COVID-19 Cases in Free Fall (January 21, 2021)

In my blog post of 12-28-2020, I noted that we were entering the beginnings of herd immunity due to the combination of actual cases and masking/social distancing/quarantining. While the true effects of this decline were difficult to observe at first (due to the holidays), COVID-19 cases are now beginning to fall rapidly. As of today, the 7-day moving average of recorded cases has fallen for 9 straight days with a total decline of 22 percent during that period. The graph below shows the number of daily new cases over the past 90 days. Please note that the two dips prior to the recent case decline are due to the drop in testing (and reporting) due to the Thanksgiving and Christmas/New Years holidays (this affects the 7-day moving average because cases fall during the holidays and then rise suddenly the following week or so).

Covid-19 Cases for Last 90 Days

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Using the latest number of deaths and reported cases from Worldometers (and estimating 400 actual cases for every death and 7.06 actual cases for every reported case as discussed in my 12-28-2020 blog post), approximately 50-53 percent of Americans have had COVID-19. Adding in the effects of wearing masks and social distancing (about 30 percent) brings the effective percentage towards herd immunity to about 80 percent (however see below why this number is somewhat high). Further, as of today, 17.2 million dosages of the COVID-19 vaccines have been administered. Using an estimated effectiveness of 50 percent of those vaccines (since each dose equates to approximately 50% effectiveness), that would mean that an equivalent of 8.6 million Americans  are immune to the virus by virtue of the vaccine. This equates to almost another 3 percent towards the effective herd immunity percentage.  Unfortunately, many vaccine recipients have already had COVID-19 and thus, using the estimate of 50-53 percent of Americans having already had the vaccine, to date the vaccination has added only about 1.5 percent towards herd immunity (in my opinion we should test everyone for antibodies and prioritize those without antibodies – this would accelerate the march towards herd immunity and save lives).

I recognize that the numbers above are probably a little optimistic. This is because:

• Testing has continually gotten more accessible, so that the ratio to actual cases to recorded cases has probably fallen below 7.06 and

• Quite a few deaths have been reported as resulting from COVID-19 but were, in fact, due to other causes but recorded as COVID-19 cases because the person also had COVID-19.

The net effect is that presently we are probably closer to an effective herd immunity percentage of 65-70 percent instead of 80 percent.

Obviously, not every state is the same. Below is a plot of death rates and case rates for each state.

Recorded Deaths and Cases by State

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I have added two lines to this plot. The vertical line in red indicates roughly where (the beginning of) herd immunity is reached based upon the number of reported cases. The horizontal green line indicates roughly where herd immunity is reached based upon the number of reported deaths and is considered more accurate than the red line because deaths are more discernable (or easily measured) than cases.

States with more than 1,000 deaths per 1,000,000 population should have hit the beginning of herd immunity. Note that the states that experienced high levels of COVID-19 early in the pandemic, such as New York, New Jersey, Louisiana, Massachusetts, etc. have high death rates and thus the red line (i.e., using reported cases) is more accurate in determining whether those states have reached the beginning of herd immunity (as noted in my previous blog post).

Both South Dakota and North Dakota are in the far upper right of this plot indicating that they are well into developing herd immunity. Sure enough, both states are at a point where the number of new cases is very low and should soon be negligible. Some of the states at the lower left or in the center of the plot are just now reaching the beginning of herd immunity and seeing new reported cases begin level off.

Some of the states, despite being below the green line, have falling numbers of new cases due to a variety of reasons (a more rural population, better practices of social distancing and wearing masks, etc.).

The good news is that almost all states are seeing cases either leveling off or falling. Some states, such as Texas, are just now seeing cases peak, a fact that I attribute to the great reluctance of Texans to wear masks and social distance. However, Texas is near the center of the plot so that the number of new cases in the state are not too different than what would be predicted. California, which has imposed strong measures to combat the virus, finds itself in a little better position than Texas, despite being slightly below the green line. It is interesting however, that California is in only a little better shape than Texas, relative to where it falls on the plot, despite these strong measures.  This indicates that strong measures help, but not dramatically.

The new administration is requiring masks on federal property and encouraging people to wear masks for 100 days. Unfortunately, this will have little effect on the fall in COVID-19 cases because the vast majority of the population (92 percent) are already wearing masks when they are most needed. 

The new administration is also pledging to give 100 million vaccinations in 100 days (1 million doses per day) however, at the rate that vaccinations are ramping up (including the pre-planned expansion of vaccine sites to include pharmacies, etc.), this goal should be reached even if the new administration does not do anything.  We are already administering over 900,000 doses per day.  In addition,100 million doses  would add only about 8 percent to the effective level of herd immunity.  A better, and still achievable, goal would be to administer 300 million doses in 120 days. I state this not to criticize the new administration, but to emphasize that we are already well along the way towards herd immunity and hopefully they can administer vaccines more aggressively than their easily reached target. What is most needed, is for people to be tested for anti-bodies, and for vaccines to be given only to those without anti-bodies, at least during the first few months of vaccinations.

It should be noted that what is being discussed here is only the beginning of herd immunity. For herd immunity to be reached, people will need to continue to wear masks and social distance until the combination of actual cases and vaccinations reaches a sufficient level, probably close to 85-90 percent of the US population.  The good news is that, at the present rate of case decline, we should see a dramatic improvement of COVID-19 cases by late March.  

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