A Chinese study (I know, I know) of 391 primary COVID cases did some good work sorting out the incidence of transmission to the patients' aggregate 3,410 close contacts--about 9 close contacts per index patient. It found that only 3.7% of those close contacts caught the disease. Of that 3.7%, 6.3% of cases were asymptomatic, 16.8% were mild, 73.1% were moderate, and 10.1% were severe or critical. That means less than half a percent of the close contacts of the original patients picked up a severe or critical disease as a result.
There was considerable difference in the kind of contact that encouraged transmission as well. The transmission rate to household members was 10.3%, much higher than the average 3.7% rate. The transmission rate to healthcare workers was 1.0%, much lower than the average rate. The transmission rate on public transportation was even lower: 0.1%.
It also makes a big difference whether the index case is mild or severe. For asymptomatic index cases, the transmission rate was only 0.3%. For mild index cases, it was 3.3%; for moderate cases, 5.6%; and for severe or critical cases, 6.2%. The highest transmission rate was for index cases "with expectoration," 13.6%. The overall transmission rate for all kinds of cases without expectoration was 3.0%.
The lesson here is that the transmission rate is surprisingly low, even for obviously ill index patients, and the biggest societal risk factor is the size of their group of "close contacts." If infected people managed to keep their close contacts under 9, they'd be spreading their illness even less on average. That might prove difficult for severely ill people who require intensive care, but it shouldn't be that hard for anyone with a moderate case. The spread rate for asymptomatic cases is so small--a tenth or twentieth of the spread rate for symptomatic cases--that it barely figures into public policy.
The bottom line is that the disease will bounce off of 86% even of people in close contact with an "expectorating" COVID patient. It will bounce off an astounding 99.7% of people whose only exposure is to a completely asymptomatic COVID patient.
But, but--Trump colluded with Putin to remove all of our mailboxes and thereby eliminate remote and properly franked communications, which in turn exacerbated up close, in your face close contacts via his BFF Xi.
ReplyDeleteEric Hines
"The transmission rate to household members was 10.3%, much higher than the average 3.7% rate. The transmission rate to healthcare workers was 1.0%, much lower than the average rate. The transmission rate on public transportation was even lower: 0.1%."
ReplyDeleteI'm not sure how to interpret these number...does that 1% transmission to healthcare workers mean in a single encounter, say a single visit to a doctor or a nurse stopping by a hospital room one time? Or does it mean the transmission probability over some period of time?
All very encouraging, except we know that some people do get the disease. Somehow.
ReplyDeleteDavid--Considering 391 index patients in close contact with a total of 3,410 people, some portion of the 3,410 people were healthcare providers. Only 1% of the healthcare providers in contact with these 391 index patients caught COVID from them. A larger percentage, 10.3%, of household members in contact with these index patients caught COVID from them. Overall, 3.7% of the close-contact group caught it.
ReplyDeleteAVI--well, sure, the study doesn't say that no one gets it. If 391 index patients are in close contact with 3,410 people, and 3.7% of the 3,410 people catch COVID, that's 126 people catching it from 391 initial sufferers. It's going to spread, especially if each sick person is in close contact with a lot of other people. It's just that the odds even for these close-contact people are surprisingly low when viewed from the perspective of an individual in close contact with a sick person. They're even lower if the public's only close contact is with an asymptomatic sick person. If each sick person (especially a symptomatic sick person) tries hard to be in contact with a lot less than 9 people, the spread will be even less.
PS, David, I took them to have looked at close contact over a period of time, i.e., the presumptive contagious period of an active case.
ReplyDeleteTrump colluded with Putin to remove all of our mailboxes
ReplyDeleteOutdated.
Trump is appointing Putin to Postmaster General and has traded Congress for the Duma effective 10/1.
Rumor has it that Trump is negotiating to trade New York State for Kazakhstan before 12/31
Rumor has it that Trump is negotiating to trade New York State for Kazakhstan before 12/31
ReplyDeleteHe should make Putin take California, too, which already has historical ties with Russia.
Eric Hines
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ReplyDeleteOkay, I read the paper, which is better-written than many such things are. If I understand right, the probabilities are per-event...ie, what is the chance that a person will become infected if at home with someone who has it (about 10%), is a healthcare working involved in treating someone who has it (1%), or is close to an infected person on public transportation (.1%).
ReplyDeleteBut for the healthcare case and the transportation case, events are repeated...if a healthcare worked is involved in treating 20 infected people per month, then his chance of getting the disease is about 18% during that month, if I remember my probability class correctly. If a person rides public transportation 250 times a year, twice a day...and 10% of the time he is near an infected person, then he has a 5% chance of getting it.
Those still are pretty low probabilities, especially compared to the initial estimates, which led to what turns out to be overreactions.
ReplyDeleteBut these probabilities are only part of the story. There remains the so what part. Given an infection, who cares? Mortality rates are very much under 1% for most age groups, and only in the 3%-5% range for geezers, assuming underlying good health.
Of course there is a who cares--it's no fun to get sick, whether the degree of sickness ranges from a bad cold to a bad flu, and it's even less fun--and more expensive--to get sick enough to warrant hospitalization. But that's a far cry from most of the experts' initial estimates, which were based on wholly inadequate initial data, much of which were falsified.
Eric Hines
David--but these statistics represent the entire period of exposure for this group of index patients and their close contacts. The probably don't have to be multiplied to give an accurate picture; the repetitive contacts are already built into the observed result.
ReplyDelete"but these statistics represent the entire period of exposure for this group of index patients and their close contacts"..that would seem to be the case with people living in the same home...but on public transportation?...there are going to be different contacts every trip.
ReplyDeleteSimilarly for healthcare workers...even if they are counting the entire time of exposure to all patients that a particular HC worker is working with, what about next week or next month when that same HC worker has a new group of patients?
I think the writeup is somewhat ambiguous on these points, will go back and re-read it.
For a public transportation worker or rider to be in the "close contact" group at all is a little hard to understand, but it's got to be a single person, therefore if there were, say, 100 of them in the group of 4K or so, and only x% of them got sick, it's still a snapshot of the risk over the entire period, one way or another.
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