As Cases Rise

Miller Report for the Week of June 29, 2020

By William Miller, MD – Chief of Staff at MCDH

As Cases Rise Elsewhere in US, Where do We Stand?

As we follow the news from the rest of the US, it is notable that California is one of four states that have the highest new daily cases, the others being Arizona, Texas and Florida.  A valid question is, “Do these actually represent a spread of the virus or just reflect an increase in testing?”  Also, how do you compare a state like Texas with 29 million to Arizona with 7.3 million.  Obviously, a larger state will be expected to have more total cases per day.    

There are two ways to address this.  The first, and perhaps most important, is to look at the rate of hospitalizations for COVID.  Afterall, that is what we are most concerned about, the number of people who are actually getting sick, not just the number of people testing positive.  The number of US hospitalizations for COVID had been steadily declining since it peaked in March, now with a notable upswing in the past two weeks.  Unfortunately, in Arizona, Texas and Florida, the hospitalization rates are climbing significantly.  Projections for Arizona are that it may be as bad as New York in March in just another couple of weeks from now.   This suggests that the increase of cases in those states does represent a rise in actual infections as well as the rate of spread and not just expansion of testing.

In California, the overall hospitalization rate has been slowly trending down according to the CDCs COVID-Net website.  This, despite a significant increase in southern California counties.  In Mendocino County, we still have only a rare patient admitted to the hospital for COVID and none thus far in Ft. Bragg.  This argues that we continue to have an extremely low prevalence out here on the Coast and that this is not dramatically changing, at least for the time being.

The second way to look at the increase in cases is to adjust for the number of tests being done.  When we do this, we get the percentage of positive tests out of all the tests.  This should not change based simply on the number of tests done.  In other words, if 5% of a population are infected, the number of random tests that are positive should be about 5% regardless of how many tests you perform; provided you have an adequate sample size.  If the positivity rate is increasing over time, then the change represents an increase in the rate of infection, that is to say that the epidemic is accelerating,  and not a change in the availability of testing.  By this method, we can compare the positivity rate for these same four states.

The positive rate in Arizona has doubled from 12% to 24% in the past six weeks, while Florida has tripled from about 5% a month-and-a-half ago, to now 15%.   Texas also doubled from about 6% to now 14% in that same length of time.  This supports that the increases in Arizona, Texas and Florida represent actual acceleration in the spread of the virus and not simply an increase in testing.  However, in California, the positive rate has held steady at about 6% for the past six weeks.

Interpreting the data for California is less easy because the state has such a large population that is clumped into several different discrete regions. The largest clump is in LA, with one-third of our 39.5 million Californians living in the Greater Los Angeles Metropolitan Area.  If we look at recent positivity rates for counties in that area we find Los Angeles County at 10.2%, San Bernardino County at 9.8%, and Ventura County with 5.4%.  This compares to Mendocino County with only 0.8% of tests being positive. Six weeks ago, LA County had an impressive 34% positive rate, San Bernardino was at 10.4% and Ventura about the same at 4.0%.  These numbers seem to argue that the increasing numbers of cases do not represent an escalation in the transmission of the disease.  In Mendocino County, our positivity rate has not changed over the past six weeks and remains at 0.8%.

To be sure, in southern California, new daily cases means the disease is spreading and that new daily cases most likely does represent new people getting infected.  However, I would argue that with the positivity rate decreasing or staying the same, there is not an exponential increase as being seen in those other states.  

Meanwhile, up here in Mendocino, we remain fortunate in having both a low prevalence as well as a low rate of spread.  I suggest we strive to keep it that way with continuing a strict policy of face mask wearing, social distancing and hand washing.  

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