Recommendations for Industry

COVID-19 Testing and Risk.

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While a month has passed since many states began reopening, we have continued to monitor for a “second peak” of potential illnesses.  At the same time, testing capacity has increased in almost all states. Therefore, it is essential to understand if an observed increase in cases is the result of increased testing or the beginning of an outbreak.

Unfortunately, we can’t answer this question by only looking at the absolute number of people testing positive in a county or state. We must also consider some additional measures to make a meaningful interpretation of the dynamics of the outbreak.

Some useful data measures include:

• Infection rate:  This number tells us how many people, on average, someone with COVID-19 will infect.  This number is calculated based on the biological infectivity of a virus and the control measures that are in place to reduce transmission.  An infection rate of 1.0 means that, on average, a sick person will infect one other person while a value of greater than 1.0 means that one person will infect more people, thus allowing the outbreak to continue to grow.  Different mathematical models are used to calculate this number, and it’s a good indicator that cases and death are increasing in a city or state.

• Positive test rate:  This measure tells us the percentage of tests that are positive from all the tests taken over time and is a good predictor of an increasing outbreak for COVID-19 when calculated as a 7-day rolling average.  A positive test rate of less than 3% suggests that there are adequate testing and a low-level of illness in an area.  When positive test rates exceed about 12%, there is likely inadequate testing, the outbreak is growing, or both.  Some locations experiencing outbreak growth may have adequate testing and still have high positive test rates.  Johns Hopkins is publishing the change in weekly positive rates and this data shows which states have experienced increases or decreases in their positive test rates from week to week.  The chart below shows the current positive test rates and the number of tests conducted per 100,000 people in all 50 states. 

• Hospitalizations and ICU capacity:  A true indicator of outbreak severity is the number of people hospitalized or admitted to the ICU.  If an area observes increasing case counts and increasing hospitalizations, this is a reliable indicator that the area is a hotspot and that increasing cases are not just a result of more testing.  The CDC has created a 50-state dashboard for tracking hospitalization and ICU rates.