It is the most important number in New York City right now, a critical threshold that triggers restrictions by state and local governments in response to the coronavirus. The mayor of New York shut down public schools at 3 percent. The governor says that a sustained 3 percent level in the city would result in banning indoor dining, closing gyms and hair salons, and placing a 25-person cap on attendance at houses of worship even as the holidays approach.
But as important as that number is, it seems the city and the state can’t agree on whether we’re there yet.
That’s the situation that has played out over the past week, with Mayor Bill de Blasio saying 3 percent has been breached, while Gov. Andrew M. Cuomo said it is well below that. Each relies on his own statistics, which are compiled and reported in different ways, varying even on which tests to include in the calculation.
The discrepancy can be striking: On Sunday, for instance, the city said its seven-day average was 3.09 percent. Mr. Cuomo’s office, however, put the city’s rate at more than half a point lower, at 2.54 percent.
In one way, of course, not all numbers are equal, as Mr. Cuomo’s statistics from the state Department of Health govern a wider array of activities and businesses in regions all across New York. But Mr. Cuomo granted local school districts the right to set their own parameters for school shutdowns, and Mr. de Blasio, who controls the school system, set 3 percent as that level.
And so, in the end, it was the city’s numbers that caused the temporary pause of in-person learning in the nation’s largest school system.
Why can’t they agree on whether we are at 3 percent?
The cause of the discrepancy lies in both the tests that are included and the time frame in which statistics are reported, leading to the mayor and the governor giving different numbers each day.
It is the latest discordant message between two rivals that has played out over the entire pandemic, adding a level of dysfunction and confusion to the response.
On Sunday, Mr. Cuomo suggested the city’s calculation of its positive rate was “confusing and unnecessary,” and also “irrelevant” because the state numbers would govern any broader restrictions.
The state and city health departments do, in fact, have several different accounting rules for tracking the spread of the virus. The state treats a new case as arising on the day the test results came in. The city dates each new case to the day the sample was provided.
So if an infected person goes to a clinic to have his nose swabbed on Monday, that sample is often delivered to a laboratory where it is tested. If those results are reported to the health authorities on Wednesday, the state and city would record it differently. The state would include it with Wednesday’s tally of new cases, while the city would add it to Monday’s column.
The 3 percent threshold is based on a seven-day rolling average. It matters what day a new case is registered.
Another factor contributes to the discrepancy as well, which has received little attention so far: antigen tests. New York State includes the tests in its official metrics. But while they are generally faster, they are less likely to detect the infection in people with a low viral load.
New York City, however, does not include antigen tests, preferring a more sensitive one known as a polymerase chain reaction test. The city only includes P.C.R. tests performed in a laboratory in its count. That’s why the state — which is counting both antigen and P.C.R. tests — may have a higher tally for overall cases in New York City but a lower percentage of positives.
P.C.R. tests in a laboratory have long been considered the gold standard because they are unlikely to miss any infections. But some public health experts say that much of the P.C.R. testing for coronavirus is too sensitive, resulting in coronavirus diagnoses for people who are carrying relatively insignificant amounts of the virus and are probably not contagious.
Antigen tests, which can be performed rapidly and cheaply, detect bits of coronavirus proteins. But they are more likely to miss cases, including people recently infected who have lower viral loads.
The difference in sensitivity between the two types of tests can contribute to a gap in the positivity rates between the city and state.
In fact, the positivity rate of the same group of people — in this case, New York City residents — can vary depending on how many receive antigen tests versus traditional P.C.R. tests. Antigen tests can miss some cases when the amount of virus is still low.
Say 1,000 people get tested. Let’s assume all have a P.C.R. test, and that 30 tests are positive, for a positivity rate of 3 percent. Now let’s assume half get an antigen test and half get a P.C.R. test. Maybe only 25 tests come back positive, for a positivity rate of 2.5 percent.
“We believe in our number and our methodology,” said Bill Neidhardt, the mayor’s spokesman, before alluding to the mayor’s decision on Wednesday to declare the 3 percent threshold reached, rather than adopt the state’s metric in a last-minute bid to keep schools open.
“Switching up a public health standard in the middle of the morning is not a good idea for clarity and public confidence,” he said.
Gary Holmes, a spokesman for the state Health Department, said that the state reported both types of tests “to provide the clearest possible picture of individuals diagnosed with Covid in a given time period.” A positive result on either a P.C.R. or an antigen test require an individual to quarantine.
On Sunday, Mr. Cuomo said the state does “hundreds of thousands of antigen tests per week,” noting frequent tests of nursing home workers, adding that excluding those would “significantly reduce the database.” Such speedy results may also be in high demand in coming weeks as people seek tests before the holidays, the governor’s office said. “It’s nice to know quickly,” Mr. Cuomo said.
