Trump Allies Say the Virus Has Almost Run Its Course. ‘Nonsense,’ Experts Say.

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 percent 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, Dr. Crotty said, but that has not been proven. There is no evidence that they protect against becoming infected with it.

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