COVID-19 Update: What’s Going On in Arizona – Part 2

After posting part one of this overview, I had an idea of how to gather data on the thing that I was most curious about, the age groupings of the new cases. I was fortunate that Fox10 in Phoenix had been taking screenshots of the daily cases and breakdowns from Maricopa county, so I went through their blog and pulled the numbers out by hand. It turns out that this was a very valuable activity, so I scrounged around on the internet until I found a similar situation for Pima County. Combined, these account for most of the population of the state, so their numbers should be representative of what’s happening.

Case Breakdown by Age Groups

Case Demographics from Maricopa County since 4/14
Case Demographics from Pima County since 4/09

These two plots tell a very interesting story since they contain data from the state lockdown (started easing on 5/1) as well as data from the post-lockdown. There are some obvious things to see:

  1. Exponential case growth starts around 5/28 in all but one age demographic. Case acceleration is largest in both counties by far in the 20-44 year group. There is slight acceleration in the other groups (note that the Pima chart separates the 45-54 and 55-64 brackets whereas Maricopa does not).
  2. The one exception to the above is the 65+ bracket, which seems to have had the same case slope since 4/9. This is interesting, because it makes the case that the accelerating case growth in the state since 5/28 has not impacted the most vulnerable population (~80% of deaths and ~70% of hospitalizations).
  3. The lockdown seems to have succeeded in maintaining linear growth in cases for all age groups. The lockdown started gradually easing in about 1 week increments on 5/1. The exponential growth started about 3 weeks after easing started. Hard to take much away from this other than the first steps of easing (opening up some restaurants, etc.) appears to not be responsible for the exponential growth by themselves.

Conclusion from this data

The main conclusion I take away is that the factors driving 65+ case growth do not seem to have been affected by the state lockdown. Why do I make this conclusion? The 65+ group is the ONLY age group with linear growth in cases at a constant slope on this graph. The lockdown did not affect the slope and the easing has not affected the slope. This needs to be studied more, but it makes the case that the state lockdown in AZ was not effective in achieving the goal of protecting the most vulnerable age group. Since this group drives the hospitalization numbers (which we are all concerned about, of course), it would seem that whatever approaches we take to COVID-19 should be targeted at decreasing their COVID-19 infection slope.

To the above point, evidence continues to grow that superspreading events are responsible for most of the case growth. See this preprint for the latest research on this effect. According to this research from Hong Kong, 20% of the cases were responsible for 80% of the COVID-19 transmission and 70% of infected people never transmit the disease at all. They also claimed that social events were more responsible for transmission than family or work (unless you work in a meat packing plant, I guess…). This does seem to give some weight to the notion that better managing the events that might be associated with superspreading (including the oversight of workers in group homes, etc.) would have more effect on the hospitalization and death rate than economic lockdowns, which appear to have little to no effect on over 65 case growth rates.

Hospitalization Data

I was fortunate that Maricopa county is also collecting data on hospitalization by day and was able to build a time series plot there as well. This data is presented for the current date in the state’s Tableau Dashboard, but I haven’t found anyone yet that has captured that data every day so that trends could be analyzed. So all I have is Maricopa County, but what it shows is interesting.

What do we see here? First, we note that as the number of cases started going exponential (accelerating) on 5/28, hospitalization just took a small tick up and then stayed flat. There may be a few reasons for this:

  1. At some point during the lockdown, hospitals stopped doing elective surgeries. Sometimes these surgeries end up populating the ICU. This is bad for the hospitals because elective surgeries are fairly lucrative economically for the hospital. Therefore, as soon as it made sense after easing started hospitals went back to elective surgeries. My guess is that they’re quite good at managing their hospital bed usage (ICU or otherwise) due to the economic effects of being good at doing so. This might explain how flat the hospitalization growth is in the chart. It would seem that once COVID-19 cases started growing again on 5/28 the hospitals saw a bump up in their numbers as COVID patients started flowing again into ICU’s which had been collecting patients from elective surgeries, etc., during the easing. Once absorbed, the rates have flattened off again (at a higher number, but perhaps that’s for the time being). I’m curious about the unusually high hospitalization cycle time for COVID (I hear 14 days…) and how that impacts the hospitals’ abilities to manage flow through their ICU beds.
  2. Since hospitalization rates aren’t really growing (and certainly not at an exponential rate, which is what everyone has feared), the chart above makes the case that all the concern about AZ case growth rates is misplaced (though I suppose it makes for a good story on a slow news day) as nearly all of the cases each day appear to be either asymptomatic or at least low enough symptoms to not require hospital admission. This is good and probably is exactly what the case and hospitalization data would look like in flu season if we were to track it this closely.
  3. We know that ~70% of COVID-19 hospitalizations are in the 65+ group, so I think this strengthens the case that most of the new cases in the younger demographics are not overly symptomatic. Add this to the testing results I discovered in part 1 of this series and it seems clear that probably close to all of the new cases that are contributing to the growth were discovered during the huge testing blitzes that have been happening and are not reasons for concern.


