No, seriously: Does New York have herd immunity?

No, seriously: Does New York have herd immunity?

If you think the spin about New York’s alleged “success” in managing the pandemic is insufferable now, wait until it turns out that NYC has herd immunity because the virus was allowed to rip through the city unchecked in early March while Cuomo pulled his chin and wondered what to do.


The media will be looking to give him a Nobel prize for medicine.

He’s been posting daily updates of New York State’s latest case count every morning for months. And every time he does, I look at this curve, rub my eyes, and think, “This can’t be luck.” There has to be an explanation:

Day after day after day after day, for two months, New York’s postivity rate is one percent or less. Granted, schools and many businesses there are still shut down. Granted, after their horrific ordeal this past spring, New Yorkers are probably more diligent about wearing masks, washing their hands, and keeping their distance than the average American is. Even so — not a single outbreak. Not one little blip at the tail end of that curve even though surely there are many younger New Yorkers who are partying, socializing, and otherwise bending or breaking the rules about proper pandemic behavior. Not one brief rough patch in two months.

Other places that were ravaged by the virus haven’t been as fortunate. Here’s Spain’s curve, which looks a lot like New York’s … until it doesn’t:

Sweden’s curve took longer to develop than New York’s and they’ve seen great progress lately too — but note the slight rise in cases lately:

There is one country whose curve looks a lot like New York’s. That would be Italy, which endured its own apocalyptic COVID outbreak in February, only to recover and drive cases down nearly to nothing. Even here, though, there’s been an upswing in the past two weeks, with yesterday’s case count (478) well more than twice the lowest number (190) recorded on June 29:


Only New York appears to have crushed the curve and kept it thoroughly crushed (so far). Why?

Have they reached herd immunity? One reason why that seems likelier in NYC than in Italy, Spain, and Sweden is density. Those last three are nations, of course, with the population spread out across huge distances. Northern Italy was decimated by the virus this spring but there may be large numbers of Italians in the south who have yet to encounter the virus and thus are dry tinder for a “second wave” or even a more limited outbreak. That’s less true in NYC, where people are packed cheek by jowl on a small island and its surrounding boroughs. The odds that any random resident of New York City has encountered SARS-CoV-2 surely must be higher than that of any random Italian, Spaniard, or Swede.

Is the city immune? The Times reports today that scientists are asking themselves this same question, and not just about New York. London and Mumbai are under the microscope too. Large dense metropolises that have experienced ferocious outbreaks may have turned the corner on COVID-19.

In interviews with The New York Times, more than a dozen scientists said that the threshold is likely to be much lower: just 50 percent, perhaps even less. If that’s true, then it may be possible to turn back the coronavirus more quickly than once thought…

Assuming the virus ferrets out the most outgoing and most susceptible in the first wave, immunity following a wave of infection is distributed more efficiently than with a vaccination campaign that seeks to protect everyone, said Tom Britton, a mathematician at Stockholm University.

His model puts the threshold for herd immunity at 43 percent — that is, the virus cannot hang on in a community after that percentage of residents has been infected and recovered…

Researchers in Mumbai conducted … a random household survey, knocking on every fourth door — or, if it was locked, the fifth — and took blood for antibody testing. They found a startling disparity between the city’s poorest neighborhoods and its more affluent enclaves. Between 51 and 58 percent of residents in poor areas had antibodies, versus 11 to 17 percent elsewhere in the city.


The point about efficient distribution is important. Other research has suggested that “superspreader events” are responsible for a huge number of COVID transmissions, with one estimate claiming that just 10 percent of infected people are responsible for 80 percent(!) of transmissions. Logically, one would think that many/most people who are likely to attend a superspreader event are getting infected early in the epidemic, precisely because they’re more willing to take risks, e.g., going to crowded bars. Once most of that group has gotten infected and either died or recovered and gained immunity, suddenly the “freeways” of transmission are no longer available to the virus. It has to take side streets instead, where spread is slower and less efficient.

That is, it’s not just that a random 40 percent or whatever of New Yorkers has gotten infected at this point. It may be that it’s the 40 percent that’s most likely to infect others. Now that those people are immunized, there’s an outsized effect of reduced transmission across the entire remaining population that’s vulnerable.

