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Home » The question of asymptomatic spread of COVID-19, as well as its reproduction number

The question of asymptomatic spread of COVID-19, as well as its reproduction number

The New Neo Posted on by

I keep seeing and hearing references to the supposed fact that COVID-19 is known to be spread by asymptomatic people infected with the virus, in addition of course to symptomatic people. It was one of the first things I heard about the illness, and one of the most worrisome for obvious reasons.

I’ve continued to check to see if there’s official word on whether data has borne that out, and so far that doesn’t seem to be the case – and that, if such transmission happens, it’s rare.

Now, you don’t have to believe the officials. But I’m certainly more inclined to believe them than I believe some random person online, or in the MSM, or even a supposedly expert talking head on TV. Even without asymptomatic spread, of course, it’s wise to be cautious. I think in the next month or so much more will become known about the disease and about what the scope of it will be, and we may even learn that asymptomatic spread is definitely a thing.

But at the moment, this is typical of what I’m seeing from more official sources:

When can an infected person spread COVID-19 virus to others?

While not completely known, its likely mirrors that of other coronaviruses, in that spread occurs with fever and respiratory symptoms. While there have been initial reports of asymptomatic transmission, the CDC states that the data are not clear at this stage that asymptomatic spread occurs.

On a more general topic connected to COVID-19, another thing that’s good to remember is that, as the number of diagnosed cases goes up in the US, the number of infected people is partly a function of more testing. So beware of news that says something like “the number of cases doubled in 2 days” without also telling you what the change was in the number of tested people, and why. Of course, we should expect the number of cases to go up, but taking a certain reported rate of increase and imagining that’s the real rate of increase is premature.

As the CDC site says:

In addition to CDC, many public health laboratories are now testing for the virus that causes COVID-19.

With this increase in testing, more and more states are reporting cases of COVID-19 to CDC.

And while we’re at it, on the subject of the statistic known as the virus’ reproduction number – this article is helpful in learning about the meaning of such a figure and how it’s arrived at (hat tip: “Art Deco”). A few excerpts:

The formal definition of a disease’s R0 is the number of cases, on average, an infected person will cause during their infectious period…

The basic reproduction number represents the maximum epidemic potential of a pathogen. It describes what would happen if an infectious person were to enter a fully susceptible community, and therefore is an estimate based on an idealized scenario.

The effective reproduction number depends on the population’s current susceptibility. This measure of transmission potential is likely lower than the basic reproduction number, based on factors like whether some of the people are vaccinated against the disease, or whether some people have immunity due to prior exposure with the pathogen. Therefore, the effective R0 changes over time and is an estimate based on a more realistic situation within the population.

It’s important to realize that both the basic and effective R0 are situation-dependent. It’s affected by the properties of the pathogen, such as how infectious it is. It’s affected by the host population – for instance, how susceptible people are due to nutritional status or other illnesses that may compromise one’s immune system. And it’s affected by the environment, including things like demographics, socioeconomic and climatic factors.

The article then goes into some estimates of the reproduction rate of COVID-19. I’ll skip that, because the article is old and I don’t know whether those figures are outdated, but I’m going to assume they are. But these principles are basic ones:

…[P]rojections of the future number of cases of coronavirus are fraught with high levels of uncertainty and will likely be somewhat inaccurate.

The difficulties arise for a number of reasons.

First, the basic properties of this viral pathogen – like the infectious period – are as yet unknown.

Second, researchers don’t know how many mild cases or infections that don’t result in symptoms have been missed by surveillance but nevertheless are spreading the disease.

Third, the majority of people who come down with this new coronavirus do recover, and are likely then immune to coming down with it again. It’s unclear how the changing susceptibility of the population will affect the future spread of infection…

Finally, and likely the most important reason, no one knows the future impacts of current disease control measures. Epidemiologists’ current estimates of R0 say nothing about how measures such as isolation or quarantine efforts will influence the virus’ future spread.

We still don’t really know those things, and so we don’t really know the significance of whatever reproduction rates we are finding at the moment for COVID-19. Of course, a lower rate would be better than a higher one. But even a high rate, if it’s accompanied by a much lower ratio of serious disease or death compared to mild disease or asymptomatic infection, wouldn’t be so very bad. The very worst diseases have a high reproduction rate and a high percentage of deaths to infections.

To give you an example of that sort of illness, we have the terrible scourge of smallpox. Those of us who are of a certain age have a vaccination scar (mine is so small you can hardly see it). Those scars represent a victory of science over one of the worst diseases to afflict humankind. But I doubt that most young people have any knowledge of the phenomenon at all:

The risk of death following contracting the disease was about 30%, with higher rates among babies. Often those who survived had extensive scarring of their skin, and some were left blind…

Smallpox is estimated to have killed up to 300 million people in the 20th century and around 500 million people in the last 100 years of its existence. As recently as 1967, 15 million cases occurred a year.

So the death rate in infected people was 30%. What about the reproduction number?:

Gani and Leach have recently evaluated historical smallpox data and estimated values of 3.5–6 for the basic reproduction number R0 of smallpox (after discounting for hospital-associated cases). This means that each case would infect 3.5–6 other people on average if the population were completely susceptible.

