How Respiratory Viruses Really Spread

We got it wrong with cholera, measles, tuberculosis, and SARS-CoV-2

Source: iStock/exdez

How an infectious disease spreads from one person to another is a question so vital that if we get it wrong, we will fail to control its spread and may even make it worse than it has to be.

During the 19th century in London, people believed that miasma (‘bad air’) spread cholera, a diarrheal bacterial disease. So, stinky sewers were dumped into the Thames River, a major source of drinking water. This move ended up killing far more people, as in fact cholera spreads via contaminated food and water.

We have made similar mistakes with measles and tuberculosis, which are aerosol infections that we thought, for decades, were spread by droplets. And most recently, we have done the same with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the culprit behind the Covid-19 pandemic.

Droplets vs. aerosols

In July 2020, the World Health Organization (WHO) stated that SARS-CoV-2 spread via respiratory droplets that people emit by sneezing and coughing to object surfaces or another person within <1 meter apart. The same went for the U.S. Centers for Disease Control and Prevention (CDC), which also did not acknowledge that SARS-CoV-2 can spread via aerosols until October 2020, although only as a secondary route to droplets.

It was only recently that the WHO and CDC finally accepted aerosols as one of the main modes of SARS-CoV-2 transmission, on April 30, 2021 and May 7, 2021 respectively. The CDC is a step ahead of the WHO in this regard, stating that “surface transmission is not the main route by which SARS-CoV-2 spreads, and the risk is considered to be low.” But no news conferences were held, so the update did not get much attention initially.

Droplets are large particles that are heavy and drop from the air quickly, whether indoors or outdoors. Direct contact with such virus-laden droplets is from people; it’s indirect contact when it comes from fomites (contaminated objects and surfaces).

Aerosols, in contrast, are small particles that are light and linger in the air for some time, especially in poorly ventilated and crowded indoors. Aerosols thus…