June Activity Drives the Arizona Summer Surge
In Arizona there was abundant activity in June, largely among a younger cohort (20 to 50-year olds), and this caused a significant summer surge. The virus has penetrated deeply into this young and middle-aged cohort and as a result significant progress toward herd immunity is being established. A fall/winter wave is unlikely as the infection dynamics are largely driven by this age bracket. Death rates will continue to decline so long as we continue to protect the vulnerable and at-risk elderly individuals continue to take precautions proportionate to their risk factors.
This analysis replaces the study from late June: Arizona: Second Wave Infection Analysis. The Arizona second wave was more intense than anticipated due to high connectivity in the month of June. We have improved our calibration approach and algorithms to better match the data trends.
Updated Recovery Models
We examine 4 recovery models which reflect the likely range of outcomes for Arizona. All cases reflect dynamic age balanced mobility and social distancing (connectivity) proportionate to risk. All cases reflect continued protection of the vulnerable.
- Case 1: Active June, slow July, risk balanced active Fall, schools open.
- Case 2: Active from June onward, flat reference case.
- Case 3: Active June, August activity increase.
- Case 4: Similar to Case 3 with additional October activity increase.
- Arizona residents are generally modulating their behavior in proportion to risk.
- Over 25% of Arizona residents have been infected by late July.
- Infection rates are approaching 40% among younger individuals in the 20 to 50 age range.
- Total deaths will be in the range of 700/million (compare to NYC ~2,800/million, Sweden ~750/million)
- Death rates continue to be heavily tilted to elderly populations.
- Returning children to school and extracurricular activities do not cause a significant increase in deaths or infections.
- Risk proportional mobility lowers herd immunity; reached when 30-40% of 20-50 year olds are infected.
- Vulnerable and elderly populations are at risk and should continue to take precautions and avoid high risk situations.
- The summer surge will eventually die out, we will not see a fall/winter infection peak.
- Additional of deaths are possible if elderly people loosen up too early, but this will not drive the infection vectors for the general population.
Case 1 is the most conservative approach for social distancing and reflects the fewest number of deaths. Case 2, 3 and 4 are not very different and indicate that among the younger cohorts driving the infection, herd immunity will be established. The take away is that until the infection has damped down, elderly and vulnerable people should continue to avoid risky situations and practice social distancing. In all cases the infection dies out by winter.
In general, we see the peak of new infections in late June. The percentage of the population currently infected shows a peak a couple weeks after the peak of new infections. With the delay from infection to test, and from test to results, we would expect to see the peak of positive tests to be later in July. As the average age of infection shifts to the lower mortality rate cohorts, we see the infection fatality rate drop to ~0.2%. As older people become more mobile in Case 3 and 4, we see the infection fatality rate rise to ~0.4% later in the year. Overall we estimate between 1% and 1.5% of the population are actively infectious in early August, and this will continue to decline rapidly. The cumulative percent of population infected ultimately will be in the range of 30% to 40%.
Date of Death Extraction
Determination of actual date of death is critical to calibration of the analysis. The graph below shows the derived date of death the reported death date data sets found at the COVID Tracking Project. Our approach is far more accurate than a rolling average of reported deaths and is detailed here: Reported versus Actual Date of Death. The most recent week represents an incomplete count.