A few sentences have shaken a century of science.
A week ago, more than a year after the World Health Organization declared that we face a pandemic, a page on its website titled “Coronavirus Disease (Covid-19): How Is It Transmitted?” got a seemingly small update.
The agency’s response to that question had been that “current evidence suggests that the main way the virus spreads is by respiratory droplets” — which are expelled from the mouth and quickly fall to the ground — “among people who are in close contact with each other.”
The revised response still emphasizes transmission in close contact but now says it may be via aerosols — smaller respiratory particles that can float — as well as droplets. It also adds a reason the virus can also be transmitted “in poorly ventilated and/or crowded indoor settings,” saying this is because “aerosols remain suspended in the air or travel farther than 1 meter.”
The change didn’t get a lot of attention. There was no news conference, no big announcement.
Then, on Friday, the Centers for Disease Control and Prevention also updated its guidance on Covid-19, clearly saying that inhalation of these smaller particles is a key way the virus is transmitted, even at close range, and put it on top of its list of how the disease spreads.
There was no news conference by the C.D.C. either.
But these latest shifts challenge key infection control assumptions that go back a century, putting a lot of what went wrong last year in context. They may also signal one of the most important advancements in public health during this pandemic.
If the importance of aerosol transmission had been accepted early, we would have been told from the beginning that it was much safer outdoors, where these small particles disperse more easily, as long as you avoid close, prolonged contact with others. We would have tried to make sure indoor spaces were well ventilated, with air filtered as necessary. Instead of blanket rules on gatherings, we would have targeted conditions that can produce superspreading events: people in poorly ventilated indoor spaces, especially if engaged over time in activities that increase aerosol production, like shouting and singing. We would have started using masks more quickly, and we would have paid more attention to their fit, too. And we would have been less obsessed with cleaning surfaces. [Continue reading…]