There are other differences in how the city and state calculate the positivity rate that also help explain why the city’s rate is higher.
For instance, if a person tests negative repeatedly within a seven-day period, the city counts only a single test when calculating the positivity rate. But the state includes all negative tests from different days in its calculation.
Confused by the terms about coronavirus testing? Let us help:
- Antibody: A protein produced by the immune system that can recognize and attach precisely to specific kinds of viruses, bacteria, or other invaders.
- Antibody test/serology test: A test that detects antibodies specific to the coronavirus. Antibodies begin to appear in the blood about a week after the coronavirus has infected the body. Because antibodies take so long to develop, an antibody test can’t reliably diagnose an ongoing infection. But it can identify people who have been exposed to the coronavirus in the past.
- Antigen test: This test detects bits of coronavirus proteins called antigens. Antigen tests are fast, taking as little as five minutes, but are less accurate than tests that detect genetic material from the virus.
- Coronavirus: Any virus that belongs to the Orthocoronavirinae family of viruses. The coronavirus that causes Covid-19 is known as SARS-CoV-2.
- Covid-19: The disease caused by the new coronavirus. The name is short for coronavirus disease 2019.
- Isolation and quarantine: Isolation is the separation of people who know they are sick with a contagious disease from those who are not sick. Quarantine refers to restricting the movement of people who have been exposed to a virus.
- Nasopharyngeal swab: A long, flexible stick, tipped with a soft swab, that is inserted deep into the nose to get samples from the space where the nasal cavity meets the throat. Samples for coronavirus tests can also be collected with swabs that do not go as deep into the nose — sometimes called nasal swabs — or oral or throat swabs.
- Polymerase Chain Reaction (PCR): Scientists use PCR to make millions of copies of genetic material in a sample. Tests that use PCR enable researchers to detect the coronavirus even when it is scarce.
- Viral load: The amount of virus in a person’s body. In people infected by the coronavirus, the viral load may peak before they start to show symptoms, if symptoms appear at all.
On the other hand, when someone tests positive repeatedly, the city includes each positive test when calculating the positivity rate. The state, however, only counts the first positive test and ignores subsequent ones.
That last variable — whether repeated positives are counted or not — can have a big effect on the positivity rate. Gareth Rhodes, a member of Mr. Cuomo’s coronavirus task force, estimated that about 15 or 20 percent of positive P.C.R. tests currently emerging from New York City were for someone who tested positive in the past.
Why does it matter?
If New York City hits 3 percent based on the state’s metrics, the governor has suggested it will enter into a so-called “orange zone,” the middle of three color-coded restrictions that the governor enacted in early October.
Under the governor’s plan, a so-called “red zone” is subject to the toughest restrictions, with bans on mass gatherings, indoor dining, and in-person learning. Nonessential businesses would close and religious services would be limited to 25 percent capacity of houses of worship or 10 people, whichever number is lower.
“Orange zones” — which the governor says the city is in danger of falling into — allow slightly large gatherings and outdoor dining, but schools shut down as do “high-risk” nonessential businesses like barbers and gyms. Precautionary zones — appropriately yellow — allow in-person classes but ramp up testing, and put some less stringent caps on dining and gatherings.
Four of the city’s five boroughs — Queens, the Bronx, Brooklyn and Staten Island — already have yellow zones, and Mr. Cuomo said Upper Manhattan may have one imposed later this week. Staten Island may also face intensifying restrictions.
These zones are re-evaluated after two weeks, and the metrics for entering and exiting each of these zones involve a complicated mix of data and a dash of deliberation: Restrictions can be modified on “expert advisement,” the state says, and include determinations based on local hospitalization rates, or whether outbreaks can be traced to a single source (like a prison, gathering or group residence).
Other factors can include “compliance and enforcement actions taken by local government” as well as more vague standards like “community cooperation to reduce viral spread.”
Which, of course, could render a mere number — such as 3 percent — less important.
Can the city act on its own?
Based on its metrics, New York City has already hit the 3 percent mark. Yet it may still be some time before businesses face any restrictions.
Not only do state statistics put the city’s seven-day rolling average below that number, but New York City must breach that threshold for 10 consecutive days before the city would enter the “orange zone.”
Technically, the city could use several workarounds to target businesses it decides are contributing to virus spread. For example, the city’s Department of Health could try to close any restaurant by declaring it a public health threat.
But the governor has broad emergency powers over a range of businesses, trumping Mr. de Blasio. At news conferences, the mayor sometimes sounds a resigned note, saying that certain decisions about closures must be left to the governor.
So, for now, New Yorkers will continue to wait for the city to hit 3 percent … again.