COVID-19 swept across the world far more effectively than previously thought, with a stunning number of cases left unreported as recently as the summer of 2021, a new World Health Organization (WHO) study says.
About 3 out of every 5 human beings carried antibodies against COVID-19 in their bloodstream as of September 2021, according to a review of millions of virus blood tests.
That translates to 10.5 actual COVID infections for each case reported between June and September of 2021, researchers estimate.
In other words, public health officials found out about only 9.5% of all COVID cases during that period, the study says.
"Particularly in the beginning as COVID struck, there was a great reluctance for people to attribute illnesses to COVID," said Dr. William Schaffner, medical director of the Bethesda, Md.-based National Foundation for Infectious Diseases. "There was a great sense at the time that the reported cases were exaggerated. Actually, they were vastly underreported."
The global scale of COVID-19 infections has remained a mystery, researchers noted.
More than 1 million are dead from COVID in the United States and more than 6.5 million worldwide, but no one can say exactly how many people have been infected by the novel coronavirus.
"This virus can produce a lot of infections without symptoms or mild symptoms," said Schaffner, who was not part of the study.
"Indeed, there are many cases of serious illness in parts of the world where testing is simply not readily available, and so you underestimate the number of cases, and you underestimate the capacity of this virus to spread rapidly and comprehensively throughout a population," he added.
Blood antibody tests are considered the gold standard for measuring a population's exposure to a virus, either through past infection or through vaccination.
So WHO researchers set out to paint a more accurate picture of COVID's infectiousness, based on data from 965 different blood antibody studies conducted around the world between January 2020 and May 2022. The studies involved more than 5.3 million people tested.
Blood tests show that reported cases always lagged actual COVID infections, particularly early in the pandemic, the study found.
Between July and September 2020, there were more than 51 COVID infections for every case reported to health officials, the researchers estimate. Only about 2% of cases were reported.
Underestimates varied between regions of the world, according to the study. Poorer regions were more likely to underreport COVID cases, and more likely to have positive antibody tests due to infection than from vaccination.
In the high-income Americas, nearly 30% of infections were reported in mid-2020, while in the Eastern Mediterranean only 0.5% of cases were reported.
By mid-2021, nearly 56% of cases were being accurately reported in the Americas, but only 0.6% of cases were reported in Africa.
"As we enter the third year of the COVID-19 pandemic, implementation of a global system or network" for extensive viral blood testing "is a crucial next step to monitor the COVID-19 pandemic and contribute to preparedness for other emerging respiratory pathogens," the researchers, led by Isabel Bergeri of WHO, concluded.
The results also show that more than one-third of the world's population remains unexposed to COVID, and therefore, is at risk for future infection, the researchers noted.
The fact that COVID is so widespread, and that Omicron can reinfect even people with antibody protection, means that SARS-CoV-2 has become a part of human life going forward, said Dr. Aaron Glatt, chief of infectious diseases at Mount Sinai South Nassau in Hewlett, N.Y.
"We're reaching a point where people will have to consider that this is going to be something that is around like flu," Glatt said. "Having had prior COVID probably protects you somewhat from more serious illness, especially if you're vaccinated, but the fact that you're immune doesn't mean you won't become ill. It means you won't get more serious illness, and that's the real critical point."
"Omicron can infect people previously infected and people who've been vaccinated. Now, you get mild illness, but our vaccination and our previous immunity does not inhibit very much the spread of this virus," Schaffner said. "So you'll notice that we in public health have stopped talking about herd immunity a long time ago."
The new study was published Nov. 10 in the journal PLOS Medicine.
The U.S. Centers for Disease Control and Prevention has more about COVID-19.
SOURCES: William Schaffner, MD, medical director, National Foundation for Infectious Diseases, Bethesda, Md.; Aaron Glatt, MD, chief, infectious diseases, Mount Sinai South Nassau, Hewlett, N.Y.; PLOS Medicine, Nov. 10, 2022