During the 20-day interval between June 15 and July 5 the daily rate of confirmed covid diagnoses in the US more than doubled. How much of that increase is an artifact of more testing, and how much is a real increase in infections?
According to trend data from Johns Hopkins, the daily number of covid tests conducted in the US has been increasing at a steady rate since the beginning of the epidemic. The percentage of tests confirming infection with coronavirus peaked in early April, then declined steadily, bottoming out at 4.4% in early June, before going back up to the present level of 7.6%. During the most recent 20-day interval the testing rate is up about 30 percent while the test-positive rate has increased 70 percent. It would be reasonable to discount the increase in testing rate as an artifact of expanding technical capacity, focusing specifically on the test-positive rate as an accurate measure of the incidence of viral infection in the population. Tentatively then we’d infer that the real rate of covid increased 70 percent over the past 20 days.
But there are other factors to consider. Between June 15 and July 5 the covid death rate decreased by 30 percent. Death is a lagging indicator: all else equal, a declining death rate results from a declining infection rate. It’s possible that the spike in test-positives will eventually be mirrored by a jump in deaths. But how long is the lag? Go back up the timeline to the preceding 20-day interval, from late May to mid-June: the test-positive diagnosis rate was flat, while the death rate dropped 35 percent. Go back another 20 days, early May to late May: test-positives flat, deaths down 17 percent.
Over the past two months then, the coronavirus death rate has decreased by two-thirds, a trend that’s been either independent of or contrary to varying trends in test-positive diagnostic rate. Death may be a lagging indicator of infection, but based on the data it’s not a lagging indicator of official test-positive diagnoses.
So which is it: are infections going up, as the test-positive data strongly imply, or are infections going down, as the death counts indicate? It seems most likely that the upward trend in test-positives reflects not an increase in viral prevalence in the US, but rather an increasing level of accuracy in measuring the actual spread of the virus through the population.
It’s been widely acknowledged that, since the beginning of the epidemic, the official daily tally of covid diagnoses has grossly underestimated the true rate of contagion. Recently the CDC announced that the number of people who have been infected by the virus is at least ten times the total number of test-positives recorded in the official tallies. Together, the increases in testing rates and test-positives are closing the gap, achieving a closer correspondence between official tallies and the actual spread of the virus.
Death is a hard outcome of infection: all else equal, a declining death rate points back upstream to a declining infection rate. But what if all else isn’t equal? Deaths could drop because more young and otherwise healthy people are now getting infected, whereas the old and infirm who previously accounted for most of the deaths are successfully avoiding contagion. The data don’t support this interpretation, as discussed in the preceding post: the age demographics of covid deaths have not shifted downward since the peak of the epidemic. Deaths could also drop due to improvements in medical treatment. That’s possible, maybe even likely, but for better care to have reduced death rates by two-thirds over a matter of a month and a half, absent any new treatment that dramatically reduces symptoms or that cures the disease? Not likely.
If death rates are an accurate lagging indicator of infection rates, then the spread of the virus has been reduced substantially over the past two months. So, as the test-positive rates are going up, the actual infection rates are going down. Will the twain ever meet, with the number of test-positives finally catching up to the actual number of new infections in the population?
It might be happening.
Based on the small number of well-designed random serology surveys, I’ve been using an age-adjusted covid mortality rate of 0.6 percent for the US. Death is a lagging indicator of infection: based on the mortality rate estimate, each death results from 1/.006 = 167 infections. For July 4 and 5 the death counts were 250 per day — the lowest levels since the beginning of the epidemic in late March. 250 x 167 = 42 thousand: that’s the number of new infections per day that would result in 250 deaths per day. For July 4 and 5 the test-positive counts were 45 thousand per day — just about equal to the infection rate estimate based on deaths. The daily test-positives likely won’t climb any higher unless/until the daily deaths go up. Or unless the 0.6% mortality rate is too high…