This single number decides how fast N.J.’s reopens. How is N.J. measuring it?

For months, New Jersey’s full reopening has been tied to what seems like a simple target: The number 1.

Coronavirus transmission and its complicated relationship with the number 1 has become more important in the debate about indoor dining and gyms than the number of New Jerseyans in hospitals and on ventilators. It’s possibly even more important in guiding future decisions than the number of people dying from COVID-19.

The coronavirus transmission rate, or Rt, is a count of how many people, on average, a person with coronavirus will infect.

If that rate is above 1, the virus will grow over time; below 1 and it eventually will peter out.

Simple enough, right?

In theory, perhaps. But calculating the number is anything but straightforward. Different states and agencies use different models to arrive at the transmission rate, which means the Rt in a state like New Jersey will be different, depending on which organization did the calculation. And only the state’s calculations hold the key needed to reopen New Jersey during the pandemic.

So how does the state calculate this crucial number?

Donna Leusner, a spokeswoman at the state Department of Health, said New Jersey’s methodology was developed by Imperial College London. Its transmission rate is based on the number of positive tests from the reverse-transcription polymerase chain reaction test — PCR test — the most common test currently being done. Leusner said the figure is used in most transmission rate models.

New Jersey’s model also assumes the average amount of time that passes between transmission from one person to the next is six and a half days.

Based on those figures, plus a slew of other in-the-weeds assumptions that are part of the methodology, NJ Advance Media now can replicate the state’s calculation, providing an important check and balance for residents. Previously, an NJ Advance Media developer came close to New Jersey’s Rt numbers, but without knowing the methodology and all assumptions, the models had not aligned.

The transmission rate has become one of the primary figures Gov. Phil Murphy uses to control New Jersey’s reopening. In early July, Murphy delayed the state’s reopening indefinitely when the transmission rate hit its highest point in two and a half months, at 1.03.

For comparison, Murphy said the Rt was closer to 5 in March when the coronavirus was rampaging across the state. The rate had climbed back up to 1.35 in late July – a concern to many who track the numbers – but it since has fallen to 0.83.

NJ Advance Media asked several New Jersey-based epidemiologists and researchers to review the state’s methodology.

Their takeaway: Despite some minor issues — and any methodology has its own issues — the state’s method is providing a consistent measure that officials can follow.

“You have touched on a complex question, ‘Which method is better?’ The mostly unsatisfying answer is that none are better,” said Henry Raymond, a Rutgers epidemiologist. “Each method has its own biases which may go in different directions. The really key point is that they have settled on a method and are consistently using it.”

Part of the trouble modeling viruses — using New Jersey’s method or otherwise — is that it’s impossible to know the exact number of new coronavirus cases on a given day. That means models must use a lot of assumptions to mitigate unknowns.

“The common limitation is a reliance on knowing who the new cases are and that all cases are detected, which brings us back to problems with testing, in general, and to the lags we’ve been seeing in test results coming back, in particular,” said Stephanie Silvera, an epidemiologist at Montclair State University.

These limitations exist regardless of which model is favored, but experts say the important thing is the state sticks with its model.

“While it might be ‘off’ from the ‘truth’… if it’s used consistently, it gives at least a consistent indicator of what the situation is,” Raymond said. “In plain language, while it might be ‘off’ it’s likely to be ‘off’ consistently and thus using it to understand the direction of the trend is likely to be fairly right on target.”

That brings us back to the number 1.

With New Jersey playing jump rope with that threshold over the last two weeks, epidemiologists said Rt is crucial to our understanding of how the virus is behaving, particularly as we navigate questions about schools reopening and the possibility of moving dining back indoors.

But Rt by itself does little to help officials control the virus. That comes from other measures, such as social distancing, restrictions on gatherings and mask mandates, plus a robust testing and contact tracing infrastructure.

Samuel Wang, a Princeton University neuroscientist, said earlier in the pandemic that he will remain concerned about transmission rate until it is much lower than 1, comparing an Rt around 1 to a wildfire burning out too slowly.

“Further control,” Wang said, “will require intensive contact tracing — ideally coupled with isolating new cases.”

CORRECTION: A previous version of this story stated that officials had not disclosed the state’s methodology for calculating Rt until last week. That was incorrect. The state released its methodology earlier this month.

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Payton Guion may be reached at [email protected]. Follow him on Twitter @PaytonGuion.

NJ Advance Media developer Arjun Kakkar may be reached at [email protected].

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