The Covid-19 epidemic that raged through New York City in late winter and early spring is starting to feel like ancient history. The disease has mostly moved on to other locales. Life in the city is far from normal, but tons livelier than it was in March and April.

One of the many advantages to having the coronavirus in the city’s rearview mirror (for now at least) is that it’s getting easier to see how the biggest urban explosion of the disease in the U.S. (so far at least) actually played out. Thanks to antibody surveys conducted by the state of New York and a study released in preliminary form last month by a large group of researchers at New York City’s Icahn School of Medicine at Mount Sinai, it is now possible to craft a rough estimate of how many people were infected with the disease in the city and when. Turns out it’s a lot different from the standard picture provided by confirmed-case counts!

A Lot Different From The Standard View
No epidemiologist or public health official would dispute that there were a lot more actual cases of Covid-19 in New York City than confirmed ones, or that the peak in new infections occurred before the peak in confirmed cases, but the exact ups and downs of the disease depicted above are the product of a bunch of assumptions that may not be entirely correct. I will describe those assumptions and their possible flaws below, but I’m convinced that this is nonetheless a big improvement over the confirmed-cases chart, which misses most early cases of the disease because there simply wasn’t enough testing capacity in March and early April. The very different picture of the New York City Covid-19 epidemic that my estimate paints can thus better inform our understanding of how the disease spread and what may have stopped it.

What enables such estimates is that the virus that causes Covid-19 leaves traces in the blood of those who recover from it. In one study of New York area Covid patients by Mount Sinai researchers, antibodies to the new coronavirus appeared in blood samples from 621 of 624 of those who had previously tested positive for the disease via nasal-swab tests, with a median time from symptom onset to antibody appearance of 20 days.

Some of those same researchers then tested blood samples from patients at Mount Sinai Health System hospitals in the city from mid-February through mid-April to compile weekly estimates of antibody prevalence among patients who had been admitted to the hospital from the emergency room and those there for other reasons. Even the latter group is not necessarily representative of the overall populace, of course, but the 19.6% of non-emergency Mount Sinai patients testing positive for antibodies during the week ending April 19 was close enough to the 22.7% prevalence estimated for New York City from a large state survey conducted April 19-28 that the Mount Sinai researchers concluded that their weekly measures did convey useful information about Covid-19’s presence in the city. A Centers for Disease Control and Prevention survey based on blood samples taken in late March in the New York metropolitan area also delivered results compatible with Mount Sinai’s.

The Evidence Of The Coronavirus In New Yorkers' Blood
Given the aforementioned 20-day median delay from the onset of symptoms to the production of antibodies and the 6-day median lag from infection to symptom onset estimated by the CDC, this implies that a whole lot of New Yorkers got Covid-19 in March. With a city population estimated by the Census Bureau at 8,336,817, the 22.7% prevalence from the late-April state survey would mean that nearly 1.9 million New Yorkers had already contracted the disease as of early April. The 10.1% prevalence estimated from the Mount Sinai blood samples for the week ending April 5 implies that 842,000 already had it as of March 10.

Then again, the 2% prevalence estimated for the week ending March 1 implies that 167,000 New Yorkers had already contracted Covid as of Feb. 4. This is incompatible with pretty much all other research on the early spread of the disease here, which mostly depicts it first arriving in late January or early February and taking off thereafter. So there is clearly error in these estimates, and the numbers derived from the early Mount Sinai blood samples in particular shouldn’t be taken too seriously because they’re within that margin of error.

Even the numbers from the bigger state surveys aren’t perfectly reliable. For one thing, the samples were collected at grocery stores, meaning that cautious sorts who had all their groceries delivered were excluded. The estimates have also shifted over time. New York Governor Andrew Cuomo initially reported the late-April New York City percentage as 21.2%, then revised it down to 19.9% after the survey was completed. The researchers at the state Department of Health and University at Albany who designed the survey subsequently revised that up to 22.7% for a paper published in the Annals of Epidemiology, which reflected their estimate of cases that were missed because the test they used turns up false negatives from time to time.

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