Have COVID-19 Strains become Less Virulent?

Virulence: Virulence is a pathogen’s or microorganism’s ability to cause damage to a host. In most contexts, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to cause disease—is determined by its virulence factors. (Wikipedia)

Here’s some Images from the Arizona Dept. of Heath Services data dashboard that I think tell a story that could indicate decreased virulence of the Delta variant.

  1. COVID Cases by Day in Arizona – Entire Pandemic: In the image below we see the cases per day since around April of 2020. You can easily see three surges of cases. The first happened in the summer of 2021 and coincided with a huge, relatively uncontrolled outbreak in Northern Mexico. Many of the cases during this time occurred in border counties of Arizona. The second surge occurred in the winter of 2020 where the entire U.S. saw a spike of cases that correlated with the average daily low temperatures dropping to below 40 degrees. The latest surge corresponded with the more-transmissible Delta variant and has seen two spikes. This surge has been less of a spike and more of a “slog” where perhaps we are seeing the combination of the arrival of the Delta variant in the late summer merge with the more traditional cold-weather pattern for a virus where the night-time temperatures drop. Understandably, the lack of relief is wearing out health care workers and challenging hospitals. Note that the number of cases per day for the second spike of the Delta outbreak is roughly equivalent to the first summer outbreak.
COVID-19 Cases by Day (https://www.azdhs.gov/covid19/data/index.php#confirmed-by-day) – 12/21/21

2. Hospitalization – Cases by Day: Below you can see hospitalization for the three major outbreaks. The winter outbreak hospitalization by day far exceeded the first summer outbreak. Likewise, the first summer outbreak’s hospitalization per day is just under double the peak of the Delta variant outbreak. The only problem with the Delta outbreak is that it is lingering. Similar cases per day and less hospitalization per day. Just over a longer time. This naturally creates problems in hospitals processing sick people through their system due to the need to navigate bottlenecks that form. Just like in a factory, bottlenecks are going to be less of a problem in a quick surge of production than they are in long, tiring runs of production where errors and inefficiencies compound.

3. Deaths per Day: In the image below, we see similar patterns to hospitalization. If you look closely, you can see that the peaks of the deaths are a week or two behind the peaks of hospitalizations. Again, we see the same pattern as we see with hospitalization. Though cases during the Delta wave are roughly equal to the first summer wave, the deaths are around half.

COVID-19 Deaths by Date of Death (https://www.azdhs.gov/covid19/data/index.php#deaths) – 12/21/21


Does this data show that Delta variant is less virulent than the preceding variants?

Perhaps. It’s quite possible that during the first summer wave we did a worse job of measuring cases. COVID tests are pretty ubiquitous now in late 2021 and maybe we’re collecting a higher percentage of the cases. Conversely, it’s also possible that people have inferred or imagined that Delta is less of a risk to them and are not getting tested if they experience mild symptoms. Either of these could be true and both would impact the usefulness of the case number. Additionally, the new variable of COVID vaccinations that was introduced in early 2021 has certainly reduced the impact of the Delta variant. It would take some work to decipher whether the virulence of Delta to unvaccinated people was equal or less than previous variants.

This is one of the challenges of measuring cases for the purpose of scientific analysis. It is very hard in a real-world study to control for the measurement variables across numerous regions and measurement authorities (governments, hospitals, universities). This is one of the reasons why we still don’t know much about this virus, despite having measured it for around a year and a half.

My Opinion: Oftentimes the concerns around measures will balance out when data is considered in very large batches (“big data”). My suspicion is that human nature is the constant across the measurement of all of these surges and we can take what is presented to us and assume that Delta is less virulent than the previous strains, either due to the virus itself or due to the boosts to our immune systems from either natural immunity or the COVID vaccines that most people have received.

Omicron and the future: We’ll continue evaluating the hospitalization and death metrics in the context of cases. My suspicion is that as Omicron arrives, it will dominate and gradually eliminate Delta and previous variants still lingering out there. If Omicron is less virulent, perhaps then we’ll see a leveling off of the cases to some background number and then we can say that COVID-19 has become endemic. If Omicron is not less virulent, then we’ll have a rough month or two ahead of us.

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