Relative Epidemic Momentum
UPDATE: This article was previously titled Epidemic Energy as the metric presented here is analog to energy. As a metric intended to communicate the state of situation to the general public, the concept of momentum is more prevalent and easier to relate with the concept presented here. The term “relative” was added.
Where are we? Our location is what location services figure out for us, and we can express it in relative terms such as 100 meters to the left, 1 mile north, and so on. When it comes to the current pandemic, we have many metrics designed and used for specific purposes. All metrics are related and are sometimes used simultaneously in dashboards to help us assess our current situation, to help us find out where we are.
At the beginning of the pandemic, I started studying trend metrics related to #COVID19 and was immediately hooked on the subject. My search for simple metrics led me to create the #Covímetro(1), representing the daily growth of COVID-19. It worked, but it needed context. For example, a 2.5% daily growth when we have 10 cases per day is not the same as a 2.5% growth when we have 1,000 cases per day. Adding that average volume to the Covímetro gave it the context it needed to assess the current situation.
Knowing the (most) current daily volume of detected infections (cases) combined with the daily growth, and adjusted with the positivity rate to account for undetected infections, gives us something new: Epidemic Momentum, a proposed metric to assess relative epidemic conditions.
Why “momentum”? Daily cases can be seen as analog to mass — daily cases (detected infections). Growth can also be seen as analog to velocity — change per day. Therefore, “mass” multiplied by “speed” is a measurement of momentum. As these two metrics correspond to epidemic metrics, I refer to the product as Relative Epidemic Momentum(2).
The graph presents a relative value as the 100% can be set at any date for comparison with another date, as the latest value shown in the graph.
In Puerto Rico, the maximum Relative #EpidemicMomentum, as proposed here, occurred November 7, 2020 — a few days after the 2020 elections. Analysis of the latest data, as of April 19, 2021, puts the latest maximum momentum at 65% above our previous maximum, a point we reached in a third of the time it took to get the 2020 peak.
The metric does not have a unit but one can be developed for it. Further development may also include the addition of weighing factors to the growth and positivity variables, population-normalized volume data, and growth calculated for presumed-active cases.
Conclusion: The proposed metric allows analysts to objectively compare a specific date status with respect to other points in time. Further development can be focused on establishing a standard unit for the epidemic momentum to develop an Absolute Epidemic Momentum.
Notes and revisions:
(1) Covímetro, a growth index, is the daily percentage growth of recently accumulated cases. It compares a rolling accumulation total between day 101 and day 100. The range establishes a 1% as stable growth, values above it at sustained growth, and values below it as sustained reduction.
(2) The relative epidemic energy is equal to multiplying the average daily volume times the daily growth times the positivity rate. The positivity rate normalizes daily volumes, thus reducing the effect of testing variations and testing constrains.
(3) This publication was updated April 13, 2021. The two graphics were update with the most current data.
(4) This publication was updated April 14, 2021. The title of the publication was changed from Epidemic Energy to Epidemic Momentum. The change was inplemented in the document. As explained, the intent is to use a descriptor that is easier to understand by the general public. The two graphics were update with the most current data.
(5) This publication was updated April 16, 2021. The Epidemic Momentum concept illustration was added.
(6) This publication was updated April 18, 2021. The title now includes the term “Relative” and the concept for “Absolute Epidemic Momentum” is introduced in the conclusion. Othe rminor corrections were made.
(7) This publication was updated April 19, 2021. Minor grammatical changes were made, and weighing factors added as a metric development option.The Epidemic Momentum graph was updated with recent data.
