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.