Role of aerosol transmission in respiratory viruses

Abstract

Respiratory viruses cause considerable morbidity and mortality each year. These viruses transmit from one person to another when viable virions escape the respiratory tract of an infected person and successfully reach the respiratory tract of another person while maintaining viability through that journey. Transmission of virus is thought to occur through direct transfer of respiratory secretions, through contaminated surfaces, and through the air. Transmission through the air would occur via larger virus-laden respiratory droplets at close range, or via smaller virus-laden aerosols at close or longer range. The relative contribution of each transmission mode of respiratory viruses remains controversial, particularly the aerosol route. The overall aim of this thesis is to understand the role of aerosols in the transmission of common respiratory viruses by assessing the potential risk of transmission in influenza and other respiratory virus infections from virus-laden droplets and aerosols in hospital rooms with patients undergoing normal activity or medical procedures that may be aerosol-generating, and assessing the risk of influenza virus infections via airborne route. In addition, I evaluated the effectiveness of certain non-pharmaceutical interventions (NPIs), particularly respiratory etiquette and surgical face masks, in reducing the laboratory-confirmed influenza virus infections in the community.

In a series of air sampling studies, I found that both influenza A and B virus RNA can be recovered in droplet and aerosol fractions of the air particles collected in general patient rooms and well-ventilated isolation room. In addition, respiratory syncytial virus (RSV), human adenovirus (HAdV) and human parainfluenza viruses (HPIVs) were also detected in both droplets and aerosols, while RSV and HAdV were found in air particles <1µm. However, a caveat of these studies is that recovery of respiratory virus RNA in aerosols could occur without viable virus being present. The role of aerosols in nebulizer treatment, a medical procedure that is considered to be potentially aerosol generating, remained unclear, due to the lack of detection in both baseline and experimental sampling sessions. Future research is required to provide information on which medical procedures can generate respiratory aerosols that contain infectious pathogens.

In a risk assessment model, I showed that susceptible adults were at higher risks of being infected with influenza virus via inhalation of influenza virus-laden aerosols than children if they were exposed in the same room for equal length of time, as a result of higher inhalation rate in adult than in children. I also showed that increasing the ventilation rate could reduce the infection risk of all exposed persons in the patient room. Finally, I conducted a systematic review on the effectiveness of NPIs for influenza virus infections. I identified 10 randomized-controlled trials of surgical face masks which compared the influenza infection rate in individuals with and without the use of surgical face mask (combined with the practice of hand hygiene in some studies), but I did not observe any significant reduction in the reduction of influenza virus infections with the use of surgical face masks. I did not identify any literatures describing the protective effect of respiratory etiquette.