2 March 2021 Blog: Improving COVID-19 Metrics in Los Angeles County

Yesterday Los Angeles County reported fewer than 1,000 new daily cases and a 7 day average of 3% test positivity – both significant and encouraging signs of improvement.  On the test positivity rate, Dr. Barbara Ferrer, County Public Health Director commented, “That’s actually the lowest its ever been since we’ve been offering testing in the community.”  (link:  https://ktla.com/news/local-news/l-a-county-says-rate-of-positive-covid-19-tests-is-lowest-its-ever-been/)

And she is right – at the outset of the pandemic, test positivity rates were in the 9-12% range and even in the lull of August and September rates hovered between 3-5%.

While the test positivity rate is but one metric to follow, I prefer to look at prevalence rates which represents the percentage of the population with the infection at a given point in time. It is calculated as a function of the test positivity rate and the new case rate and, because of the nature of the time between infection and detection of a new case, is about two weeks delayed.  Nevertheless, the current prevalence rate is at 0.4% down significantly from the 1/5/2021 peak of 24.4% (Figure 1).  That prevalence rate is truly mind boggling – of 100 individuals in the County, 24.4% had the infection at that time.

The County’s data dashboard only has information current to 2/28/2021 so I could only calculate rates through the week ending 2/23/2021.  However, new daily cases stand at 9.1 per 100,000 which is similar to rates seen in September (Figure 2). 

Mortality rates however are falling more slowly than case rates and remain 5-6 times higher than rates seen in September/October (Figure 3). This is not unexpected given that these rates have an inherent lag.

Ideally, we would like to see the new daily case rate continue to drop, preferably to below 1 to be on track for containment.  This, according to the Harvard’s Global Health Initiative (link: https://globalepidemics.org/key-metrics-for-covid-suppression/).  Nevertheless, a rate from 1-9 moves us from “accelerated spread” (stay-at-home orders recommended) to community spread (rigorous test and trace recommended).  This is all welcome news after a trying three months.

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