Nearly six in ten Americans had evidence of a prior SARS-CoV-2 infection by February 2022, according to a new Centers for Disease Control and Prevention analysis that shows how dramatically the Omicron wave expanded the country’s cumulative exposure to the virus. The estimate rose from 33.5% in December to 57.7% in February, and among children and adolescents it reached roughly three-quarters of the population.

The CDC’s MMWR report, released online April 26, uses antibodies against the virus’s nucleocapsid protein to identify prior infection. Those antibodies are not produced by the vaccines used in the United States, allowing researchers to distinguish infection-induced antibodies from protection generated solely by vaccination. The survey therefore captures many infections that never appeared in routine case counts because they were asymptomatic, untested or never reported.

Omicron sharply increased cumulative infection

Seroprevalence changed little from September through November 2021, then rose rapidly after Omicron became dominant in December. By February, the estimated prevalence of infection-induced antibodies was 75.2% among children ages 0–11, 74.2% among adolescents ages 12–17, 63.7% among adults ages 18–49, 49.8% among adults ages 50–64 and 33.2% among people 65 and older.

The age gradient is notable. Younger Americans, who were less likely to have been vaccinated early in the campaign and have extensive contact through schools and households, showed the highest cumulative infection. Older adults had the lowest measured infection-induced seroprevalence, consistent with higher vaccination coverage and more sustained efforts to avoid exposure because of their greater risk of severe disease.

The findings complement the CDC’s commercial laboratory antibody surveillance, which draws on blood specimens submitted for routine clinical testing across the country. The approach cannot identify the exact date of infection or determine whether antibodies remain high enough to prevent reinfection, but it provides a population-level view that ordinary case surveillance cannot.

Confirmed cases captured only part of the wave

Reported infections reached record levels during the winter, as shown in the agency’s case-trend data, but laboratory-confirmed case counts necessarily miss infections among people who never test or who use home tests that are not reported. The serology results suggest that the true number of infections during Omicron was substantially larger than official case totals.

A contemporaneous preprint analysis examining U.S. infection and vaccination histories similarly estimates that a large share of the population entered 2022 with some combination of vaccine-derived and infection-derived immune exposure. The exact protective effect varies by age, time since exposure, variant and vaccination status, so antibody prevalence should not be treated as a measure of complete immunity.

The CDC report explicitly cautions against interpreting a positive antibody result as evidence that a person no longer needs vaccination. The FDA’s vaccine guidance continues to support authorized and approved COVID-19 vaccination because vaccines reduce the risk of severe disease, hospitalization and death even when prior infection has occurred.

Children show the largest increase

The rise among children is especially significant because vaccination eligibility came later for younger age groups. Children ages 5–11 did not become eligible for Pfizer-BioNTech vaccination until November 2021, and those under 5 remain ineligible. By February, three-quarters of children under 12 had infection-induced antibodies, indicating that exposure spread widely through an age group with limited vaccine coverage during much of the Omicron wave.

That high infection prevalence does not mean pediatric COVID-19 is uniformly benign. A separate CDC hospitalization analysis found that weekly COVID-19-associated hospitalization rates among children ages 5–11 reached a pandemic high during Omicron circulation, with unvaccinated children hospitalized at substantially higher rates than vaccinated children. Most infected children do not develop severe illness, but the enormous number of infections means even a small percentage of serious cases creates a meaningful clinical burden.

Population denominators for the antibody estimates rely in part on demographic benchmarks such as the Census Bureau’s American Community Survey. CDC researchers weighted laboratory specimens by age, sex and metropolitan status to make the sample better reflect the U.S. population, while acknowledging that people who obtain clinical laboratory testing may differ from the general population in ways that cannot be fully corrected.

Prior infection changes the immunity landscape but not the need for protection

Evidence continues to show that previous infection can provide meaningful protection against severe outcomes, although that protection is neither uniform nor permanent. A Lancet systematic review of prior infection found substantial protection against reinfection and severe disease before Omicron, while also documenting waning over time and variation by variant. Omicron’s ability to infect people with prior immunity demonstrates why cumulative exposure does not translate into a simple endpoint for the pandemic.

CDC surveillance has also found lower hospitalization risk among many people with prior infection compared with those without documented prior exposure. An April MMWR analysis of hospitalized adults examined how vaccination and previous infection interacted during Delta and Omicron periods, underscoring that immunity is increasingly a mixture of vaccination, infection and both.

The new 57.7% estimate therefore changes the baseline from which public-health officials must evaluate future waves. By February, a majority of Americans had evidence of infection, and an even larger share had either infection-induced or vaccine-induced immune exposure. That should reduce the pool of people with no prior immune encounter with SARS-CoV-2, but it does not eliminate the possibility of reinfection, severe disease or new variants.

The most immediate implication is that confirmed case counts alone no longer describe the full scale of transmission. Omicron infected tens of millions of Americans beyond those captured in official statistics. The United States is entering the next phase of the pandemic with much broader immune experience than it had only months ago, but also with large differences by age, vaccination status, health condition and time since the last infection or dose.