurs, but it cannot be ruled out.
32 Influenza Transmission and the Role of PPRE: An Assessment of the EvidenceCHAPTER 3 – PROTECTIVE MEASURES AGAINST INFLUENZATRANSMISSIONThe second element of the charge to the panel is the following:Based on the conclusions of this review (of the various modes of transmission),what is your assessment of the contribution that N95 respirators or surgicalmasks will make to the prevention of transmission 15 of seasonal and pandemicinfluenza?In light of the conclusions of Chapter 2, a surgical mask or respirator will mitigatethe transmission of influenza only if it does one or more of the following:• reduces the amount of infectious material that is introduced into thesurroundings from an infected person;• prevents ballistic particles from landing on the mucous membranes of apotential host;• prevents self-inoculation of mucous membranes by contaminated hands;• prevents nasopharyngeal, tracheobronchial and alveolar-sized particles frombeing inhaled by a potential host.Surgical masks and respirators need to be evaluated and compared based on theirdesign, function and ability to fulfill these criteria. This will establish what thetheoretical role of each could be in preventing influenza transmission. Thesecharacteristics will then need to be examined within the context of a generalhierarchy of control and practical mitigating factors in order to evaluate theincremental protective contribution of respirators and surgical masks respectively.THE HIERARCHY OF CONTROLThe only interventions which have been tried and shown unequivocally to reducethe spread, and to mitigate the impact, of influenza in populations are vaccinesand antivirals. The panel recognizes that the most important line of defense againstinfluenza is vaccination, but that other interventions are needed because vaccinationwill not be 100 per cent effective, and because a vaccine is not likely to be availableduring the first wave of a pandemic. Prophylactic antivirals are animportant adjunct to vaccine in the control of seasonal outbreaks of influenza in15 The contribution is to be assessed on the context of the infection control hierarchy; the componentsof which include, engineering controls, administrative controls and personal protective equipment.
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