New York has a way to tell when its coronavirus outbreak has run its course.
When a population has immunity or social distancing measures are in place, epidemiologists look to a figure called the "effective reproduction number" (R), which represents the average number of people whom a single coronavirus patient is expected to infect.
The effective reproduction at a specific point in time, called the R(t), shows how fast the virus is currently spreading within a population. It offers a real-time guide for states looking to ease social distancing or stay-at-home restrictions.
New York's R(t) is now 0.8, Gov. Andrew Cuomo said at a press briefing on Sunday. That means, right now, every person is infecting fewer than one other person on average. Taken on its own, this figure suggests that New York's outbreak is temporarily contained.
But the rate could easily rise if social distancing measures are relaxed.
"Short term, the numbers are on the decline. Everything we have done is working," Cuomo said. But he cautioned against "getting too far ahead of ourselves."
If the rate of transmission rose above 1.2, he said, cases could spike again dramatically.
"If we just said, 'OK, guys, we're all tired of staying home — we're going to open the schools and everybody can go and hang out in the cafés,' I would expect we'd have another big rebound and it would not be pretty," Dr. Elizabeth Halloran, a biostatistician at Fred Hutchinson Cancer Research Center and the University of Washington, previously told Business Insider.
New York's stay-at-home restrictions are scheduled to last until May 15. On Sunday, Cuomo outlined a plan for reopening the state:
- Phase 1: Resume construction and manufacturing. Wait two weeks to see if cases spike.
- Phase 2: Reopen low-risk businesses that have a plan to keep transmission low. Schools could reopen as well.
- Prohibit attractions that would draw large crowds from outside the local area.
Cuomo said upstate regions are more likely to reopen sooner than downstate areas like New York City. The decision to reopen, he added, ultimately hinges on whether hospitalizations decline for two weeks.
"The federal guidance from the CDC is that before you start reopening, the state and regional hospitalization rate must be in decline for 14 days," Cuomo said. "In this case, I think the CDC guidance is right."
The fact that cases are declining is a testament to the actions of New Yorkers, he added.
"The great achievement in this period has been that when people get the facts and they trust the facts and they understand the facts, they do the right thing," Cuomo said. "That is a lesson that I hope people remember after this is all over."
