Most COVID-19 infections evolve from exposure to positive tests to hospitalizations to deaths. This well-defined pattern allows us to forecast the pipeline of confirmed cases to hospitalizations to deaths with a fair amount of accuracy. For example Thanksgiving travels and gatherings exposed a large portion of the population to the novel coronavirus causing positive test cases to rise now, 7-10 days afterward. 5-7 days after that the serious cases go to hospitals. Another week later they result in a favorable discharge or death.
When the community infection rate is high and the positivity measure (percent of tests returning positive) is high, only the serious cases get tested as in the early days of the pandemic this spring. The average lag time from cases to deaths was only 5 days and the case fatality rate (CFR = deaths/cases) was also high (see figure above). As testing availability improved and the positivity rate dropped and more asymptomatic and mildly symptomatic cases were identified, the lag time between cases and deaths increased to 15 days in the second wave. The median lag time between case date and death date is reported is now close to 20 days as some states such as Florida take up to 4-6 weeks to actually report some deaths. The longer the lag time the more difficult it is to model CFR and forecast the daily death rate. The summer peak in cases led to some deaths that were reported in a timely fashion but a large number of cases took many weeks to resolve due to real extended times for treatment but also due to inefficiencies in the reporting process. When we compare cases to the actual date of death, the relationship becomes much tighter (see figure below).
The peak in daily death count actually followed the peak in case count by 15 days as did the rise and fall in this second wave. The potential pitfall in this comparison is that deaths associated with the third wave is barely seen in the data as death counts bottomed at the beginning of November and are just beginning to rise (with the data for the last 5 weeks is still incomplete. Some COVID deniers use this to say that the actual death count is still flat for the third wave – discounting the reality that it is flat because the count is incomplete.
Comparing cases to the actual date of death also allows us to measure the CFR better. During the first wave when little was known about COVID-19 and only severe cases were treated and lots of older patients died the CFR was near 4.5%. During the second wave, younger and asymptomatic and mildly symptomatic cases were uncovered and the overall CFR dropped to near 2.0%. The third wave involved even younger patients as schools reopened and the CFR is currently near 1.8%. CFR also improved over time due to improvements in treatment plans (proning) and therapeutics (remdesivir, dexamethasone, and bamlanivimab). Some of this improvement though could be reversed if hospitals and health resources in the USA become overwhelmed as they are in certain regions of the country.
Thus far Florida has fared much better than the rest of the country in this third wave partly because of the Sunshine State’s milder weather and hospitals are only filled to half of the peak this past summer (see figure above). But the increase in national travel for the holidays and a decrease in temperature in Florida could make the third wave worse than the first two in Florida.