(Non)Spread from asymptomatic cases - Printable Version
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(Non)Spread from asymptomatic cases - M T - 06-09-2020
I'm sure everyone has seen the WHO's statement that tracking of asymptomatic cases has led to very rare transmissions. Many people are surprised by this.
The first I saw of the frequency of asymptomatic cases was an early study in China saying that the confirmed cases were not sufficient to explain the spread to other areas. I've seen the frequency of asymptomatic cases be the result of models based on the spreading of the disease.
Is there another explanation of the overall spread if asymptomatic cases almost never spread the disease? Is R0 much greater than previously thought? Does this suggest that infection-at-a-distance is more frequent than thought? Just where is the community spread coming from?
Or, is it possible that WHO's statement needs to be interpreted to mean that in the world of masks & social distancing, asymptomatic cases aren't spreading the disease but symptomatic still is? This makes some sense to me. The non-pharmaceutical interventions (NPI) may not be enough if a person shedding heavily is close by, but it is enough for a lightly shedding person is 6' away with two masks in between. This allows for spread from asymptomatic cases before a society adopted social distancing and masks.
RE: (Non)Spread from asymptomatic cases - burger - 06-09-2020
WHO more or less retracted the statement on asymptomatic spread. https://www.statnews.com/2020/06/09/who-comments-asymptomatic-spread-covid-19/
Quote:A top World Health Organization official clarified on Tuesday that scientists have not determined yet how frequently people with asymptomatic cases of Covid-19 pass the disease on to others, a day after suggesting that such spread is “very rare.”
Spread from pre-sympomatic people is still a real thing. So, unfortunately, we can't ditch our masks and hand san yet. :)
RE: (Non)Spread from asymptomatic cases - magnus - 06-09-2020
(06-09-2020, 12:24 PM)burger Wrote: WHO more or less retracted the statement on asymptomatic spread. https://www.statnews.com/2020/06/09/who-comments-asymptomatic-spread-covid-19/
Quote:A top World Health Organization official clarified on Tuesday that scientists have not determined yet how frequently people with asymptomatic cases of Covid-19 pass the disease on to others, a day after suggesting that such spread is “very rare.”
Spread from pre-sympomatic people is still a real thing. So, unfortunately, we can't ditch our masks and hand san yet. :)
This sounds like when the CDC said that spread along surfaces is not easy...but then a day later said, it's still possible and to take appropriate precautions.
Leftcoast - Leftcoast - 06-09-2020
Messaging is very important - I’ve already had the WHO statement (mis)quoted to me as proof shelter-in-place was unnecessary. “See? Temperature checks are all that was needed!”
lex24 - lex24 - 06-09-2020
It’s also important to accept good news every once in a while. And the WHO’s statement is good news. Is it a certainty? Of course not. What is?
The messaging on this has been all over the place. It wasn’t that long ago that Santa Clara’s Health Director proclaimed that she believed we could all get the virus.
I tell you where this is most important- school. If you want to further dividing the education gap between people with means and people without means then one great way to do that is to keep kids at home. Those that have strong educational backgrounds in their family, money etc. will do just fine. Those who don’t will suffer tremendously.
RE: (Non)Spread from asymptomatic cases - M T - 06-10-2020
Now, back to my questions. IF we accept that WHO was somewhat accurate in reporting that tracking of asymptomatic cases lead to few transmissions, how does that fit in with what was seen previously. (You can define asymptomatic by any particular level of maximal symptoms. I do NOT want to include pre-symptomatic.)
My guess would be that "asymptomatic" would apply to people who had (perhaps barely detectable) virus in their system but whose immune system kept the virus from replicating more than a certain amount. For those people, the shedding was minimal, resulting in some virus in the environment but not as much as the people in which the virus was replicating a lot (and triggering all the symptoms and immune responses). Because of minimal virus and minimal immune response(?), these people would have minimal symptoms.
At this time, all publicly announced tests are binary. Either Yes or No. I understand that the PCR tests involve up to a certain number of iterations in order to detect the virus. If detection happens within some fixed number N of iterations, the verdict is Yes, otherwise the verdict is No. The number of iterations to detect the virus could be reported, but isn't. I believe the number of iterations could be used as a rough estimate of how much virus was in the sample (which is a function of the amount of virus in the patient along with how well the sample was taken). If the results could give us a finer measure (such as High, Medium, Low level) of the virus present, that might be useful.
So, a person exposed to an "asymptomatic" case would have fewer virus to inhale. The use of masks would help to lower that as well. In either case, the person's body has to react to fewer virons and maybe can do so well enough to keep them from replicating too much.
