From the New York Times:
By J. David Goodman and Michael Rothfeld
April 23, 2020, Updated 6:35 p.m. ET
One of every five New York City residents tested positive for antibodies to the coronavirus, according to preliminary results described by Gov. Andrew M. Cuomo on Thursday that suggested that the virus had spread far more widely than known.
If the pattern holds, the results from random testing of 3,000 people raised the tantalizing prospect that many New Yorkers — as many as 2.7 million, the governor said — who never knew they had been infected had already encountered the virus, and survived. Mr. Cuomo also said that such wide infection might mean that the death rate was far lower than believed.
While the reliability of some early antibody tests has been widely questioned, researchers in New York have worked in recent weeks to develop and validate their own antibody tests, with federal approval. …
In New York City, about 21 percent tested positive for coronavirus antibodies during the state survey.
Most estimates of the herd immunity level would therefore put NYC at about 1/3rd of the way there, which is substantial. On the other hand, NYC, when it’s functioning, doesn’t function as an isolated herd, but as the crossroads of the world. So it’s hard to see how herd immunity short of virtually 100% could work for NYC without the virus dying out in the rest of the world.
The rate was about 17 percent on Long Island, nearly 12 percent in Westchester and Rockland Counties and less than 4 percent in the rest of the state.
State researchers sampled blood from the approximately 3,000 people they had tested over two days, including about 1,300 in New York City, at grocery and big-box stores.
So the respondents were out and about shopping. It would be nice to know if they collected any data from a survey of people in the study to see how their behavior compared to a survey of people online.
… Dr. Howard A. Zucker, the state health commissioner, said the lab had set a high bar for determining positive results … He said that while concerns about some tests on the market were valid, the state’s test was reliable enough to determine immunity — and, possibly, send people back to the office.
“It is a way to say this person had the disease and they can go back into the work force,” Dr. Zucker said. “A strong test like we have can tell you that you have antibodies.”
But he cautioned that the length of any such immunity remained unknown. “The amount of time, we need to see. We don’t know that yet,” he said, adding, “They will last a while.” …
Dr. Demetre C. Daskalakis, the city’s top official for disease control, wrote in an email alert on Wednesday that such tests “may produce false negative or false positive results,” pointing to “significant voids” in using the science to pinpoint immunity. …
Dr. Michael Osterholm, an infectious disease expert at the University of Minnesota, praised the overall intent of New York’s study, but said the results in this case probably skew to a higher estimate than is real because a survey of grocery store shoppers in a pandemic would not be representative.
A small test of people out on the street in the Chelsea barrio of Boston found 1/3rd infected.
The sampling may disproportionately include those who have either already had the illness, or those who naturally tend to go out more and so are more likely to be exposed to the virus, he said.
Say there are 300 people in NYC:
A. 100 go grocery shopping 3 times per week and are 24% infected.
A. 100 go grocery shopping once per week and are 8% infected.
B. 100 never go grocery shopping and have what they need delivered and are 1% infected.
So, a random sample of grocery shoppers would be 75% group A, 25% group B, and 0% group C, adding up to 21% infected.
But the actual population is about 11% infected.
Two warnings: I made up all these numbers and I did the arithmetic in my head.
It would miss children, teenagers and older adults who may be sheltering in place.
“It’s not a criticism. It’s more of a comment that we have to be careful about interpreting supermarket customers as a representative sample when the state was in lockdown,” he said.
State officials said the test had been calibrated to err on the side of producing false negatives — to miss some who may have antibodies — rather than false positives, which would suggest a person had coronavirus antibodies when they did not. …
Mr. Cuomo on Thursday did not talk about any potential for immunity among those previously infected.
But he did suggest, based on the survey, that if as many as 2.7 million New Yorkers had the virus, the death rate in New York from Covid-19 would most likely be far lower than previously believed, possibly 0.5 percent of those infected.
More than 15,000 people have died of the virus in the state, a figure that does not include an additional 5,000 people in New York City who were never tested but were presumed to have died from the disease.
As of yesterday, New York City was at 15,740 official CV deaths, which include 5,000 who weren’t tested for CV. The population of New York City was 8.4 million people in 2018, so 15,740/(.21)*8,400,000 = 0.9% Infection Fatality Rate in NYC if this 21% Infection Rate is representative.
Now there are two time related problems.
First, antibodies don’t develop immediately, so the cumulative Infection Rate would likely be higher than the 21%. That would tend to lower the final IFR.
Second, and pushing in the opposite direction, not everybody who will die of an existing infection has already died. That would push the final IFR higher.
How to balance out the two factors is beyond me at present.
Somebody needs to figure how many more of the currently infected are likely to die in NYC. There are currently about 15,000 people in New York State hospitalized with CV.
If NYC is 1/3rd of the way to herd immunity, then 3 times the current 15,740 deaths would be about 47,000 deaths in NYC. (But maybe the equation would be more like 3 times 25,000 or 75,000 total deaths.)
NYC is about 1/40th of the U.S. population, so 40 times 47,000 deaths is almost 1.9 million.
… Mr. Cuomo said antibody testing results, along with hospitalization numbers, would influence the state’s reopening strategy, noting that the number of people being hospitalized was still too high to consider easing restrictions.
The state’s plan would involve tracking infections as restrictions are loosened on gatherings and businesses. Antibody testing would be used, Mr. Cuomo said, for identifying coronavirus survivors who can donate convalescent plasma — the part of the blood that contains antibodies.
Plasma treatment is quite promising, although I’m not aware of it being proven yet to work in this case. The challenging logistics of it are easier to make work later in an epidemic when there are large numbers of recovered blood donors.
Governor Cuomo tweeted demographic statistics for the whole state (in which 13.9% had antibodies):
Percent positive by demographic:
Upstate New York is much whiter than New York City, so it’s hard to tell offhand how much this is a race difference vs. how much it’s a difference of geography.