More than 140 million Americans have had the coronavirus, according to estimates from blood tests that reveal antibodies from infection — about double the rate regularly cited by national case counts.
The data goes through late January, when the omicron variant of the coronavirus was causing more than 500,000 cases a day, meaning the number of Americans now infected is considerably higher. The data comes from 72,000 blood samples taken in January.
The highly infectious variant has caused case counts to skyrocket. As of late November, just before the omicron variant began spreading in the United States, the blood test study estimated that 103 million people had been infected. By that measure, 37 million new people caught the virus over two months ending in late January.
Every two weeks, the CDC gathers tens of thousands of blood tests analyzed by commercial labs nationwide for reasons unrelated to the coronavirus, such as checkups or other medical treatment. Those samples are also tested for coronavirus antibodies. The percentage of people with antibodies is known as seroprevalence.
The blood test study includes infections throughout the pandemic but counts each person only once. Daily coronavirus case rates tally every known infection, so many people who have had reinfections are counted again and again. The estimated 140 million is well over double the number of people included in counts by The Washington Post or government agencies as of late January.
The blood tests count only antibodies from natural infection, including asymptomatic cases, not from vaccination. The study measures the presence of antibodies. It does not indicate whether there is strong protection against subsequent infection.
Infection rates are much higher for children and younger adults, the study found. It estimated that 58 percent of children up to age 11 have antibodies from natural infection, along with the same share of children age 12 to 17.
Just under half of adults up to 49 have been infected, the CDC estimates. The rate drops to 37 percent for people 50 to 64 and 23 percent of people 65 or older.
The coronavirus is much deadlier for older people. Half of the people who have died of covid-19 have been age 75 or older. Vaccination and other preventive measures have been concentrated on the most vulnerable older people. Younger people have much lower vaccination rates and are much more likely to be working in jobs or attending school, where they risk exposure.
At least half of the population in 14 states has been infected, blood tests showed, from 56 percent in Wisconsin to half in Georgia. The other states with a majority infection rate, in descending order, are Iowa, Illinois, Ohio, Wyoming, Texas, Indiana, Mississippi, Nebraska, Michigan, New Jersey, Tennessee and Louisiana. New York is just under that level, at 49 percent.
The lowest infection rate is in Vermont, with 18 percent. The next-lowest states in ascending order are Hawaii, Maine, Oregon, Washington, New Hampshire, Virginia and Massachusetts, which tops the lowest states with 37 percent.
Nevada was not included in the study, and there was insufficient data for North Dakota, Arizona and Utah.
Initially, tests for seroprevalence that included both natural infection and vaccination could show whether the country was reaching a level of herd immunity to control spread of the disease. But protection from infection or vaccination is strongest for about six months, the CDC has found, after which the risk of infection grows again. The delta and omicron variants — spaced about six months apart — both infected people who previously had the virus.
Just before the omicron variant, a seroprevalence study that included both vaccination and natural infection found antibodies in more than 90 percent of adults, but that did not prevent omicron’s enormous infection rates, hospitalization and death, said Kristen Nordlund, a CDC spokeswoman.
Vaccination or exposure to the coronavirus may have reduced the deadliness and severity of the omicron wave, Nordlund said.
Scientists still can’t explain why some people get the virus and others do not. A CDC study of disease spread within households during the omicron wave found that 53 percent of people in a household with an infected person also caught the virus. Vaccination, mask-wearing and isolation within the household all reduced transmission and protected other members of the household. But even in households without isolation, vaccination or mask-wearing, not everyone caught it.
Another study of seroprevalence that examined a community after a coronavirus wave also found that it picked some people and skipped others, said William Powderly, head of the Institute for Public Health at Washington University of St. Louis.
The wave dissipated even though a vast majority of people tested showed no antibodies, Powderly said, so they were susceptible to the disease raging around them but didn’t catch it.
“Shouldn’t it just cut through everybody and find everybody who is vulnerable?” he wondered. “The transmission dynamics of this virus are not completely worked out. … There is variability in the virus and variability in the population.”
As to what will happen next, he said, scientists have to acknowledge “the necessary humility. I just think we don’t know.”