We knew from the beginning how the end would come. Eventually, the coronavirus will not be able to find enough sensitive hosts to survive, disappearing from where it briefly appears.
To achieve so-called herd immunity – the point at which the virus can no longer spread because there are not enough vulnerable people – scientists have suggested that perhaps 70 percent of a given population must be immune, either by vaccination or by surviving the infection .
Now some researchers are struggling with a hopeful possibility. In interviews with The New York Times, more than a dozen scientists said the threshold is likely to be much lower: just 50 percent, maybe even less. If this is true, then it may be possible to recover the coronavirus faster than once thought.
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The new estimates result from complicated statistical modeling of the pandemic, and the models have all taken divergent approaches, yielding inconsistent estimates. It is not certain that every community in the world now has enough inhabitants now immune to the virus to counter a second wave.
But in parts of New York, London and Mumbai, for example, it is unlikely that there is already substantial immunity to the coronavirus, scientists said.
“I am very willing to believe that there are pockets in New York City and London that have substantial immunity,” said Bill Hanage, an epidemiologist at the Harvard TH Chan School of Public Health. “What happens this winter will reflect that.”
“The question of what it means for the population as a whole, however, is very treacherous,” he added.
Herd immunity is calculated based on the so-called reproductive number of the epidemic, R0, an indicator of how many people each infected person spreads the virus.
The initial calculations for the threshold immunity threshold assumed that each member of the community had the same susceptibility to the virus and randomly mixed with everyone else in the community.
“That does not happen in real life,” said Dr. Saad Omer, director of the Yale Institute for Global Health. “Re-immunity can vary from group to group, and subpopulation to subpopulation,” and even by zip codes, he said.
For example, a neighborhood of older people may not have much contact with others, but will soon follow the virus if they encounter it, while teens may attach the virus to dozens of contacts and still keep themselves healthy. The virus moves slowly in suburbs and rural areas, where people live far apart, but spreads through cities and households crowded with people.
Once such real variations in density and demographics are accounted for, the estimates for herd immunity fall. Some researchers even suggested that the figure may be in the range of 10 percent to 20 percent, but they were in the minority.
Assuming the viruses are the most outgoing and susceptible in the first wave, immunity to a wave of infection is spread more efficiently than with a vaccination campaign that seeks to protect everyone, said Tom Britton, a mathematician at the University of Stockholm.
His model sets the threshold for immunity against herds at 43 percent – that is, the virus can not hang in a community after that percentage of residents has been infected and recovered.
Still, that means many community residents are sick or dead, a high price to pay for herd immunity. And experts like Mr. Hanage have warned that even a community that may have achieved immunity to herds may not be self-sufficient.
The virus can still spread here and there, even if its general spread is tuned. It is also unclear how long someone who has recovered may be immune, and for how long.
Virus-resistant communities?
The coronavirus crashed this year’s Purim celebrations in New York City’s Orthodox Jewish neighborhoods, tearing through the parades and masquerades in Brooklyn on March 9 and 10.
Schools and synagogues soon closed to stop the spread, but it was too late. By April, thousands in Brooklyn communities were infected, and hundreds had died.
“It’s like a black hole in my memory because of how traumatic it was,” said Blimi Marcus, a nurse living in Borough Park who was hit hard by the virus.
But everything has changed now, Ms Marcus added: “The general feeling is one of rumor, that we somehow had it all and we are safe.”
Is it possible that some of these communities have herd immunity? In some clinics, up to 80 percent of the people tested had antibodies to the virus. The highest prevalence was found among teenage boys.
But people in clinics often show symptoms and are therefore more likely to be infected, said Wan Yang, an epidemiologist at Columbia University’s Mailman School of Public Health in New York. Randomized household surveys would likely find lower rates – but still well above the 21 percent reported average for New York City, she said.
Investigators in Mumbai conducted just such a random household survey, knocked on every fourth door – like, if it was locked up, the fifth – and took blood for anti-antibody testing. They found a startling difference between the poorest neighborhoods of the city and their more affluent enclaves. Between 51 percent and 58 percent of residents in poor areas had antibodies, against 11 percent to 17 percent elsewhere in the city.
The lowest-income residents are tightly packed together, share toilets and do not have much access to masks. “These factors contributed to a spread of silent infection,” said Dr Jayanthi Shastri, a microbiologist at Kasturba Hospital in Mumbai who led the work.
Most researchers are afraid to conclude that the hardest-hit neighborhoods of Brooklyn, or even those in affected areas of Mumbai, have reached herd immunity or will be spared future outbreaks.
But models like Mr. Britton indicate that it is not impossible. Other researchers have controversially suggested that herd immunity can be achieved at rates of immunity as low as 10 percent to 20 percent – and that whole countries may have already achieved that goal.
“We are nowhere near normal in our daily behavior,” said Virginia Pitzer, a mathematical epidemiologist at the Yale School of Public Health. “To think that we can just stop doing it and go back to normal and not see a rise in cases that I think is wrong is wrong.”
A second wave might also hit groups like weeks that were spared by the first, and still devastated, she said. Immunity, for example, is a patchwork quilt: antibodies were present in 68 percent of people who visited a clinic in the Corona district of Queens, for example, but in only 13 percent of those tested in a clinic in Cobble Hill section of Brooklyn.
But another group, led by mathematician Gabriela Gomes of the University of Strathclyde in Britain, took responsibility 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 per cent to 20 cents. per cent.
“At least in countries where we have applied it, we could never get a signal that threshold immunity thresholds are higher,” Gomes said. “I like to have this horizon that it could only be a pandemic for a few months.”
Other experts cautioned, saying that these models are flawed, as are all models, and that they simplify the circumstances on the ground.
Jeffrey Shaman, an epidemiologist at Columbia University, said it was not clear to him that Ms Gomes’ model offered only one possible solution. And he was suspicious of the great reach between the four countries.
“I think we would be playing with fire if we suggested we were done with this,” Mr Shaman said.
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