I could imagine that the use of masks increases the percentage of "asymptomatic" cases. Perhaps people are getting exposed to very minimal number of virus (wafting on the winds from far away) and their immune system is training on them. Then when exposed closely to an "asymptomatic" case, with masks, they get enough dose that they get a level of virus that could be detected. If that does is low enough that their body can deal with it, they too become "asymptomatic". If they get a big dose from someone shedding a lot, their immune system doesn't keep the virus in check, so they get a full blown case, with symptoms.
These dynamics could explain the impressions of some doctors that subsequent infection waves have been less severe.
So, that's a theory (from a lay person) of what might be happening. It's your turn to point out where I'm wrong. Have at it.
RE: (Non)Spread from asymptomatic cases - dabigv13 - 06-10-2020
It is a tempting theory that the amount of initial inoculum of virus dictates the clinical course, with people exposed to lots of virus having more severe courses than those who had a small inoculum.
As of yet, there is no good evidence to suggest this is how covid19 works. Smallpox can behave like this, but I don't believe we have any evidence of this effect in respiratory viruses in general, in humans. Certainly there is a level low enough that exposure will not generate an infection, but I doubt those very low level exposures would generate a protective immune response either, since the innate immune response was presumably what took care of it before the adaptive response even needed to go into gear.
Would welcome any more evidence on this, as I am certainly no expert in immunology.
RE: (Non)Spread from asymptomatic cases - akiddoc - 06-10-2020
(06-10-2020, 02:32 PM)dabigv13 Wrote: It is a tempting theory that the amount of initial inoculum of virus dictates the clinical course, with people exposed to lots of virus having more severe courses than those who had a small inoculum.
As of yet, there is no good evidence to suggest this is how covid19 works. Smallpox can behave like this, but I don't believe we have any evidence of this effect in respiratory viruses in general, in humans. Certainly there is a level low enough that exposure will not generate an infection, but I doubt those very low level exposures would generate a protective immune response either, since the innate immune response was presumably what took care of it before the adaptive response even needed to go into gear.
Would welcome any more evidence on this, as I am certainly no expert in immunology.
I also do not know of any dose related studies regarding severity of illness.
The WHO backed off the lack of asymptomatic spread statement today and claimed it was in a misstatement after Fauci stated that the WHO was wrong.
RE: (Non)Spread from asymptomatic cases - magnus - 06-28-2020
More damning commentary on the WHO in this article about the evolution of asymptomatic spread awareness.
https://www.nytimes.com/2020/06/27/world/europe/coronavirus-spread-asymptomatic.html
RE: (Non)Spread from asymptomatic cases - M T - 06-28-2020
(06-28-2020, 12:28 AM)magnus Wrote: More damning commentary on the WHO in this article about the evolution of asymptomatic spread awareness.
https://www.nytimes.com/2020/06/27/world/europe/coronavirus-spread-asymptomatic.html
That article feels as if it was written to try to rescue Dr. Roche's reputation. Among other things, events are mentioned out of order, distorting cause and effect.
The letter (with lead author Dr. Roche) published January 30 in NEJM (original version here) contained critical findings supported only by hearsay evidence, that was not indicated as such. It was called out as flawed on February 3 for reporting the Patient 0 (a woman) as being asymptomatic (based on hearsay evidence from other patients) when she did have symptoms. Only on February 5, did Dr. Roche's team speak to the patient who they asserted was asymptomatic.
The Böhmer paper referenced in the NYT is here.
The January 30 letter states "During her stay, she had been well with no signs or symptoms of infection but had become ill on her flight back to China"
and "However, it is notable that the infection appears to have been transmitted during the incubation period of the index patient, in whom the illness was brief and nonspecific."
The nature of her illness and symptoms seems stated as a fact known to the authors, rather than the authors reporting second-hand information, as done later about Patient 1, "He reported ...".
The authors could have said "she appeared well with no signs or symptoms", or "Patient 1 reported Patient 0 appeared well ..." to make it clear.
Whether the case was presymptomatic or symptomatic can be argued, but that's not the point. She reported various pains and tiredness and temperature issues, and took some acetaminophen while in Germany. (Contrary to the NYT, the interview doesn't suggest she took the acetaminophen for jet lag. A quick look around the Internet didn't show acetaminophen as suggested for jet lag.) But she did wind up with fever and a cough in China.
By the way, "became ill on her flight back to China" is not correct either.
The NYT article seems to suggest that the authors wrote their article in part to provide info WHO had called for:
Quote:Within hours, it was online. It was a modest clinical observation at a key time. Just days earlier, the World Health Organization had said it needed more information about this very topic.but I note the letter doesn't mention WHO at all.
The NYT article seems to suggest that the info used in the letter about the patient came from a call on January 30. However, the February 4th alert on the misinformation indicated that one of the letter's authors said they used info reported by the other patients. The information in the letter is consistent with info published on January 28 (including the incorrect info about becoming ill on the flight back).
To me, the article feels as if it was written to push an opinion, rather than factually presenting the information.