8 APRIL — The US case counts have plateaued over the past seven weeks, with daily fluctuations falling well within the expected range of random variation. However, as has been well and repeatedly documented, case counts greatly underestimate the number of new infections. For several months the ratio held steady at a rate of around 2.7 new infections for every new dx test-positive. Now, however, there’s evidence to suggest that the ratio is ticking upward — which would mean that the infection rate likewise is climbing. Here’s the rationale:

Covid deaths are the most accurate indicator of covid infections, with death lagging about 3 weeks after diagnosis.

The covid fatality rate varies based on age. The older a nation’s population, the higher its fatality rate. The median age in the US is 38, with an age-adjusted fatality rate of 0.65% of covid infections.

US covid vaccines have been administered disproportionately to the older, more vulnerable people. Immunizing most of the elderly effectively reduces the average age of the US population still vulnerable to infection.

Assume that the median age of unvaccinated Americans at risk of covid infection is 36 years — 2 years younger than the overall population. The age-adjustment algorithm reduces the estimated US covid fatality rate from 0.65% to 0.55%.

Using this revised fatality rate as the basis for a revised estimate of infection rate, then over the past month there have been 3.2 new infections for each new dx test-positive case — an increase of 20 percent over the previously stable ratio of infections to cases.

These calcs imply that the plateaued daily case rate disguises a 20% *higher* rate of new covid infections over the past month or so. That makes sense: younger people who get infected tend to experience less severe symptoms, and so are less likely to see a doctor and less likely to be tested for the virus. The current rise in infections will almost surely exhaust itself over the next two or three months as vaccinations are made readily available to all US adults and herd immunity is approached.

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Another way of looking at it… Over the past two month around 2M vaccinations per day have been administered in the US: that’s about 120M, or 36% of the US population. Assume a little more than half were first shots, bestowing 80% immunity on recipients, while the other half got their second shot, adding another 15% immunity: that’s an average of 50% immunity per shot. Around 30% of the population has already been covid-infected. So, the additional population immunity achieved through vaccination over the past two month was .36 x .5 x .7 = 13%. All else equal, we’d expect that the number of new infections over the past month would have decreased by 13%. But the case count stayed flat, which means that the number of new dx-positives among the vulnerable population went up by 1/.87 = 15%.

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