That link reports on a study conducted on historical data from an outbreak in 1967 among a Nigerian group that refused vaccination:

The basic reproduction number, R0, was 6.87 (95 percent confidence interval (CI): 4.52, 10.1) for the whole course of infectivity, and RF = 0.164 (95 percent CI: 0, 1.31) for the fever period that preceded the rash.

You can see how a disease with such lethality and such high contagion could cause devastation. You can also see, from that last quote, how the reproduction can differ between an early (although symptomatic) stage of a disease and a later stage. It’s not a fixed amount.

I don’t know about you, but when I read about something like smallpox, I am very very very grateful for that little scar on my arm – and for all of us who were vaccinated back when smallpox still haunted the earth, and therefore played our tiny parts in its becoming a historical curiosity. Let’s keep it that way.

Comments

The question of asymptomatic spread of COVID-19, as well as its reproduction number — 26 Comments

  1. Sorry, off topic, from Instapundit

    IRAN: NOTHING TO SEE HERE, MOVE ALONG: Coronavirus burial pits so vast they’re visible from space: Iranian authorities began digging a pair of trenches for victims just days after the government disclosed the initial outbreak. Together, their lengths are that of a football field.

    Two days after Iran declared its first cases of the novel coronavirus — in what would become one of the largest outbreaks of the illness outside of China — evidence of unusual activity appeared at a cemetery near where the infections emerged.

  2. Is it reasonable to guess that next year, when the virus starts up again, the transmission factor will be smaller because the virus will continually encounter hosts who already have antibodies from this year?

  3. JimNorCal:

    Yes, very reasonable, except that would depend on how many people get infected in the first wave. If it’s a very small number, their immunity would have a smaller effect.

  4. JimNorCal:

    I don’t doubt that in Iran the situation involves a lot of illness. But how much, really? And at what rate are graves usually dug there? What does this digging really represent? Are they just trying to be prepared? The report I read said it involves roughly 100 yards of graves. How many people can be buried in 100 yards? Depends how you bury them, of course, but if they are buried conventionally it wouldn’t be that many.

  5. 1,323 confirmed current number (will update when they update) in the US
    38 deaths in US / 8 recovered..

    the numbers outside of Wuhan are topping… meaning we met inflection point days ago, so most of what your seeing is after the horse runs away, we get the idea to close barn doors.

    however, you can be sure that every left leaning politico and person in authority is doing what they can to change outcomes in a small but negative way regardless of what happens to the little people they are so good at pretending to champion.

    The biggest ill will be in the future, when a quick response will matter and no one will listen given what will happen after this…

    Speaker Nancy Pelosi Caught Trying to Include Abortion Funding in Bill to Combat Coronavirus

    CNN is talking about respirators in gyms as if this was the old polio pandemic and talking that this is Trump, and when Joe comes back, this will bring back the Obama wonks that are really competent…

    Judge suspends federal jury trials, grand jury proceedings for 30 days over coronavirus concerns

  7. The Islamic Republic regime is lying to its people about what is happening and what to do, which of course makes things worse. Reports from the opposition estimate the number of coronavirus deaths in Iran at upwards of 3,600. They have been burying people in mass graves. Now they are digging bigger mass graves. This Iranian blogger (ex-patriot, maybe; I can’t tell) has a lot of updates on twitter (lots of detail here, and some disturbing images) https://twitter.com/HeshmatAlavi

  8. Sarah Rolph:

    Did you see this comment of mine?

    I have zero trust in the leaders of Iran to tell the truth, and I do believe the situation is worse there than reported. But the nature and extent of it is unknown.

  9. What is curious to me is the pattern of infections here in Washington in nursing homes. The seeding factor (visitor, employee, or ?) for the outbreak at the nursing home in Kirkland has not been disclosed – at least I have not seen it defined anywhere. Today we learned that three residents at a nursing home in Stanwood are positive. How did the virus get in there? Humans are the main transmitters, although I guess a virus could come in on a package, letter, bottle, or some other item, but that seems unlikely. In trying to contain a virus such as this, I would think nursing homes, hospitals, and doctor’s offices would be doing everything possible to stop the spread.

    I am scheduled for a minor out-patient surgery on the 23rd and am wondering if I ought to postpone it. I’m sure the operating facility will be sterile, but I’ll be sitting in a post-op room with several other patients while our removed cancer samples are examined (It’s MOHS skin cancer surgery) to see if they got it all. Could be safe, but should I take a chance? Decisions, decisions.

  10. neo: Yes, I was trying to reply to your comment. I thought you might be interested in that report and perhaps in that source. Information does leak out, so we can get a glimpse. You’re certainly right that we can’t know much.

  11. “Unfortunately, though, in this day and age, for many people, “under control” means zero risk. Zero risk is impossible.” — Neo

    I blame it all on “Man of La Mancha.”

  12. Neo,

    Did you come across anything regarding the fecal-oral route as a possible disease vector in asymptomatic people? I seem to recall that SARS and MERS both had fecal-oral transmission.

