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Has the stay-at-home order affected the COVID-19 growth rate?

The County Commission gathers online to advise Commission Chair Hutchinson on amendments to the stay-at-home order

BY LEN AND JENNIFER CABRERA

When Alachua County issued a stay-at-home order on March 23, the County Commission feared that coronavirus was an existential threat, a once-a-century pandemic that would see exponential rates of infection and death. 

It’s been two weeks since the stay-at-home order, and we haven’t seen either exponential growth or any discernible difference in the rate of new cases since before the order was issued. The general understanding of the progression of COVID-19 is that new cases take up to 14 days to show up, so results of any change should show up around 14 days later—but we haven’t seen any change at all.

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Looking at data from 3/19 to 4/7 shows a linear relationship in the total number of COVID-19 cases in Alachua County.

Cumulative positive cases in Alachua County

The chart starts on 3/19 because that was the first day the Florida Department of Health reported the number of positive, negative, and pending tests by county per day. For the analysis, we only considered the number of tests each day that had a positive or negative result, assuming pending results would be included in future days.

The graph for the total number of cases looks very similar to the graph for the total number of tests. There is no exponential growth in total tests, which could be because there are only a limited number of tests, but the results of those tests do not suggest exponential growth in COVID-19.

Cumulative number of test results returned in Alachua County

Presumably COVID-19 tests are only given to those mostly likely to have the disease. (To our knowledge, Alachua County has not instituted randomized testing to identify the rate of asymptomatic infections.) If COVID-19 were in fact growing exponentially, not only should we see an accelerating number of cases, we should also see a higher percentage of positive results from the limited testing resources. That’s not what the data show.

The graph below shows that new cases by day are not increasing at an increasing rate as they would if there was exponential growth. Despite the slightly positive trend line, the regression model slope is not statistically different from zero (p-value is 0.36). There is also no discontinuity in the graph, suggesting the stay-at-home order had no effect on the number of cases.

Positive test results per day in Alachua County

The graph below showing the percentage of positive tests per day also has no significant trend over time. Taken day-by-day, the average positive test rate is 5.5%. This is about the same as the overall rate computed by dividing total cases by total positive tests (5.3%).

Percentage of test results that are positive, by day

During the March 24 online County Commission discussion, Commissioner Ken Cornell said the cases were doubling every two days, which would lead to 8000 cases after 20 days. He said he hoped that the stay-at-home restrictions would lead to a doubling only every 4 days, or 2000 cases after 20 days. We don’t know when his 20 days started, but 28 days after the first case was reported, there are 139 cases in Alachua County, and our rate of new cases has been consistently linear, not exponential. 

The bottom line is that there has been no statistically-significant change in the number of positive tests per day or the percentage of positive tests per day since the first testing data became available from the state on March 19.

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