Small respiratory droplets that remain airborne for long periods account for the spread of more than half of influenza A infections within households. This finding, published in Nature Communications, has implications on the possible measures to reduce transmission, which have, in the past, focused on reducing contact or large droplets transmission.
Influenza A viruses spread through exposure to respiratory droplets expelled during coughing or sneezing from infectious individuals. Epidemiologists distinguish between larger droplets that settle to ground within 1-2 meters from the source and smaller infective droplets with a diameter below 5 micrometres that can remain airborne for longer periods; the latter are referred to as aerosols. Benjamin Cowling and colleagues, build on the hypothesis that clinical presentation of influenza is different depending on the mode of transmission, with aerosolized influenza A viruses causing more ‘typical influenza-like disease’, with fever and cough and large molecule based influenza A presenting just fever. From this they develop a mathematical model inferring the contribution of each mode to transmission of influenza A virus in households. They use data from two randomized trials of hand hygiene and face masks in families in Hong Kong and Bangkok to validate the model and find that the hand and face related measures used to prevent transmission did not substantially change the overall risk of infection for household contacts. This suggests that aerosol transmission is possibly the predominant mode of influenza A virus spread within cohabiting individuals.
The importance of aerosol transmission implies that the possible control strategies to prevent the spread of influenza within households need to be reconsidered.