quote:

Timothy Sly
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Epidemiologist, professor, Ryerson University, Toronto,


Can coronavirus (Covid-19) suddenly end on its own similar to how the Spanish flu did?

Originally Answered: Will COVID eventually smolder out similar to the influenza outbreak of 1918?

Pan/A/H1N1 (1918) ended as all viral pandemics end, with the population reaching the transmission threshold (I hesitate to use the correct term “herd immunity” because that phrase appears to have been bent out of its original meaning and hijacked for political purposes!) As there was no influenza vaccine in 1918, nor would there be for decades, the virus circulated until sufficient proportion of the population had acquired the infection and had survived with at least some immunity.

In 1918 the global population was around 1.8 B. Assuming 40% of the population were infected (720 M cases) in three waves from the end of 1917 to 1919, quite reasonable for influenza and supported by CDC estimates that more than 500 M were infected. The case fatality ratio for the 1918 pandemic has long been estimated to be close to 2.5%. Now, let’s multiply (0.025)x720 M, giving an estimated 18 M deaths, which is within modern estimates of 17–50 M deaths.

Now let’s see how this compares to the herd immunity which would have been necessary for that influenza. The R-naught for the 1918 pan-flu has been estimated between 1.6 to 1.7. From that we find the immunity threshold (herd immunity) as between (0.6)/(1.6) to (0.7)/(1.7), or 37% to 44%. Compare this targeted herd immunity needed to slow and stop the pandemic with the estimated 40% of the population who had been infected (- minus the 2.5% mortality - giving 37.5%). The figures will never be known precisely, but we can see that herd immunity could well have been attained by 1920, and this is most likely the mechanism which brought the pandemic to an end.

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