CNYs restart after coronavirus hinges on a number you never heard of and cant count – syracuse.com

Syracuse, N.Y. The opening of Central New Yorks economy will depend on measuring a largely unmeasurable value once known only to epidemiologists.

At Upstate Medical University on Tuesday, Gov. Andrew Cuomo announced that one of two gates to open the economy was how fast the novel coronavirus is spreading. Thats known as the rate of transmission, or Rt for short. It might be the first time that value has been used to guide government actions, said an Upstate scientist.

Rt is the average number of people who catch the virus after being exposed to an infected person. An Rt of 2, for example, means that every infected person infects two more. Cuomo has said the rate needs to fall to 1 or below for a region to reopen and stay open.

You wouldnt start reopening unless you had a transmission rate below 1.1; really below 1, Cuomo said. If the transmission rate hits 1.1, thats what they call outbreak. That means its going to spread much, much faster. Youre in trouble.

When Rt is 1, the virus keeps spreading. People will recover at the same rate others are getting sick, so the virus will hover in the background. When the Rt drops below 1, the epidemic slows down and will, theoretically, die out over time.

Rt cant be directly measured. Its not a blood test that gives clear results, or a simple count of the number of people lying in the hospital. Its a scientific concept critical to predicting how fast a disease spreads, but it can only be derived from computer models that rely on imperfect data and assumptions.

Its not clear if Central New York has met Cuomos threshold. Cuomo said Sunday that Upstates rate was 0.9, but he did not break it down by region nor explain how it was calculated. His office did not reply to questions.

Upstate has devised its own model of the viruss spread locally, which two weeks ago estimated the Rt value at 1.3. That would still be too high for a restart under Cuomos requirements.

The Upstate model will likely be updated this week, said Dr. Kathryn Anderson, one of six Upstate professors who created it.

Cuomos decision to use the rate of transmission makes sense and might be unique, Anderson said.

As a scientist I find his approach reasonable, she said. Im not aware of other disease where people used Rt as a metric for guiding policy decisions with this level of impact.

Cuomo said the other number the state will track for a reopening is the percentage of open hospital beds. A region would need to have at least 30% of its beds open in case of a surge in patients suffering from COVID-19, the disease caused by the virus.

The percentage of hospital beds is easy to measure: Count the number of beds with patients in them and divide by the total beds.

Likewise, its easy to do a nasal swab to see if someone is infected, or a finger prick to see if someones blood has antibodies.

The rate of transmission isnt like that. Theres no test, no simple count. The swirling movement of people in society makes it impossible to know for sure how many got sick from every sick person.

Its very rare that we have a situation where we can study this person infected this person who infected these people and onwards, Anderson said.

Figuring out the Rt for other diseases is more straightforward. For example, HIV, the virus that causes AIDS, is transmitted only through direct contact, primarily sex and sharing of intravenous needles.

A respiratory virus like the new coronavirus, called SARS-CoV-2, can also be spread unknowingly through air or on surfaces. A person sneezing in a restaurant or touching an office door handle can pass it on. To take that into account, epidemiologists have to consider dozens of questions: How many people were in close proximity to the infected person? How far away, and for how long? Did the people exposed then touch their face before washing their hands?

There is no way to know those things, so they have to be assumed based on previous studies of viruses and of human behavior. Scientists can only estimate Rt by plugging those assumptions and current data into a computer model.

Every day (Onondaga County executive) Ryan McMahon shows the number of new cases per day, and with some assumptions we can look at the data and get a sense of what R is, Anderson said. We see a rise in cases over time and we have to make some pretty powerful inferences with our data.

One of those assumptions is how long it takes for someone to become sick after being exposed. The Upstate model assumes four to five days. Another assumption is how well residents are abiding by social distancing measures like avoiding gatherings. Cell phone tracking data is a proxy for that, but its not perfect.

Upstates model and the cell phone data have shown that Central New Yorks Rt number has been consistently too high for Cuomos cutoff of 1.1. The latest Upstate modeling in mid-April showed the Syracuse area at R of 1.3, meaning the virus was still spreading too fast for a reopening.

Upstate will issue new model updates this week, Anderson said.

Cuomo has said that regions of the state where the virus was under control could start reopening businesses as long as social distancing rules were enforced. McMahon has said he and several other counties are working on a regional plan to submit to the governor.

Scientists have estimated that without social distancing measures, the Rt for the coronavirus would initially be 2 to 3. The seasonal flu is about 1.3, a high enough rate to cause about 8% of Americans to get the flu each year.

At the other end of the scale is measles, in which an infected person can pass the virus to 12 to 18 others. That explains why measles can spread so quickly when children arent vaccinated.

The rate of transmission of a disease falls as more people become immune from getting a vaccination or the disease itself. Those people then cant catch and pass on the virus.

Its too soon to know if people who have had the novel coronavirus will develop immunity. That doesnt matter much right now in Central New York because only about 1.3% of us have had it, according to state testing. That means nearly 99% of us are still susceptible to catching, and transmitting, the virus.