Fewer than 10 percent of Americans have had coronavirus and developed antibodies that might protect them from reinfection, the first nationally-representative study of its kind found.
Somewhere between 8.2 and 9.4 percent of Americans have antibodies to coronavirus, according to new data on dialysis patients analyzed by Stanford University researchers.
That puts the US far afield of public health experts estimates for the herd immunity threshold - at which point enough people would have some protection against infection that the virus's spread would be very limited - of around 60-70 percent.
The new figures align with Centers for Disease Control and Prevention (CDC) director Dr Robert Redfield's estimate that 90 percent of Americans are still vulnerable to infection.
They also underscore the possibility of a dangerous second wave of coronavirus cases and deaths this winter.
A vaccine could stem the devastation of another wave, but worry is mounting that President Trump will push through the approval before the November 3 election - whether or not it has been sufficiently vetted and proven safe.
The proportion of people with coronavirus antibodies was lowest in states in the West (light green), while about a quarter of Northeasterners have already had COVID-19 (dark green)
The Stanford University analysis included data on more than 28,000 Americans undergoing dialysis for kidney failure across the US.
The scientists chose dialysis patients as their sample population because they routinely go to health clinics, and have their blood regularly drawn any any way. The researchers figured that their demographics and risk factors were fairly representative of the US on the whole.
Nationwide, the prevalence of antibodies came out to an average of 9.3 percent of the US population.
But the proportion of already-infected residents varied widely from state-to-state, region-to-region and among different demographic groups.
Only about five percent of people in the Western US had antibodies, while 25 percent of people tested in the Northeast had the immune proteins to coronavirus.
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