Jan. 5 (UPI) -- More than 60 million people in the United States may have been infected with the new coronavirus by mid-November, but that does not mean the population is nearing "herd immunity," according to an analysis published Tuesday by JAMA Network Open.
The new estimate means about 14% of the U.S. population had been diagnosed with COVID-19 as of Nov. 15, roughly five times as many as the official case count -- just under 11 million -- reported by the U.S. Centers for Disease Control for that date, the researchers said.
However, even this higher total falls well short of the 60% that has to have had COVID-19 or been vaccinated against it to achieve herd immunity, they said.
"Reported COVID-19 cases underestimate the actual number of ... infected persons," study co-author Frederick J. Angulo told UPI.
"Even with the large continued wave of COVID-19 infections, the U.S. remains a long way from acquiring the level of herd immunity necessary for disrupting ... transmission," said Angulo, lead epidemiologist with the COVID-19 Medical Team at Pfizer Vaccines.
The estimates are based on findings from seroprevalence surveys, in which groups of people are tested for antibodies against the virus and the prevalence is applied to larger populations, according to the CDC.
The apparent discrepancy in case totals between the official count and the seroprevalence estimates is likely due to the fact that 40% of those infected with the coronavirus remain asymptomatic and don't know they have COVID-19.
"Immunity is acquired by infection or immunization," said Angulo, whose employer, Pfizer, has developed one of the two COVID-19 vaccines approved for use in the United States, with biotech firm Moderna producing the other one.
Through the end of December, just over 1% of the U.S. population had been vaccinated, according to data from the CDC.
Herd immunity occurs when a sufficient percentage of a population is immune to an infection, reducing the likelihood of transmission and infection for individuals who lack immunity, according to the Johns Hopkins School of Public Health.
At the current vaccination and infection rate, up to 1 million more people in the United States could die from COVID-19 before herd immunity is achieved, researcher Dr. Laura Nicholson told UPI.
"Assuming we are now close to a 20% infection rate in the U.S., if we could rapidly vaccinate 40% of our population ... we could get [to herd immunity] without so many more deaths," said Nicholson, an associate professor of molecular medicine at Scripps Institute in San Diego who was not part of the JAMA study.
"We must find a way to accelerate vaccination and continue aggressive masking and distancing," she said.
For this study, Angulo and his colleagues analyzed data from five surveys conducted by the CDC in April, May, June, July and August.
Based on the prevalence of COVID-19 antibodies in these surveys, between 38.2 million and 60.8 million people nationally had been infected with the virus through the fall, they estimated.
"Even when using statistical models to try to capture mild and asymptomatic cases, the majority of the U.S. population remains at risk," infectious disease specialist Dr. Ross M. Boyce, who was not part of the JAMA study, told UPI.
"The idea that we would achieve any sort of level of herd immunity in the absence of a vaccine was never a realistic approach, and I am somewhat skeptical that we will achieve the high levels of [vaccine] coverage required to put a definitive end to the pandemic in a timely manner," said Boyce, an assistant professor at the University of North Carolina-Chapel Hill.