Gauge of transmission shapes policy decisions
Epidemiological score influences orders to lift restrictions
As the Bay Area inches toward reopening the economy, a single number that defines the state of the coronavirus outbreak is gaining new status: the R0.
That’s the reproduction value, pronounced Rnaught. It’s an epidemiological number that, in simplest terms, represents how many people an infected individual will transmit the virus to. An R0 of 2 means that every infected person will infect two others, and those two people will each infect two more, and so on.
An R0 of 1 means every person will infect one other, and a disease outbreak will neither grow nor decline. To slow down spread, the R0 needs to drop below
1. California as a whole is hovering just below 1, but the number varies from county to county. Santa Clara County — an early hot spot in the Bay Area — has an R0 of about 1 at the moment; San Francisco is just under.
Calculating the R0 is complicated science, based on often muddy data and imperfect disease models. But it’s also a convenient way of describing the state of an epidemic, and Bay Area health officers are closely following the number as a marker of whether it’s safe to start emerging from shelterinplace.
“We’ve been holding pretty steady with an R0 of 1, sometimes it goes a little lower,” Dr. Sara Cody, the Santa Clara County health officer, told the Board of Supervisors at a meeting this week. Efforts to contain the outbreak, including aggressive testing and investigation of new cases, should be able to knock that number down, she added.
“That should give us some headroom, some money in the bank, to loosen our very strict social distancing measures we have in place,” Cody said. “Even a small increase in the R0, like 1.1 or 1.2, you can see that (case) counts will begin to grow. It’s really important that we keep our R0 below 1.”
The R0 of a virus — or bacterium or any other infectious agent — is calculated based on several factors, including innate features of the pathogen such as whether it spreads by an exchange of respiratory drops or some other mechanism, and how long individuals are infectious.
Other factors depend on human behavior and the environment in which the virus is transmitting — whether people tend to take a crowded subway, for example, or drive to work. So the R0 often varies from country to country, and even city to city.
When the coronavirus was first identified, its inherent R0 was thought to be about 2 to 3, which is fairly high. It means an outbreak can quickly double or triple in size. That’s higher than seasonal influenza, which is typically just over 1.
But that number can be pushed down by altering human behaviors. That’s the point of social distancing. The more people are kept apart, the less the virus can reproduce, until eventually the R0 drops below 1. At that point, an outbreak can be contained and the virus can potentially be wiped out entirely, although that’s unlikely to happen with the coronavirus, infectious disease experts said. It’s just too good at transmission to vanish without a vaccine.
“Anything less than 1, we’re doing OK,” said Dr. Lee Riley, an infectious disease expert at UC Berkeley. “But the speed of the reduction is certainly related to how far below 1 the reproductive number is. The lower the number, the faster the epidemic will finish.”
A team of amateur scientists, led by founders of Instagram, developed a model for calculating the R0 for all U.S. states that shows the value dropping below 1 in most places after shelterathome orders were put in place, or after residents were at least advised to stay home as much as possible. Their R0 results are online at rt.live.
Six weeks ago, according to their calculations, only 14 states had R0 values under 1. As of Thursday, only two states — Wyoming and Minnesota — were over 1.
But infectious disease experts warn that these numbers are nuanced, and the models that produce them are imprecise. Most of the states under 1 are just barely there, which means their success in keeping the reproduction value down is still precarious.
“We crank these disease models out all the time. The R0 is just one estimate that emerges as an indication of how different mitigations might be working,” said Shannon Bennett, chief of science at the California Academy of Sciences.
A week ago. Dr. Grant Colfax, director of the San Francisco Department of Public Health, said during a virtual town hall with UCSF that the city’s R0 was about 0.9 or 0.95 — too close to 1 for anyone to feel comfortable easing up social distancing restrictions.
Dr. George Rutherford, a UCSF infectious disease expert who has been advising the city’s public health response, said models have put the number at 0.8 or 0.85 more recently. That’s great news as far as keeping the outbreak contained, and suggests the city may be able to ease some restrictions, he said.
But those should be small steps, he added, so public health officials can carefully watch R0 — and other markers of the local outbreak — to make sure the virus isn’t spreading widely again.
“As we release from shelter in place, we want to be really careful about what the R0 is,” Rutherford said. “We want it to be less than 1. It’s the best measure of what’s going on.
“But there’s also a lot of uncertainty to the number,” he said. Like most public health experts, and most Bay Area residents, he’s watching several markers to help him understand how the outbreak is progressing. “I still look at the case counts every day,” he said.