Only once the share of immune people crosses a certain threshold — for simulitis it is about 80 percent — are most of the healthy people protected from the sick people.

Scientists are still determining the herd immunity threshold of SARS-CoV-2, the coronavirus that produces the deadly disease covid-19.

Yet even if coronavirus antibodies prevent repeat infections — an assumption still being studied and debated — evidence suggests the United States remains far from achieving herd immunity.

One way to measure the gap between herd immunity and reality is through seroprevalence surveys, in which researchers draw blood from thousands of people and check to see how many of them have coronavirus antibodies.

Because people infected with the coronavirus often fail to show symptoms, these surveys can reveal more people with antibodies than case numbers alone would suggest.

Compare those results to a 60 percent herd immunity threshold, the midrange of current estimates.

New York City, by far the closest to achieving herd immunity, was somewhere between one-half and one-third of the way there.

The CDC data is based on blood tests conducted months ago, so it is likely that the country has moved closer to reaching the herd immunity threshold.

“There’s just way too little seroprevalence in all of these states to come anywhere close to achieving herd immunity," said Marcus Russi, a Yale epidemiologist who helped build the model.

Even the higher range of the estimates, which suggest seroprevalence in the United States could be as high as 25 percent, remain far from the herd immunity threshold.

To estimate how much worse, just three numbers are needed: the total population, the coronavirus’s herd immunity threshold and its fatality rate — the percentage of infected people who die.