One key figure helps countries decide when their coronavirus outbreaks are over - but scientists say it's a moving target

• Scientists use a figure called R0 (pronounced R-naught) to determine how much the coronavirus is spreading within a population.
• R0 represents how many people an average person with a virus infects.
• When an outbreak is over, R0 will drop below 1, meaning every person will infect fewer than one other person on average.
• But R0 could rise and fall depending on what social distancing measures are in place.
• Lifting a lockdown too soon could cause the R0 to spring back up, resulting in a second wave of infections.
• For the latest case total and death toll, see Business Insider's live updates here.
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The end of the coronavirus pandemic will likely be tied to the development of a vaccine - a process that could take around 18 months. But countries under lockdown could resume normal activities before then, based on how much the virus is spreading.

To determine how readily a virus gets transmitted, epidemiologists look to a figure called the "basic reproduction number," or R0 (pronounced R-naught). It represents the average number of people that a single patient is expected to infect, among a group that has no immunity to the virus.

In February, Chinese researchers estimated that the R0 in Wuhan (where the outbreak started) was around 2 to 2.5, meaning the average coronavirus patient infected at least 2 others. More recently, researchers have determined that the virus was likely more contagious back then, with an R0 of 5.7.

When the outbreak has run its course, the R0 will drop below 1, meaning every person will infect fewer than one other person on average.

China has already reached this point, since its infections have tapered off. The country has seen an average of 75 infections per day in April, compared to more than 15,000 at its peak in mid-February, according to official data.

By contrast, daily cases in the US have exceeded 24,000 since March 31. The vast majority of these cases come from New York, which is now the epicenter of the US outbreak.

In New York, new infections appear to have reached a plateau, suggesting that the state has flattened its curve - in other words, the rate at which new people get infected is slowing.

"The curve going up means people are infecting on average more than one person at the inflection point. When it starts to come down, you're infecting on average one person. When it is coming down, that means you're infecting on average less than one person," Dr. Elizabeth Halloran, a biostatistician at Fred Hutchinson Cancer Research Center and University of Washington, told Business Insider.

In Seattle, she said, the curve has already started to drop due in large part to strict social distancing ordinances.

"The R0 depends on how fast people are interacting with each other," Halloran said. "In our case in Seattle, everybody's behaving themselves and people are really distancing."

But that doesn't mean that Washington, or any other state, can re-open just yet. The R0 could easily rise again if lockdowns are lifted too soon.

Longer lockdowns could make sure the R0 stays low

China resumed transportation into and out of the city of Wuhan on April 8, but many health experts worry that the country could see a second wave of cases. On Sunday, China reported more than 100 new coronavirus cases - its highest number since March 5.

Singapore has also seen cases sharply rise in April, despite slowing the spread of the virus with extensive screening and contact tracing at the beginning of the outbreak. The entire country was placed under lockdown last week.

State officials have expressed concern about a similar scenario playing out in the US.

"Go look at other countries that went through exactly this, started to reopen and then they saw the infection rate go back up again," New York Gov. Andrew Cuomo said at a press briefing on Tuesday. "The worst scenario would be if we did all of this, we got that number down, everybody went to extraordinary means and then we go to reopen and we reopen too fast."

Halloran agreed that lifting lockdowns now would open the door to a second wave of infections.

"If we just said, 'Okay guys, we're all tired of staying home. We're going to open the schools and everybody can go and hang out in the cafes,' I would expect we'd have another big rebound and it would not be pretty," she said.

Halloran outlined a checklist for bringing the R0 below 1 and keeping it there.

First, the US must be able to test everybody for possible infection, she said. From there, infected people who don't require hospitalization should be taken to a center where they're unlikely to spread the virus to others. Officials should also trace and quarantine each infected person's contacts, similar to efforts underway in San Francisco and Massachusetts.

"If we want to open things up now, we'd need to be able to slow things down by finding the people who are actually infected and getting them out of circulation as quickly as possible," Halloran said.

Officials should also trace and quarantine each infected person's contacts, similar to efforts underway in San Francisco and Massachusetts. Finally, Halloran said, the US should provide personal protective equipment for every frontline worker. Doctors and nurses throughout the country are currently reporting shortages of gloves, gowns, and masks.

"All of those things are basically available," Halloran said. "They're just not available in [enough] quantities yet."

Implementing these measures across the country, she added, could allow the US to lift lockdowns before a vaccine comes on the market.

"You'd still have some infections, but we wouldn't have to completely stifle the economy waiting for a vaccine, which might or might not be available in 18 months," she said. "The infection would sort of limp along in the population."

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