Variation is key to the SARS-CoV-2 ‘waves’

Posted onAuthorIan M Mackay, PhD (EIC)

Just a short note to highlight that viruses changing their ‘look’ through selection and expansion of variants in response to our immune system’s pressure on their evolution has been the one constant driver of every wave in Australia, and probably everywhere, so far.

If you click on the image below to make it a bit bigger, you should be able to see that each peak of a ‘wave’ (new epidemic) of COVID-19 hospitalisations has been driven and dominated by a genetically different variant of the virus we call SARS-CoV-2.

COVID-19 hospitalisation & ICU peaks and troughs (= “down to” 1,200 admissions) by month, MAR2020 to 14APR2023. The predominating variants are indicated at the peaks. Every peak = a new variant, as one would expect, given epidemics are first & foremost driven by pathogen novelty.

That may seem obvious to some of you, but it isn’t obvious to all. The graph above of SARS-CoV-2 hospitalisations over time clearly highlights the association.

In my opinion, it’s also a causal relationship.

So as long as the virus keeps escaping our immunity, we’ll keep seeing epidemics. Should SARS-CoV-2 split into two (or more) quite genetically distinct yet stable variant groupings that each continues evolving, that would also be a way for the virus to go on.

Lots of viruses change with time and have other tricks to remain relevant

Influenza viruses, respiratory syncytial viruses, parainfluenza viruses, other coronaviruses, rhinoviruses, and more are always spreading somewhere in the world. If they weren’t, they would cease to exist because viruses must infect living cells to replicate and spread. These viruses then return to our population with epidemic force when one of these things occurs:

  1. The population has been refreshed by newborns who have never been infected and so have no immunity (at least once maternal immunity wanes) to protect them.
  2. The virus has changed enough to escape the population’s pre-existing immunity to older variants.
  3. There is more than one member of the virus species, and each takes its “turn” in alternating years. They all continue to evolve during that as well.

2021 was still Australia’s pre-pandemic era as far as population disease was concerned

Depending on where you are in the world, you may also be wondering why Australia had so few hospitalisations in 2020 and most of 2021. That’s because we had strict travel restrictions and border controls, including quarantine at the border until, well, until we didn’t.

This time was used to fully vaccinate (two doses back then; it should really be three or more) the Australian population.

It took until November 2021 to get 80% of the 12-years of age and older Australian population vaccinated. New Zealand did a very similar thing.

This approach of protecting the population meant we essentially skipped the ancestral and several variants of concern from leaving an immune ‘scar’ on the population. We also prevented the extra deaths, harms, and economic hits other parts of the world were suffering during this period and because of these variants.

Data from Australia’s ABC website on COVID-19 vaccine coverage.

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