WASHINGTON (NYTIMES) – In the last week, leading epidemiologists from respected institutions have, through different methods, reached the same conclusion: About 85 per cent to 90 per cent of the American population is still susceptible to Sars-CoV-2, the virus causing the current coronavirus pandemic.

The number is important because it means that “herd immunity” – the point at which a disease stops spreading because nearly everyone in a population has contracted it – is still very far off.

The evidence came from antibody testing and from epidemiological modelling.

At the request of The New York Times, three epidemiological teams last week calculated the percentage of the country that is infected.

What they found runs strongly counter to a theory being promoted in influential circles that the United States has either already achieved herd immunity or is close to doing so, and that the pandemic is all but over. That conclusion would imply that businesses, schools and restaurants could safely reopen, and that masks and other distancing measures could be abandoned.

“The idea that herd immunity will happen at 10 or 20 per cent is just nonsense,” said Dr Christopher J.L. Murray, director of the University of Washington’s Institute for Health Metrics and Evaluation, which produced the epidemic model frequently cited during White House news briefings as the epidemic hit hard in the spring.

That belief began circulating months ago on conservative news programs like those of Rush Limbaugh and Laura Ingraham. It has been cited several times by Dr Scott W. Atlas, President Donald Trump’s new pandemic adviser.

It appears to be behind Trump’s recent remarks that the pandemic is “rounding the corner” and “would go away even without the vaccine”.

But it is also gaining credence on Wall Street and among some business executives, said prominent public health experts, who consider the idea scientifically unfounded as well as dangerous.

Its most vocal adherents are calling for mask-wearing and social distancing to end just as cold weather is shifting social activity indoors, where the risk of transmission is higher.

Even in places where the pandemic hit especially hard – a French aircraft carrier, the Brazilian city of Manaus, the slums of Mumbai, India, and a neighbourhood in Queens, New York – infections did not noticeably slow down until almost 60 per cent of the inhabitants were infected.

And even those levels may not suffice, given that cases are increasing again in Brazil and in Brooklyn areas that had seen cases spike and then drop off.

“Immunity in 2020 is no closer to being just around the corner than prosperity was in 1930,” said Dr Thomas R. Frieden, a former director of the Centres for Disease Control and Prevention.

“The route to immunity without a vaccine would be through graveyards filled with hundreds of thousands of Americans who did not have to die.”

In April, in an opinion piece for The Hill that endorsed letting the virus circulate, Atlas cited the conventional wisdom that herd immunity requires that about 60% of people have antibodies. But last week he said on “The Ingraham Angle” that “most of the immunity for this virus is felt to be due to T-cell immunity” and speculated that such immunity was why children rarely became dangerously ill and why Asian countries did well against the virus.

(Many reputable medical experts think that children, who suffer many colds, may be protected in that regard. But Asian countries contained the virus through early travel bans, lockdowns, masks and rigorous contact tracing.)

The calculations that 85 per cent to 90 per cent of all Americans are vulnerable come from numerous sources.

On Friday, the CDC, citing still-unreleased data from blood samples collected at commercial laboratories across the country this summer, said that less than 10 per cent of samples contained antibodies to the virus.

Also Friday, in a study published in The Lancet, Stanford University scientists examined 28,500 blood samples from dialysis centres in 46 states and found antibodies in just over 9 per cent.

And the epidemic-modelling teams, at the request of the Times, used their models to calculate what percentage of the country is infected. The models were based not on blood sampling but on testing and death data from all 50 states.

Murray’s institute estimated that 29 million Americans, or 9% of the population, have had the virus. The Prevention Policy Modeling Lab at Harvard’s T.H. Chan School of Public Health estimated the figure at 41 million, or 12.5 per cent.

Michael T. Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, said his team believed it was 10 per cent to 12 per cent. The Covid-19 Projections website consulted by Resolve to Save Lives, Frieden’s health advocacy initiative, calculated it at 16 per cent.

Each model “has its warts” because they use slightly different assumptions about fatality rates and test accuracy, said Nicolas A. Menzies, a collaborator at the Harvard modeling lab. But all reach similar conclusions: Although some parts of the country were heavily infected in the spring – more than 20 per cent of residents of the New York metropolitan area are believed to be immune, for example – the average across the country is far lower.

More than 200,000 Americans have already died, and models estimate that if people return to old habits, such as gathering indoors without masks, more than 300,000 and possibly 400,000 could die before a vaccine is widely available.

The chief proponents of the idea that herd immunity is somehow close at hand are American and European medical professionals who oppose lockdowns. They contend that most people in the world are immune to the virus thanks to “T-cell immunity” derived from having contracted common colds that were caused by the four relatively benign coronaviruses that have circulated for years.

But this theory is unfounded. Helper T-cells are white blood cells that, once “primed” by an initial infection, can linger in the tissues for decades until they meet the same virus again and destroy it, by triggering the production of antibodies and by summoning other virus-killers.

(Shane Crotty, a virologist at the La Jolla Institute for Immunology, offered the metaphor of a Lego car. Antibodies recognize the car’s outline and attach themselves, disabling it. But T-cells recognize individual Lego blocks, even internal ones. If the car is already parked inside a human airway cell, for instance, the cell can effectively wave a car part around to attract the attention of primed helper T-cells, which in turn can recruit “killer T-cells” to inject toxic proteins that wipe out both garage and car.)

The immunity conferred by a common cold coronavirus appears to last a year or two, immunologists say, and then a person can catch the same cold again. Antibodies against it fade away; primed T-cells remain.

Primed T-cells may lower the odds of dying from the new, dangerous coronavirus, Crotty said, but that has not been proven. There is no evidence that they protect against becoming infected with it.

During a congressional hearing last week, Sen. Rand Paul, a former eye surgeon, engaged in a heated exchange on the topic with Dr Anthony Fauci, the country’s leading infectious disease expert.

The senator argued that New York’s outbreak slowed because of T-cell immunity. Fauci quickly countered: “If you believe 22% is herd immunity, I believe you’re alone in that.” In a later interview, Fauci said that he “knew of no scientific evidence” that common cold-derived T-cells protect against infection with Sars-CoV-2. Furthermore, he added, any contention that the pandemic was dying out “makes absolutely no sense at all.”

Source: Read Full Article