I’ve put together some a simple chart and two tables to follow up on my last post. With the possibility of reopening some California counties, it’s a good idea to see how they are doing. To start, the chart below shows the top 16 counties by the number of COVID-19 deaths in California.

By both its size and number of deaths, LA County is in a category all its own. The county added 325 deaths in one week, a jump from 619 to 944 COVID-19 deaths. Table 1 below lists the data for those 16 counties in Chart 1, which include more than 95 percent of all COVID-19 deaths in California.

It is helpful to put California counties into four groups. LA County alone is a region, with about one-quarter of the population of the state and more than half the cases. The other regions are described below (note the typo in the title–the data is actually from 4/27/2020).

LA county is similar in population to both Sweden (10.23 million) and Greece (10.72 million). In LA County there have been 944 deaths. Sweden has been hailed as a model by pundits who don’t seem to have examined the data closely. Sweden has had 2,568 COVID-19 deaths, almost three times the number in LA County. A better but lesser known model is Greece, with only 140 deaths.

Closer to home, a quick check of Table 1 above reveals that LA County has a death rate of 92 deaths per million, while its neighbor to the south, Orange County, has only 12 deaths per million. This is true even though Orange County has a higher population density. However, after a crowded weekend on Orange County beaches Governor Newsom ordered them to close temporarily. We’ll see if the crowds brought an increase of COVID-19 cases and deaths to what has been a relative safe haven in crowded coastal Southern California.

The LA region and South Bay/Sacramento region have very similar death rates at about one-third the rate of LA County. The remaining 42 counties together have had 76 deaths, only 4.3 percent of the total. The death rate is low, 9.3 deaths per million. Part of the reason for the low death rate is that people are spread very thinly across those 42 counties with a population density of 73.9 people per square mile. That’s about 7.9 million people, one-fifth of the state’s population, spread across more than 106,700 square miles. If that was a rectangle 100 miles wide, it would have to be 1,067 miles tall. Old-fashioned, labor-intensive contact tracing, what’s been called “shoe-leather epidemiology,” will require lots of trained workers willing to travel many, many miles. Newer technologies may not help that much (LA Times, requires registration). Even so, Modoc County, in the far northeastern corner of the state, has reopened.

Meanwhile, construction will begin again in the Bay Area, a move that the East Bay Times questioned in an editorial. The real issues concern testing, whether we have enough test kits, how fast we can process them, and how accurate they are. The rule of thumb for a disease of low prevalence (less than 10 percent of population infected) is that if the true (unknown) prevalence of the disease is roughly the same as the false positive rate of the test, then a positive test result is wrong almost 50 percent of the time.

Here’s an example: Assume one percent of Californians in a random sample have COVID-19. A test accurately reveals the one percent that are infected (i.e. no false negatives). However, 99 percent of the people in the sample don’t have COVID-19. If the test has a one percent false positive rate, then 0.99 percent of the sample will have a false positive result. The study’s results show that 1.99 percent of the sample tested positive, yet we know the true positive rate is only one percent, and almost half the people with positive results really don’t have the disease.

This implies that people will have to be tested more than once, and tested repeatedly, just like professional athletes are tested for performance-enhancing drugs. And even if we start testing people for antibodies to the disease, either from having COVID-19 or getting (someday) a vaccination, we’ll still have to test them to verify that their immunity remains, a least until we have enough experience with the new vaccines and have vaccinated a sufficient proportion of the population.

So far, humankind has only eliminated one virulent disease by vaccination and outbreak tracing–smallpox. We are close to eliminating a second disease, polio, but mostly due to warfare and poverty in some developing countries, finishing the job is proving to be tough. Due partly to anti-vaccination hysteria, we still suffer from occasional outbreaks of whooping cough and measles. COVID-19, even with plentiful and reliable testing and effective vaccines, may be with us for years.

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