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Explainer: Why COVID-19’s Reproduction Rate Is Crucial To NJ’s Restart


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This story was written and produced by NJ Spotlight. It is being republished under a special NJ News Commons content-sharing agreement related to COVID-19 coverage. To read more, visit njspotlight.com.

Gov. Phil Murphy loves to highlight the state’s use of data in its response to the coronavirus pandemic, and last week he unveiled a new favorite statistic: the reproduction rate, or Rt.

“It’s a gauge of how fast contagions spread and with a virus like COVID-19, knowing that Rt is vital,” Murphy said at his daily media briefing last Tuesday, pointing to a new chart that showed the rate’s downward trend over the past three months. “Flattening this curve has been just as important as flattening the curve of the overall numbers of new cases, which we’ve shown you for now many weeks.”

In New Jersey — where more than 165,800 people have been diagnosed with COVID-19, including some 12,400 who have died — Murphy closed schools and initiated online learning, shut down all but essential businesses and ordered people to stay home in the weeks following the first reported case on March 4.

The shutdown has appeared to work. The daily count of new cases — and the resulting impact on health care resources — has generally declined since a peak in late April, and the state has begun to re-emerge from quarantine. Beaches and parks were reopened in advance of Memorial Day and, come Monday, outdoor dining and limited retail shopping will also be permitted.

“Social distancing works, wearing a face covering works, and even as we undertake a greater economic restart on June 15th, we will need to keep up with both of these practices,” Murphy said last week. “Our economic restart cannot come with a restart of COVID-19.”

While the state Department of Health said it has been tracking the reproduction rate of the virus throughout the response to the pandemic, Murphy had publicly focused on other metrics, like case numbers, fatalities and hospitalizations. But, over the past 10 days, updates on the Rt have been a regular element of the governor’s daily briefing.

What’s the deal with Rt?
Rt is considered a critical indicator of how quickly a virus, or any contagion, is spreading at a certain point in time — the higher the number, the faster the spread. As of Thursday, New Jersey’s Rt was at 0.62, Murphy announced, saying it was among the lowest rates in the nation. The DOH — which has been tracking Rt and other data for months — also provided a breakdown of the Rt in each county as of June 8, which range from 0.46 in Morris to 1.62 in Sussex, just next door.


While his administration declined to share past data, Murphy has said New Jersey’s Rt topped 5 in late March, when the state was beginning its lockdown. “When I issued my stay-at-home order on March 21, COVID-19 was at a nearly unstoppable pace of spread,” he said last week. “Each infected person, whether they were symptomatic or asymptomatic, by the way, was spreading COVID-19 to an average of more than five other New Jerseyans.”

What does it actually mean?

The Rt value indicates the number of people who would have been infected, on average, by a single COVID-19 case at that time. So, as Murphy said, an Rt of 5 means one person is likely to spread the virus to five others, and each of those five infect five more, and so on, resulting in exponential growth of the disease if left unchecked.

Rt “helps explain why the cases exploded throughout New York, New Jersey and through much of the country,” said Dr. Edward Liftshitz, medical director of the DOH’s communicable disease service.

With an Rt of 1, the virus will continue to spread, but at a slower rate; Murphy said it had already fallen to this level when the Garden State reached its peak caseload, in late April. When the Rt falls below 1, it means spread was on the decline and the pandemic would eventually peter out.

“If (each person diagnosed) infects one other person, you’re staying the same. If you’re infecting less than one person, you’re getting better and better as far as the outbreak goes,” Lifshitz added. “And that’s what’s happening here, and that’s what’s been happening for a while. And as the governor said, your hard work and sacrifices made this happen.”

What impacts this number?
Rt is considered the “effective” version of a more theoretical value, R0, pronounced “R zero” or “R naught.” Called the “basic” reproduction number, R0 is defined as the number of cases an infected person will trigger during their illness among a population with no existing immunity.

Calculating R0 involves complex algorithms, but the value is influenced by the power of the virus or contagion and the susceptibility of the population — including issues like housing density — and the duration of the disease, or how long it takes people to recover or die. Measles, a highly infectious disease, has an R0 between 12 and 18; a World Health Organization analysis estimated the reproduction rate for the novel coronavirus to be around 2.5 during the initial outbreak in Wuhan, China.

But R0 does not account for other critical factors, like social-distancing measures or the use of masks. That’s why epidemiologists favor Rt (or Re, for “effective,” in some circles), which is a measure of disease spread at a specific point in time. By tracking the Rt over time, the impact of these policy changes can be considered, and recommendations can be adjusted if the number ticks up again. Some experts believe we will see multiple cycles of policy changes, with tighter lockdowns if Rt spikes, over the coming year.

How does New Jersey compare?
According to Rt.live, which has compiled data from all 50 states to calculate these values, New Jersey has the second-lowest rate nationwide, at 0.79 as of mid-day Thursday. (It is not clear why it differs from the number Murphy cited, and state officials haven’t said how they calculated their rate.) The numbers range from a low of 0.77 in New York state to a high of 1.13 in Vermont, which is one of 15 states with an Rt above 1. The website, which only tracks back six weeks, suggests that 18 states in total have witnessed active COVID-19 spread at some point since May 1. The Garden State has held its position behind New York throughout that period.

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