Thread by @linseymarr

Tang & I wrote about updating transmission routes to inhalation, spray, and touch, following up on suggestions by Yuguo Li and @Don_Milton. academic.oup.com/cid/advance-ar… /1

"Current infection-control guidelines subscribe to a contact/droplet/airborne paradigm that is based on outdated understanding. Here, we propose to modify and align existing guidelines with a more accurate description of the different transmission routes." /2

"This will improve the effectiveness of control measures as more transmissible variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerge." /3

"a correct and thorough understanding of how SARS-CoV-2 transmits underpins the design and implementation of effective control strategies. Just as importantly, it reduces the time, effort, and resources wasted on ineffective actions (ie, “hygiene theater”)." /4

Definitions: "here we start with a clean slate and build on widely accepted definitions that are not encumbered by discipline-specific connotations." /5

"According to dictionaries, a droplet is “a small drop of a liquid”; no minimum or maximum size is specified. Thus, all bits of fluid that are released from the respiratory tract can be considered to be droplets initially." /6

"Depending on ambient humidity, they may partially or fully evaporate, shrinking to 20–40% of their initial diameter, in which case they may be solid or semi-solid [8]." /7

"According to dictionaries, an aerosol is “a suspension of fine solid particles or liquid droplets in a gas.” Aerosol particles, whether solid or liquid, are often referred to simply as “aerosols.” " /8

"We will refer to droplets that are large enough to fall to the ground within a few seconds, and that do not evaporate significantly within this time period, as “large droplets.”" /9

"Based on these definitions, the threshold between aerosols and large droplets is approximately 100 µm (the average diameter of a human hair) and depends on environmental conditions such as humidity and air flow velocity and direction [9–11]." /10

"This threshold is also the upper limit of particles that can be inhaled [12, 13]." /11

"To overcome these limitations, we propose a shift in how we define transmission routes, to better align them with the best-available evidence for actual mechanisms of transmission." /12

"This framework will be more accurate and more truly representative of how pathogens move through the environment, and therefore how people are exposed to them." /13

"1. Inhalation. Pathogens carried by aerosols can be directly inhaled into the respiratory tract and deposited at various sites, depending on the size of the aerosol. Aerosols are most concentrated close to the point of release (an infected person),..." /14

"and the smaller ones can remain floating in the air for minutes to hours and can be carried long distances on local air currents. Transmission can occur at any distance and is more likely when people are in close proximity." /15

"2. Spray. Pathogens in large droplets may land directly onto external mucous membranes. Large droplets follow semi-ballistic trajectories [25] and settle to the ground within a few seconds. Usually, they do not travel >1–2 m, although a sneeze can propel them farther [26]." /16

"3. Touch. A susceptible individual transfers a pathogen, usually by hand, to their mucous membranes. This may occur, for example, if the individual shakes hands with an infected person who wiped their nose or touches a contaminated object." /17

"These definitions directly correspond to different interventions: respirators and masks with excellent fit and filtration capability, along with ventilation and filtration to reduce transmission by inhalation;" /18

"distancing and face shields to mitigate transmission by sprays (where surgical masks and face coverings are also effective to some degree); and hand hygiene, surface cleaning, and gloves to reduce transmission by touch." /19

"We believe that aligning how we talk about different transmission routes, particularly within an infection-control context, with actual mechanisms of transmission, will lead to clearer communication and an improved understanding of and compliance with public health guidance."/20