Tuesday, September 1, 2020

Herd immunity threshold is likely much lower than earlier believed. Scientists interviewed by NY Times say 50 percent, even less is likely. Could explain lack of resurgence in NY City-NY Times, 8/17/20

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8/25/20, Virus’s retreat in Brazilian Amazon upends notions of herd immunity,” Washington Post, Terrence McCoy, Heloisa Traiano
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8/29/20, Up to 90 percent of “positive” Covid tests are actually negative, NY Times review finds. Most carry insignificant amounts of virus and aren’t likely to be contagious.
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Aug. 17, 2020, What if ‘Herd Immunity’ Is Closer Than Scientists Thought?" NY Times, by Apoorva Mandavilli, print edition, Section D, page 8 

“Researchers are trying to figure out how many people in a community must be immune before the coronavirus fades.”  

“We’ve known from the beginning how the end will arrive. Eventually, the coronavirus will be unable to find enough susceptible hosts to survive, fading out wherever it briefly emerges…. 

To achieve so-called herd immunity-the point at which the virus can no longer spread widely because there are not enough vulnerable humans-scientists have suggested that perhaps 70 percent of a given population must be immune, through vaccination or because they survived the infection. 

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…. 

In parts of New York, London and Mumbai, for example, it is not inconceivable that there is already substantial immunity to the coronavirus, scientists said.

[Chart: As of 9/1/20, NYC 5 boros. Covid deaths, peaked in April, flat for 4 months. “Confirmed deaths,” 80%-89% of deaths had co-morbidities] 

“I’m quite prepared to believe that there are pockets in New York City and London which have substantial immunity,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. “What happens this winter will reflect that.”…

The initial calculations for the herd immunity threshold assumed that each community member had the same susceptibility to the virus and mixed randomly with everyone else in the community.
“That doesn’t happen in real life,” said Dr. Saad Omer, director of the Yale Institute for Global Health. “Herd immunity could vary from group to group, and subpopulation to subpopulation,” and even by postal codes, he said.  

For example, a neighborhood of older people may have little contact with others but succumb to the virus quickly when they encounter it, whereas teenagers may bequeath the virus to dozens of contacts and yet stay healthy themselves. The virus moves slowly in suburban and rural areas, where people live far apart, but zips through cities and households thick with people.

Once such real-world variations in density and demographics are accounted for, the [percentage] estimates for herd immunity fall. Some researchers even suggested the figure may be in the range of 10 to 20 percent, but they were in the minority.

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….
The virus may still flare up here and there, even if its overall spread is stymied. It’s also unclear how long someone who has recovered may be immune, and for how long.   
Virus-Resistant Communities?… 

Is it possible that some of these communities have herd immunity? In some clinics, up to 80 percent of people tested had antibodies to the virus. The highest prevalence was found among teenage boys.   

But people at clinics are more likely to be showing symptoms and therefore more likely to be infected, said Wan Yang, an epidemiologist at Columbia University’s Mailman School of Public Health in New York. Random household surveys would probably find lower rates–but still well above the 21 percent average reported for New York City, she said.

Researchers in Mumbai conducted just such 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 lowest-income residents are packed tightly together, share toilets, and have little access to masks. “These factors contributed to a silent infection spread,” said Dr. Jayanthi Shastri, a microbiologist at Kasturba Hospital in Mumbai who led the work. 

But models likZAe Dr. Britton’s hint that it’s not impossible. Other researchers have suggested...that herd immunity can be achieved at rates of immunity as low as 10 or 20 percent–and that entire countries may already have achieved that goal….
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Sunetra Gupta, a theoretical epidemiologist at Oxford University, [in a] widely circulated interview…said that London and New York may already have reached herd immunity because of variability among people, combined with a theoretical immunity to common cold coronaviruses that may protect against the new one.   ....
 
“That could be the explanation for why you don’t see a resurgence in places like New York,” she said….   
 
Immunity is a patchwork quilt in New York, for instance:
 
Antibodies were present in 68 percent of people visiting a clinic in the Corona neighborhood of Queens, for instance, but in just 13 percent of those tested at a clinic in the Cobble Hill section of Brooklyn.  
 
But another group, led by the mathematician Gabriela Gomes of the University of Strathclyde in Britain, accounted for variations within a society in its model and found that Belgium, England, Portugal and Spain have herd immunity thresholds in the range of 10 to 20 percent. 
 
“At least in countries we applied it to, we could never get any signal that herd immunity thresholds are higher,” Dr. Gomes said. “I think it’s good to have this horizon that it may be just a few more months of pandemic.”  
 
Other experts urged caution, saying these models are flawed, as all models are, and that they oversimplify conditions on the ground…. 
 
The new models offer food for thought, he and other experts said, but should not be used to set policy. [But it already has been, is doing so right now. Setting policy is exactly what the US invited a non-peer reviewed computer model to do: A disastrously flawed Coronavirus model changed government policy overnight and shut down the world for months.” In addition to “policy,” the US rushed to use the computer model to justify transferring trillions of US taxpayer dollars. Acceptance of this computer model by Trump and his task force co-opted and trashed institutions carefully built over hundreds of years. Americans died and were maimed wrongly believing they were preserving these institutions. Trump also used the computer model to justify granting of dictatorial powers to 50 US states in the name of a health emergency. Representative US government was nullified overnight simply by decree of Trump].   
 
“Mathematically, it’s certainly possible to have herd immunity at these very, very low levels,” said Carl Bergstrom, an infectious disease expert at the University of Washington in Seattle. “Those are just our best guesses for what the numbers should look like.”…
Imperfect Immunity

But what about immunity at levels lower than those needed for herd immunity? 

Definitely the disease would not spread as well if it gets back into New York,” said Joel Miller, a mathematical modeler at La Trobe University in Australia. “The same level of behavior change will have more effect on the disease now than it did four months ago.” 

Thinking of a city or country as composed of subgroups, demarcated by age, race and level of social activity, might also help governments protect those with the least immunity.

That perspective also might help put a renewed focus on groups who require the higher levels of immunity, because of greater exposure levels and other inequities, including Black and Latino residents, said Dr. Manoj Jain, an infectious disease expert at Emory University. “That’s where this info is very useful,” he said. 

The models also suggest a vaccination strategy: Rather than uniformly vaccinate all groups, governments could identify and immunize those most likely to be exposed in “superspreader” events.
 
“Getting those people vaccinated first can lead to the greatest benefit,” said Dr. Michael Mina, an immunologist at Harvard University. “That alone could lead to herd immunity.”   
Vaccination schemes for other pathogens have successfully exploited this approach. For example, when children were given the pneumococcal vaccine in the early 2000s, rates of bacterial pneumonia in the elderly rapidly dropped because of a “herd effect.”…  

Back in Brooklyn, fewer than 1 percent of people tested at neighborhood clinics over the past eight weeks were infected with the virus. But there are still handfuls of cases.”…
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“A version of this article appears in print on Aug. 18, 2020, Section D, Page 8 of the New York edition with the headline: Herd Immunity Is Hard to Decipher.”
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Added: “A disastrously flawed Coronavirus model changed government policy overnight and shut down the world for months:”

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