May 15, 2020
On Monday, all of Florida will open its doors to the public on a limited phased approach. If the opening doesn’t lead to a spike in new Covid-19 cases, further reductions on restrictions are likely.
But is Florida ready for this experiment? According to most epidemiologists, there are three critical elements that are recommended for a state to open safely: substantial testing, extensive contact tracing, and a sustained reduction in daily new cases.
Let’s start with testing, since that is the most talked about requirement. On March 5th, Florida began a state-wide testing program. As of May, Florida has conducted a total of 561,057 tests or 2.6% of the state population (per capita). That equates to about 8,700 tests per day average. Some experts say that testing for Florida’s population should be closer to 32,000 tests per day.
But the number of tests maybe the wrong metric. The World Health Organization (WHO) says it’s not the number of tests that are key, but the percentage of new positive cases that matter. The WHO recommends that if a country has a positive test rate that is 10% or lower, they are conducting enough tests. If positive tests are over 10%, you aren’t testing enough.
This 10% rule is considered many epidemiologists the standard. So how does Florida look when we apply that WHO recommendation? The graph below shows that Florida is actually doing quite well.
This graph tracks the percent of new positive cases by date. The black line is the 10% level and when the red line drops below it, the number of positive cases has reached acceptable levels, which occurred around mid-April. As you can see, the red line has continued to drop. The last reading was at 7.3%.
In other words, the state is testing enough. That doesn’t mean the state can ease up, but it does mean it’s doing an acceptable job.
The most important measure for determining whether a state is ready to reduce restrictions is the daily number of new Covid-19 cases. The President’s Corona virus task force and the CDC recommended a 14 day continuous decline in new cases
Without this continuous reduction of new cases, the state has not yet contained the virus. Remember the virus spreads exponentially from person to person. This rate of spread is called the R0 (R Naught).
An a R0 number less than 1 means that an infected person is no longer spreading the virus and eventually the virus will die out. That’s what happened to the MERS virus, it just disappeared. There are multiple estimates for the R0 transmission rate, ranging from 1.9 to 3.5.
An R0 rate of 2.0 means every infected person can infect two other individuals. And those infected folks can infect another two people and so on. This creates an aggressive rise in the spread of the virus.
In the graph below, the red line tracks the daily number of new cases on a logarithmic scale, in this case the base 10. The data used here are the actual daily new cases (a linear scale), but transformed into a log10 value, that shows the rate of change of new cases which makes it much clearer because the rate of change rate is constant. Most importantly, for visual purposes it “smothes” out the data points.
The green line in the middle, is a reference line indicating the end of March when the number of new cases began to flatten out. From this point forward, the number of new cases bounced around, but in a slight downward movement.
The zig-zag movement indicates that in general the cases are going up and down at regular pace. In the graph below, the blue line with dots shows how up and down the new cases have been over the last nine days.
The first segment shows that the new cases declining for four days and then rising in the next four days. And the last case is down by only 6 cases. As I’m writing this, CNN is showing a map that has Florida’s new cases are declining. It hasn’t happened yet!
This pattern has been going on for several weeks. In other words, the state has not even come close to meeting the Federal guidelines. Florida is not alone in not having 14 consecutive days. Only a few have met this requirement.
Why does it matter? In general, it shows that Florida does not have control over the virus’s spread. Opening up at this point, could create a spike in new cases. Each new case can spread to two or more people and they can then spread it four more people and so on. This is called exponential growth and it doesn’t take long for it to get our of control.
So enjoy your new found freedom on Monday, but remember to wear your mask at all times and wash your hands as much as you can. Be safe…