Why Are Infections on the Rise Again? (Part 1, Arizona)

 

In an earlier post I mentioned the covidestim.org website. Unlike all other sites, which summarize reported numbers, this one gives the results of epidemiological modeling. 

This modeling is beneficial because, for example, some people are infected and don't know it or don't report their illness. The true number of cases can be estimated by including other data in statistical models. For example, serological analysis (which shows the presence of virus antibodies in blood samples) offers another data point for estimating how many cases there have actually been.    

Using these likely true-case numbers, covidestim.org estimates the percentage of the population that has ever been infected. Right now that number for Arizona is 75.25%. 

In addition to infections, many of us have been vaccinated. It's not as many as we'd like, but according to JHU 55.47% of Arizonans have had the jab (AzDHS says it's 60%; I don't know what accounts for the difference).

When I saw these numbers I thought to myself: OK, three-quarters of us have been infected and more than half of us have been vaccinated.  Who is left to get sick?

It turns out, there are plenty of people...if you look at actual numbers instead of percentages. If three-quarters of us have been infected, that means one-quarter of us have not. One-quarter of our population (based on Census Bureau estimates) equals 1,856,200 people.

Some of those uninfected people have been vaccinated too. We don't know how vaccinations are distributed across people who have and haven't been infected. Given that, the only reasonable thing to do is assume that they are split according to the population percentages.

Fourty-five percent are unvaccinated, and 45% of the number uninfected people from above equals about 835,290 people. That's a lot of people who we can estimate are both uninfected and unvaccinated.  

At the current rate of around 3000 new cases a day, it would take 278 days—a little over nine months, or until next July—for the virus to burn through all of them. Even that idea is based on assumptions that are probably not correct—the subject of Part 2 of this post.

Anyway this illustrates the importance of keeping up mitigation measures, like getting vaccinations/boosters, wearing masks indoors, especially in crowded indoor environments.

Image by Peggy Marco from Pixabay



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