r naught

R0, ‘R-Naught,’ The Coronavirus Number You Should be Paying Attention to

Simply put, "R-naught" measures new infections that result from a single case of a virus

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Illinois Gov. J.B. Pritzker on Monday mentioned a term that some may have never heard before — R0, pronounced "R-naught" — but one that's remarkably important in understanding the science-driven guidance state officials have sought in their plans to reopen Illinois.

Simply put, "R-naught" measures new infections that result from a single case of a virus

A value above one means that each infection causes more than one new infection, resulting in more transmission, Pritzker stated at his daily coronavirus news briefing Monday. Similarly, a value below one means that each existing infection causes less than one new infection, which in time, Pritzker said, will lead to a decline in new cases.

A value of one means that transmissions are stable.

"R-naught," which is also referred to as the reproductive number, depends on where people live and what countries do to try to slow it down, Dr. Mike Ryan, executive director of WHO’s emergencies program, previously told NBC News.

At the beginning of the coronavirus pandemic, before mitigations were put in place across Illinois, the estimated "R-naught" value was 3.6, according to state officials.

As of Monday, Pritzker said, the state's value was at one, but not below, adding that when the value goes below one, it will be "very good news for Illinois."

The governor added that mitigations must remain in order to keep the transmission rate low, and a gradual course toward reopening — as outlined in his Restore Illinois plan — is necessary.

"If we were to lift all our mitigations entirely at the end of the month, modeling shows there would be a new surge of COVID-19 and a higher R naught in each of the four regions, even in areas where we halted seemingly moderate impacts from growing," he said.

Pritzker also said Monday that modelling data now indicates that the state may not hit the so-called "peak" in the coronavirus pandemic until as late as mid-June.  

Pritzker said that the state has made progress in its fight to slow the spread of COVID-19, but that the progress has only helped to flatten the curve of the virus and potentially lengthen the time that numbers will plateau, rather than causing a decline in numbers statewide.

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