    I remember there were some initial claims of this back in February. It seems likely that since we haven’t heard more, its been discredited.

    But it seems to me that if there were asymptomatic transmission (a big ‘If’), the fecal-oral route would be the likeliest way that is happening. Because even if someone is showing no symptoms, they might still be one of those folks who refuse to wash their ****ing hands after they go to the bathroom. And then they touch stuff we all touch.

    Gives me the heebie jeebies just thinking about it.

  13. Fractal Rabbit:

    I remember seeing that, too, at the beginning. Here’s something from the CDC that I think is fairly current:

    The virus that causes COVID-19 has been detected in the feces of some patients diagnosed with COVID-19. The amount of virus released from the body (shed) in stool, how long the virus is shed, and whether the virus in stool is infectious are not known.

    The risk of transmission of COVID-19 from the feces of an infected person is also unknown. However, the risk is expected to be low based on data from previous outbreaks of related coronaviruses, such as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS). There have been no reports of fecal-oral transmission of COVID-19 to date.

  15. About those borders — why is anyone not from Mexico or South America showing up at our southern one? Because they think they can get it — and once upon a time, they could have. With free health care whether or not they have COVID-19.
    Probably prioritized over US Citizens (as suggested by Rep. Chu’s bill cited elsewhere today).
    I am somewhat sympathetic to their woeful situation and legitimate concerns about their own countries (why don’t any of the America haters ever crash the border into Venezuela?), but breaking our laws — including the new safety EOs – and endangering others breaks my emotional bond real fast.

    https://www.washingtonexaminer.com/washington-secrets/dhs-warns-150-000-immigrants-from-72-coronavirus-nations-at-border

    Bonus link:
    https://www.washingtonexaminer.com/washington-secrets/new-border-wall-blocks-90-of-illegal-crossings-up-from-just-10

  16. re: Open borders/Schengen Treaty, etc

    Europeans have, literally, become idiots. It’s a problem that’s growing exponentially until now it’s an existential threat.

    Wait.

    Am I writing on the wrong thread?

  17. Here is a page I added to one of my Astrophotography websites that I will update daily:

    https://snrcalc.now.sh/covid19

    Each day I’ll visit JHU’s GitHub repository and download the time series data for USA Confirmed Cases, Deaths, and Recoveries. I’ll import it into Excel and build Pivot Tables and Charts, then take a screenshot and upload it to my site.

    Ideally I would import just the time series data to my website and then let you visualize it with charting software. One step at a time.

  19. Brian: how did you get that link to print?
    Every time I’ve added a link to world o meters the blog’s software silently discarded my comment!

  20. https://medicalxpress.com/news/2020-03-coronavirus-quickly-people-symptoms.html

    That’s a link to a report describing a paper soon to be published in the journal “Emerging Infectious Diseases.”

    Among other things, the paper addresses the the issue of pre-symptomatic patients spreading the Wuhan coronovirus infection.

    The authors found that more than one in ten transmissions were by patients without symptoms. I don’t know whether I should feel encouraged or discouraged by that number. This is just one paper; but, for what it’s worth, here’s a paragraph describing its contents:

    “Meyers and her team examined more than 450 infection case reports from 93 cities in China and found the strongest evidence yet that people without symptoms must be transmitting the virus, known as pre-symptomatic transmission. According to the paper, more than 1 in 10 infections were from people who had the virus but did not yet feel sick.

    Previously, researchers had some uncertainty about asymptomatic transmission with the coronavirus. This new evidence could provide guidance to public health officials on how to contain the spread of the disease.”

  21. Here’s another study like the one Cornflour mentioned (although the could be the same one; newspapers especially online are remarkably lax about their citations, especially the ones that essentially plagiarize other sources without attribution).

    https://www.sfgate.com/news/article/Study-suggests-coronavirus-is-highly-contagious-15127538.php

    This one is a study abstract, and you have to speak med-talk to understand it.
    https://cmmid.github.io/topics/covid19/control-measures/pre-symptomatic-transmission.html

  22. Cornflour:

    I searched for the text of that research you mention but I could not find it. Unless I can read the actual report rather than a press release about it, I have no way to understand how they came to their conclusion about asymptomatic transmission. But from that press release, I find it hard to see how the researchers could know that it was an asymptomatic exposure rather than passing by someone sneezing in the street, for example. Here, that latter scenario would be highly unlikely, because so few people are infected at this point. But in China, it was a lot more prevalent.

  25. Neo:

    As I’ve said in both of my previous comments, the paper hasn’t yet been published. The web page for the journal issue that will include the paper provides a citation, but nothing more.

    Or maybe you’ve looked at preprint servers and didn’t find it? Is that what you mean? I checked bioRxiv, but it wan’t there.

    Anyway, I’d expect the paper to be available soon. It looks as if about half of the papers from the June issue have been published. For the time being, I’m forced to rely on the MedicalXpress report describing the paper. My first comment linked to that report — the one you accurately called a press release.

    I understand your preference for the original paper, but it’s just not available yet. When it becomes available, we now know where it will appear, if you care to read it at a later date.

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