Explaining the COVID reproduction rate — and why NC’s is climbing


RALEIGH, N.C. (WNCN) — North Carolina has one of the nation’s highest current effective reproduction rates of the coronavirus, according to a data compiled by a scientist from MIT.

The effective reproduction rate, or Rt, is a measure of the speed with which a virus spreads and estimates the average number of people an infected person will infect at a particular point in time.

A higher number corresponds to a faster spread, while a value less than one indicates the spread is under control.

According to covid19-projections.com, a website maintained by MIT data scientist Youyang Gu, only three other states presently have higher Rt values than North Carolina, which is at 1.06.

Arkansas, California and Utah were each at 1.07, according to the most recent data by Gu, whose data analysis has been a component of models used by the Centers for Disease Control and Prevention.

Pia MacDonald, an epidemiologist at RTI International, says an Rt number depends on three components — the probability of transmission, the number of contact that an infected person has over a period of time and the duration of the infectiousness.

“In its purest sense, R is, if I’m sick, how many people will I, going about my business, transmit that infection to?” MacDonald said. “One person passes it along to more than one person, so that is what’s driving the growth of this outbreak.”

The effective reproduction rate varies over time and can change depending on what measures are being used to mitigate or contain the spread of a disease. 

That makes it slightly different from the basic reproduction number — known as R0 or R-naught — which measures the number of infections caused by an infected person in a completely susceptible population. Because COVID-19 has been in the population for months, the R0 is no longer as useful of a measurement, MacDonald said.

Gu’s data indicate that in North Carolina, the changes in that number appear to have tracked with the steps taken in the state’s reopening process.

According to his data, the Rt in the state was at 1.83 on March 1 and dropped precipitously during that month when municipalities and the state issued their stay-at-home orders, falling to a low of 0.97 on April 26.

It climbed back to 1.0 on May 20 — two days before the state moved into the second phase of reopening — and has held steady at 1.06 since May 28.

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