The media has been excessively focused on case growth in Arizona, but I believe the data indicates clearly that there is no emergency happening.

  • Most of the new cases are in younger demographics, are not requiring hospitalization, and were uncovered through testing. This is good and is part of the reality of having a novel virus in the environment.
  • Hospitalization growth is not a concern and reflects something that hospitals manage every year during outbreaks of flu or other pathogens. They are very good at managing their case loads (but I’ve heard they need to manage their beds at somewhere near 80% capacity to make money anyway). Once hospital bed occupancy hit 75% the growth slowed to a crawl and now seems to be flat.
  • Case growth in the most susceptible age group does not seem to have been affected by the economic shutdown. Additionally, the growth of mask wearing in Pima County that started in May also appears to have had no impact on the 65+ growth rate. This makes me wonder if other approaches may be more effective at ultimately limiting deaths and hospitalizations that overwhelmingly come from this group. This is an area that obviously bears more study.

Questions I have

After writing this I still have some questions that are interesting to me. I will use this space to capture a few:

I note above that the lockdown seem to have had no effect on growth of cases in the 65 plus group. My evidence for this is that post-easing, we’re seeing exponential growth in all other groups but not 65+. I have a few questions that I can’t answer about this:

  1. Does the incredible growth in testing invalidate the assertion that case growth is higher after easing than before? I.e., are we catching cases now due to testing that we would not have caught before testing started? If true, perhaps there is no case acceleration, we just changed the way we measure in mid-stream. Also, if true, there may be less indication that case growth in the other demographics doesn’t affect case growth in over 65.
  2. Is the shallow, linear slope of 65+ case growth due to their increased likelihood at wearing face coverings? I have no data on this, but it would stand to reason that the group who’s lives and health is most threatened by this virus may take greater precautions. I suspect that testing in this group hasn’t increased nearly as much as it has in other groups because I’m pretty confident that most tests conducted prior to the testing blitz were only given to people who had a strong prior for being infected. That would mean that most tests would have been given to people with symptoms (and a very high percentage of the other age groups are asymptomatic)

I also have questions about the efficacy of face coverings on asymptomatic people as source control. I understand the logic that it is easier to mandate that everyone do something that appears to have benefits, but I’m not sure if the evidence indicates that there is a compelling reason to deprive liberty in this way. I keep evaluating this…

  1. Evidence continues to mount that wearing face coverings is more effective than extreme social distancing. A very recent paper published in the proceedings of the National Academy of Sciences (LINK) makes the case that it is airborne transmission via aerosols that is the dominant route for transmission of COVID and that a facemask as source control on an infected person is the most effective mechanism to prevent atomization of the virus-containing droplets. If the virus is atomized, six feet of distance is no longer protective, and indeed, some evidence indicates that cloth masks on passers-by do not filter out these aerosols. My question, however, is whether this kind of atomization can occur in a pre-symptomatic person who is not coughing? Also, can atomization be caused by a pre-symptomatic person talking loud? I have seen evidence of atomization of flu and other viruses occurring through coughing, but that is not pre-symptomatic behavior.

2 Replies to “COVID-19 Update: What’s Going On in Arizona – Part 2”

  1. I don’t know the source of your data. Have you checked out the AZ dept of health services data dashboard?
    It definitely shows increase hospitalizations due to COVID since opening up. And if you look at the ventilator usage and ICU beds specific to COVID it is also going up (all look somewhat exponential but I can’t claim to know that is tru). Numbers for 65+ may not be going up as much as younger ones but what I am seeing in my anecdotal evidence is they are staying home more. There
    Also there have been numerous studies that have show that any covering worn by infected people help reduce the spread.

    1. Hi Louisa, nice to hear from you. Yes, that’s the data I’m using. COVID-related ICU beds are up about 150 from the lowest number of the year (about 10% of the state’s total). Clearly a reason for concern, but a drop in the bucket compared to the number of new cases since that low point. My observation is that the current increase in cases can be attributed to vastly increased testing because the math works out (there’ve been about 350K tests conducted since 5/19 when COVID ICU bed use was around 300 — its 452 now — and there have been just over 20K cases recorded since that date. The advertised positive test rate has ranged from 5 to 6.9, so if you do the math, 350K*.06 = 21K.) This all assumes the AZDHS state numbers are accurate, which may be a stretch… they’ve not been very impressive so far. I see you listed as “Dance Instructor” now. How’s that going?? Lots of change for me so far due to the merger… has been fun.

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