After weeks of daily charts and updates on new coronavirus cases and hospitalizations — both crucial figures in New Jersey safely emerging from this crisis — Gov. Phil Murphy this week brought up another number that will be equally important in the state’s reopening: the virus’ reproduction rate.
The two-and-a-half months and counting of quarantine, social distancing and economic shutdown have all proved effective in bringing down this reproductive rate, according to state officials and outside epidemiologists. But now it’s the reproductive rate, along with other key figures, that will determine when the state can end the shutdown.
“When I issued my stay at home order on March 21, COVID-19 was at a nearly unstoppable pace of spread,” Murphy said on Tuesday. Each infected person, whether they were symptomatic or asymptomatic, was spreading COVID-19 to an average of more than five other New Jerseyans.
“And by the time when our hospitals were at their peak stress, we had cut the rate of spread to roughly one-to-one. And today, thank God, that rate of spread is less than one-to-one, and we need to keep it that way.”
If the coronavirus reproduction rate stays below one-to-one, the number of cases will continue to shrink and eventually peter out. Once the reproduction rate climbs above one, the virus is growing again and could again become an epidemic.
These are the sort of epidemiological concepts that have entered the lexicon in the time of a global pandemic, so let’s take a moment to explore them.
The most basic measure of viral reproduction is a number called R0, pronounced R-naught. It refers to how many people, on average, will catch the virus from one infected person. That’s where the concept of one-to-one, to use the governor’s words, comes in.
If R is equal to one, that means each infected person is, on average, infecting one other person. Cases are doubling at that rate, generation to generation. For example, if 100 people have the virus, they are infecting 100 others. Once R gets above one, that’s when cases start to multiply.
Alternatively, once R gets below one is when case numbers start to shrink. Those same 100 infected people from the previous paragraph, but now with a virus with an R value of 0.5, only infect 50 people. Those 50 people then only infect 25 people.
Murphy didn’t say specifically say Tuesday what the rate of reproduction is in New Jersey, but models from researchers at Rutgers University-Camden currently show an R value of between 0.85 and 0.95 for at least the last 20 days.
To be clear, the measure that Murphy actually referenced on Tuesday was Rt, or the virus’ actual transmission rate at a given time. Rt is basically an applied version of R0, a snapshot of viral transmission at one point in time, but they are two sides of the same coin.
New Jersey epidemiologists agree with the governor that tracking viral reproduction is important, but they caution that it’s not the end-all when it comes to fighting coronavirus.
“It’s a really useful tool ... but it’s not a magical number,” said Stephanie Silvera, epidemiologist and professor at Montclair State University, about R0. “I certainly wouldn’t base all my policy decisions on that number.”
On top of the reproduction rate, the Murphy administration has also closely been tracking other data, like hospitalizations and new reported cases, to determine how the state should come out of its quarantine.
One of the reasons epidemiologists don’t rely solely on R0 is that it’s a complex statistic, based on “hard science, observable events and a lot of guess work,” said Henry Raymond, an epidemiology professor at Rutgers. He added that as the contact tracing and coronavirus testing programs continue to get more robust in the state, which are two of Murphy’s biggest goals, the less guess work goes into calculating the reproduction rate.
While the governor is using that rate and other metrics to monitor New Jersey’s situation, the R0 being pretty close to one remains an issue.
“I would be more concerned with the fact that it’s not a lot less than one,” Samuel Wang, a neuroscience professor at Princeton University, said about the reproduction rate of the virus. "In other words, the ‘wildfire’ is only burning out gradually.
“Further control will require intensive contact tracing, ideally coupled with isolating new cases. The governor’s proposal to build a large group of contact tracers is an essential part of getting (R0) down further.”