By now, the vast majority of people worldwide have had COVID-19 at least once. Blood antibody studies, which are far more reliable than official case counts, show that global infection-induced seroprevalence had already reached about 36% by September 2021, before the highly contagious Omicron variant swept the planet. After Omicron, those numbers climbed dramatically. In the United States, SARS-CoV-2 antibodies were detected in over 95% of adults and 90% of children by the end of 2023.
Why Official Case Counts Tell Only Part of the Story
Throughout the pandemic, the number of confirmed COVID-19 cases significantly underestimated total infections. Many people never got tested, especially those with mild or no symptoms. Home rapid tests, which became widely available in 2022, were rarely reported to public health authorities. As Our World in Data has noted, the actual number of infections “is not known, not by us, nor by any other research, governmental, or reporting institution.”
This is why researchers turned to seroprevalence studies: large-scale blood surveys that look for antibodies produced by actual infection (as opposed to vaccination). These studies consistently revealed infection rates several times higher than confirmed case counts suggested. By February 2022, for example, CDC blood surveys found that 57.7% of the U.S. population already had infection-induced antibodies, a figure far exceeding the official tally at the time.
The Omicron Wave Changed Everything
Before Omicron arrived in late 2021, roughly one in three people globally had been infected. The variant’s extreme transmissibility then nearly doubled infection rates in a matter of months. CDC data from U.S. blood donors captured this surge in real time: infection-induced seroprevalence jumped from 33.5% in December 2021 to 57.7% by February 2022. In a single winter wave, about a quarter of the American population caught the virus for the first time.
University-based serosurveys told a similar story. At one large public university, antibodies indicating prior infection nearly doubled from 19.7% in September 2021 to 39.1% by March 2022. Notably, there was also a twofold increase in infection-detected antibodies among people who reported never having had COVID, suggesting many infections went entirely unnoticed.
Children Were Infected at Even Higher Rates
Children and teenagers caught the virus at rates that surprised many parents. By February 2022, approximately 75% of children aged 0 to 11 and 74% of those aged 12 to 17 had blood evidence of a prior COVID infection. That was substantially higher than the overall U.S. population rate of 57.7% at the same time point. One likely explanation: children were less likely to be vaccinated early in the pandemic, and school and daycare settings created ample opportunity for transmission. About one third of children who tested positive for infection antibodies had become newly infected just since December 2021.
Reinfections Became Increasingly Common
Having COVID once did not mean you were done with it. Between September 2021 and December 2022, reinfections accounted for 12.7% of all reported COVID cases among U.S. adults across 18 jurisdictions. That share rose sharply as the virus evolved: reinfections made up just 2.7% of cases during the Delta period in late 2021 but climbed to 28.8% during the BQ.1 subvariant period roughly a year later.
Among people who were reinfected, the vast majority (95.6%) experienced one reinfection. About 4.3% had two reinfections, and a small fraction (0.2%) had three or more. These numbers almost certainly undercount true reinfections, since they only captured lab-confirmed positive tests collected more than 90 days apart. Anyone who used a home test or skipped testing altogether would not appear in the data.
Global Variations in Infection Rates
Infection rates varied considerably by region, shaped by differences in population density, public health measures, and testing capacity. A major meta-analysis published in PLOS Medicine found that by September 2021, global seroprevalence from both infection and vaccination combined had reached 59.2%, while seroprevalence from infection alone was about 35.9%. The wide prediction interval on that figure, ranging from roughly 23% to 51%, reflects just how different the pandemic looked from one country to the next.
Africa illustrates this gap between official numbers and reality particularly well. Confirmed case counts on the continent were low compared to Europe and the Americas, but seroprevalence studies repeatedly showed infection rates many times higher than reported cases suggested. Limited testing infrastructure meant the vast majority of infections simply went unrecorded. Within Africa itself, distribution was uneven: the Southern region accounted for 43% of cumulative reported cases through the end of 2020, while Central Africa reported just 3%.
Where Things Stand Now
By the end of 2023, near-universal exposure had become the norm in many countries. In the United States, over 95% of adults carried detectable SARS-CoV-2 antibodies from either infection, vaccination, or both. The distinction between “have had COVID” and “have not” has largely disappeared at the population level. Most people have been exposed to the virus multiple times through some combination of infection and vaccination, building layered immune responses that reduce the severity of subsequent encounters. For all practical purposes, a previously unexposed adult in a high-income country is now the rare exception rather than the rule.