April 13, 2021 | Number of The Day
NJ’s latest rate of coronavirus transmission

Speaking at his coronavirus media briefing Monday, Gov. Phil Murphy noted that the rate of transmission (Rt) of the virus in the state was 0.94, notably “now back under one, which is a good sign.” The Rt value indicates the average number of secondary cases that a person infected with the virus is expected to cause. A number higher than one means each infected person is passing the virus to more than one other person — and the number of cases keeps increasing.

The last time the transmission rate was this low in New Jersey was more than a month ago, on March 1. Then it gradually rose and has been hovering around 1.1 for weeks. Regular readers of our COVID-19 data page will know that about two weeks after the pandemic first hit New Jersey, the Rt here was 5.31, meaning that each infected person was, on average, spreading the virus to more than five other people.

Some epidemiologists have been concerned about an excessive focus on the Rt, since it is an average for a population, is a lagging indicator and can hide local variations. “For instance,” according to a report in Nature, “Germany’s national Rt value jumped from just over 1 to 2.88 in late June [2020] (later revised down to 2.17) largely because of an outbreak in a meat-processing plant at Gütersloh in North Rhine-Westphalia … The Robert Koch Institute noted that national infections overall were still low, which is why the local outbreak had such an effect on the country’s Rt, which had dropped below 1 again by the end of June.”

On Monday, Murphy said, “Also, remember that the rate of transmission is calculated using a seven-day rolling average of positive test reports, so this number tells us that we are currently seeing a gradual decrease in the rate of spread.”