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Comment by Mary-Jane | Hacker News Reader

There two comparisons to other diseases appear to contradict each other. First: COVID-19 is both less communicable and less fatal than SARS. Second: it has spread faster and killed more people than SARS. I would like to understand how both can be true.

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Some quick things, the R0 doesn't specify the time it takes to spread, just the number of people that get infected per already infected person on average. A disease that has an R0 of 3 but that takes a month to become infectious would spread slower (to begin with) than one with an R0 of 2 but infectious after day 1.

A lower fatality rate in a higher population can cause more deaths. 10% of 1000 is smaller than 5% of 1000000.

But more importantly, something that is more obvious and more quickly restricted will spread less. SARS had an R0 of 2-4 but with control measures in place that dropped to 0.4 (figures from Wikipedia).

Is there another measure that takes these other factors into account? It's very easy to look at these comparison charts and conclude there shouldn't be anything to worry about.

SARS made people very sick very fast so it was easy to identify and quarantine patients. With extensive contact tracking it was able to be contained.

With covid19 many patients are able to continue their lives and infect others without being obviously sick themselves. This limited the ability to contain outbreaks without extensive testing.

So, silly question: why the basic reproduction number R0 doesn't take this into account?

R0 is what it is, but I agree, it seems like there should be a more meaningful measure that takes factors like incubation period into account.

A lot depends on whether you mean:

* R0 when the disease appears first time and people dont know about it

* R0 when people learned about disease and changed behavior.

Assuming disease is new, one person sick with SARS will infect more people then one person sick with COVID.

Due to stronger symptoms and shorter time to symptoms, we learn SARS is issue, change our behavior and quickly close small area. SARS will spread less. In covid, by the time we figure area is infected, a lot of people are infected elsewhere. It takes us more time to react.

Doesn't that mean the effective R0 value for COVID-19 should be higher then?

It’s the same as how you win in Plague Inc — you want a virus that isn’t very lethal, has a long incubation period, and causes mild symptoms in most people.

That gives your virus time and opportunity to spread.

Fast lethality means the host dies before you spread. Strong symptoms means the host stays home and you don’t spread.

Covid19 hit the sweet spot. Asymptomatic for many days, barely noticeable symptoms in 70%+ of the population, then suddenly very deadly in the few percent that get bad enough.

More deadly viruses tend to kill its victim faster, before disease can be transmitted. Also, despite being Novel, COVID-19 could be mistaken for flu or be invisible, so could go undetected for longer periods. Countries grew fatigue from the scares of earlier epidemics like swine flu and avian flu, so waited until danger was "evidently on their doorstep".

With better testing, this could've been nipped in the bud worldwide. Also, with proper pandemic preparations and alertness, countries wouldn't need to shut down. Shutting down proactive measures to save small amounts of money, costs much, much more in the longer run.

You will witness every decent country hit by unconstrained exponential contagion, shutting down.

When it's too late, or people can't be bothered, they become Italy, Spain and Worse.

The simplest explanation is that describing a disease as more or less fatal (in this context) only answers the question, “if you already have the disease how likely is it to kill you?”

The total number of fatalities caused by a disease on the other hand depends on the spread across the population which is influenced both by the communicability and by the effect of the symptoms on its spread—how long are people infected, how long do the infected live, how identifiable are the infected.

Less fatal per case, killing more because wider spread.

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