Thread by @erlichya

Are we going to see two waves of covid-19?
Let's look on previous outbreaks of respiratory viruses with similar R0s.
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Epidemiological data from previous pandemics show that typically the number of cases grows in two waves. For example, consider SARS-CoV 2003 outbreak as documented in Ontario CA. The first wave was from Mid March to early April, followed by a second wave in late May.

H1N1 hit London in 2009 in two waves:

How about A/H3N2 outbreak in the Island Tristan da Cunha? You guessed it: two phases!

Also 2013-2015 H7N9 in China. Although this one looks more seasonal with three waves.

Also the 1918 Spanish Flu showed bimodal activity in many places (with a weak initial wave):

I am not an expert in the subject so I looked at some potential explanation on why such pandemics appear in several waves. The paper below suggested a few mathematical models that includes waning immunity, viral mutation, and human heterogeneity.
journals.plos.org/plosone/articl…

Another explanation is that policy makers order massive countermeasures, which are helpful to attenuate the outbreak and lower R0<1. When this countermeasures succeed, policymakers are under huge pressure by the public to cancel them and continue with their life...

But then, R0 jumps again and there is a second wave of outbreak.

Regardless of the mechanism, if there is anything to learn from history is that a second wave is more common than no second wave for relatively closely related viruses. That means that any protection mechanisms should be continued even when we feel that the worst is behind us.