Of course, in order for herd immunity to be possible, it would have to be the case that even people with mild cases of COVID are immune afterward. Most seem to have mild infections, after all; if all of them are still susceptible to infection after recovering, or if they’re immune only briefly before becoming susceptible again, then true immunity is much harder than we think to attain. Coincidentally, though, the Times has another story about that today. As far as scientists can tell, it seems that mild cases do produce durable immunity.


Scientists who have been monitoring immune responses to the coronavirus for months are now starting to see encouraging signs of strong, lasting immunity, even in people who developed only mild symptoms of Covid-19, a flurry of new studies has found.

Disease-fighting antibodies, as well as immune cells called B cells and T cells capable of recognizing the virus, appear to persist months after infections have resolved — an encouraging echo of the body’s robust immune response to other viruses…

“This is very promising,” said Smita Iyer, an immunologist at the University of California, Davis, who is studying immune responses to the coronavirus in rhesus macaques and was not involved in these papers. “This calls for some optimism about herd immunity, and potentially a vaccine.”

So herd immunity is possible in NYC, at least theoretically — although the mechanics are still in dispute. Some scientists have proposed the possibility of cross-immunity stemming from earlier infections by other, much milder coronaviruses. The idea is that T-cells in a person with cross-immunity “remember” those previous viruses and thus “recognize” SARS-CoV-2 when it enters that person’s system, spurring a strong initial immune response that fights the virus off. A recent Oxford study speculated that that cross-immunity could help explain why the virus is having so much trouble regaining traction in hot spots like London, and could mean that the true threshold for “herd immunity” from COVID is much lower than 50-60 percent. Between the protective measures people are taking (masks, hand-washing, social distancing) and cross-immunity, Oxford suggested that herd immunity may be reached with as little as 25 percent of the relevant population having been infected and recovered.


A study by the state government of New York found roughly 25 percent of New York City residents had antibodies as of mid-April. The percentage is surely higher, maybe much higher, today. If Oxford is right then NYC’s crushed curve has an obvious explanation: They’ve reached herd immunity — at least as long as they keep social distancing in effect. What we’re really talking about in all this isn’t true herd immunity, after all, where we get to drop all precautions against the virus, but immunity when a certain level of precautions are taken. Call it, uh, qualified immunity if you like. If New Yorkers were to go back to normal and everyone was suddenly cheek by jowl again, no doubt some who had thus far managed to avoid an encounter with the virus would find that their luck had run out.

The possibility of herd immunity raises another question. If, as pretty much everyone assumes, Wuhan’s outbreak was much more extensive than the Chinese government let on, is it possible that parts of China have reached herd immunity as well? Hmmmm:

It’s also hard to avoid speculating that maybe one reason why China’s neighbors have had such dramatically better outcomes with COVID than the U.S. is that they’ve developed more extensive cross-immunity over time from exposure to other regional coronaviruses. SARS is the most famous example, but if it’s true that China’s wet markets are a virus factory, it may be that mild coronaviruses circulate more frequently in the Far East. When a not-so-mild one finally came along, much of the population may have already developed T-cell defenses for it. That’s just speculation, but I’d curious to hear what someone who knows what they’re talking about thinks of it. An obvious objection: If there’s regional cross-immunity due to proximity with China, why has Russia had such a hard time of it with COVID?


Anyway, a little back-of-the-envelope math for you. If we assume (incorrectly) that all of the recent hot spots in the U.S. plus NYC have reached herd immunity by dint of their outbreaks — that’s California, Texas, Arizona, and Florida — that would account for a population of around 105 million people or so, about a third of the U.S. It cost us 170,000 deaths to reach that point. The bad news is that it’s surely not the case that huge states like California have had the same percentage of people exposed to the virus as a tiny enclave like NYC has — that was my point about density up top — so California surely is not immune and realistically may not be close to reaching immunity. The good news, though, is that there may be sufficient immunity in each state at this point to foreclose the possibility of a truly nasty second wave this fall. We may see Spain-type second waves from time to time in those states but we’re never going to see curves as steep as we have already, one would think.

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