Yes Covid is airborne; no don’t freak out

Kind of like everything about Covid.  The evidence seems pretty clear that some non-trivial portion of the time, Covid spreads through the air not just by droplets (which typically quickly fall to the ground in 6 feet or less), but by tiny, tiny aerosols that can hang around for a while.  Here’s one recent estimate of the relative frequency of proportion of transmission:

There’s still a fair amount of controversy, but it does, nonethless, seem pretty clear that there’s indoor situations where just wearing a mask six feet apart (droplets) and washing your hands & not touching your face (fomites) are not enough and aerosols may well get you.  And, “airborne” transmission is inherently scary because it is harder to protect yourself and airborne diseases (like measles and chickenpox) tend to be highly contagious.  But, it’s clear that Covid-19 is, at least some of the time airborne, and not highly contagious.  (Really, remember the modal person with Covid spreads it to 0 other people).  Based on my amateur epidemiology I was thinking that there must just be a high dose of aerosols necessary to infect a person (which would explain why low ventilation, indoor spaces would be so key).  But, looks like it’s even more than that and for whatever reason, people with Covid-19 just don’t emit that much infectious particles compared to what it actually takes to infect another person.  This is now my favorite article on transmission I’ve read, in large part for basically saying, “yes, airborne, but, no, don’t freak out.”

Notwithstanding the experimental data suggesting the possibility of aerosol-based transmission, the data on infection rates and transmissions in populations during normal daily life are difficult to reconcile with long-range aerosol-based transmission. [emphases mine] First, the reproduction number for COVID-19 before measures were taken to mitigate its spread was estimated to be about 2.5, meaning that each person with COVID-19 infected an average of 2 to 3 other people. This reproduction number is similar to influenza and quite different from that of viruses that are well known to spread via aerosols such as measles, which has a reproduction number closer to 18. Considering that most people with COVID-19 are contagious for about 1 week, a reproduction number of 2 to 3 is quite small given the large number of interactions, crowds, and personal contacts that most people have under normal circumstances within a 7-day period. Either the amount of SARS-CoV-2 required to cause infection is much larger than measles or aerosols are not the dominant mode of transmission.

Similarly, the secondary attack rate for SARS-CoV-2 is low. Case series that have evaluated close contacts of patients with confirmed COVID-19 have reported that only about 5% of contacts become infected. However, even this low attack rate is not spread evenly among close contacts but varies depending on the duration and intensity of contact. The risk is highest among household members, in whom transmission rates range between 10% and 40%.24 Close but less sustained contact such as sharing a meal is associated with a secondary attack rate of about 7%, whereas passing interactions among people shopping is associated with a secondary attack rate of 0.6%.4

All told, current understanding about SARS-CoV-2 transmission is still limited. There are no perfect experimental data proving or disproving droplet vs aerosol-based transmission of SARS-CoV-2. The balance of evidence, however, seems inconsistent with aerosol-based transmission of SARS-CoV-2 particularly in well-ventilated spaces. What this means in practice is that keeping 6-feet apart from other people and wearing medical masks, high-quality cloth masks, or face shields when it is not possible to be 6-feet apart (for both source control and respiratory protection) should be adequate to minimize the spread of SARS-CoV-2 (in addition to frequent hand hygiene, environmental cleaning, and optimizing indoor ventilation).

To be sure, there are rarely absolutes in biological systems, people produce both droplets and aerosols, transmission may take place along a spectrum, and even medical masks likely provide some protection against aerosols.6,10 It is impossible to conclude that aerosol-based transmission never occurs and it is perfectly understandable that many prefer to err on the side of caution, particularly in health care settings when caring for patients with suspected or confirmed COVID-19. However, the balance of currently available evidence suggests that long-range aerosol-based transmission is not the dominant mode of SARS-CoV-2 transmission.

So, in the end, it’s Japanese 3C’s as much as ever regardless of the aerosol vs. droplet debate.  Wear a mask.  Avoid close contact and close spaces.  And don’t freak out because you hear it’s “airborne” and could be anywhere. And there you go.



Trump is not even doing his job

He’s so bad at being president, we hardly even notice, but loved this from David Plotz:

In the way you stop smelling the stench of garbage when you live next to the dump, we’ve stopped noticing a disturbing fact about the Trump presidency. 

The nation is beset by the worst crisis in at least 75 years. The pandemic rages because we failed to control its initial spread. The economy is grinding to a halt. Unemployment is stuck at depression levels. Mass protests demanding policing reform have transfixed the nation. An economic war with China is deepening into a Cold War.

The president isn’t doing anything about any of it, and more disturbingly: Americans don’t even expect him to do anything about any of it.

Is he chairing daily meetings of a pandemic task force? Is he meeting with school superintendents to find out what resources they need to open safely?  Is he negotiating into the wee hours with Nancy Pelosi over the shape of the next emergency relief bill? Is he calling BLM leaders to discuss law enforcement reform? Of course he’s not.

All reports indicate that he’s singularly focused on his reelection. Yet the purpose of his reelection is for him to serve the nation as president, which is the very thing he is not doing.

He spends his time watching TV, hurling random attacks at Joe Biden, and shaking up his presidential campaign. We have come to see that as perfectly normal. But it’s not normal!

And I love that Plotz started with the acclimation to a bad smell, as I’ve been using that metaphor for Trump for as long he’s been at it.

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