SARS-CoV-2 Reinfections in 18 U.S. Jurisdictions, 2021–2022

By September 2021, an estimated 150 million SARS-CoV-2 infections had occurred in the United States — a cumulative incidence of 44%. Reinfections were expected to grow as more people had a first infection, as immunity from infection and vaccination waned, and as new, more transmissible variants able to evade immunity appeared. Reinfections were generally reported to be milder than first infections, but some studies linked them to severe outcomes, especially in people hospitalized the first time. U.S. trends in reinfections and their severe outcomes had not yet been characterized; this MMWR report set out to describe them.

How it was done

Eighteen U.S. jurisdictions reported weekly counts of COVID-19 cases, hospitalizations and deaths among adults from September 5, 2021, to December 31, 2022, split into all infections and reinfections. A reinfection was a positive RNA or antigen test on a respiratory specimen more than 90 days after a previous confirmed or probable case, per national surveillance guidance; a person could be counted more than once. Results were broken down by age and by five periods in which one variant accounted for at least half of cases: Delta, and Omicron BA.1, BA.2, BA.4/BA.5 and BQ.1/BQ.1.1.

Reinfections among cases

Over the period, 2,784,548 reinfections were reported among adults — 12.7% of all 21,943,686 infections. Adults aged 18–49, who make up 54% of the population, accounted for 66.8% of reinfections and 62.0% of all infections; adults 50–64 accounted for 21.2% of reinfections and those 65 and older for 11.9%.

The rise was steepest among adults 18–49, from 3.0% of their infections in the Delta period to 34.4% in the BQ.1/BQ.1.1 period; among those 50–64 it went from 2.1% to 29.0%.

Reinfections as a share of COVID-19 cases and hospitalizations by week, age group and variant period, 18 jurisdictions. CDC figure.

Hospitalizations and deaths

Ten jurisdictions reported 43,432 hospitalizations and 17 reported 6,888 deaths associated with reinfection. The pattern matched that for cases but was smaller: reinfections rose from 1.9% of COVID-19 hospitalizations and 1.2% of deaths in the Delta period to 17.0% and 12.3% in the BQ.1/BQ.1.1 period. They made up a larger share among younger adults — 24.8% of hospitalizations and 20.2% of deaths among those 18–49 in the BQ.1/BQ.1.1 period, compared with 13.3% and 11.6% among those 65 and older.

Time between infections

In 17 jurisdictions, the weekly median gap between infections rose from 269 days in September 2021 to a peak of 411 days in mid-February 2022, near the end of the BA.1 period, fell sharply to 335 days in mid-June 2022 as BA.4/BA.5 took over, and reached 367 days by the last week of 2022. In September 2021, 90.5% of reinfected people had first been infected when the original strain predominated and 9.5% during the Alpha period. The June drop came when those first infected with the original strain fell below half of reinfections. By the end of 2022, 51.3% of reinfected people had first been infected earlier in the Omicron era (BA.1, 37.6%; BA.2, 7.0%; BA.4/BA.5, 6.6%), and the rest during the original-strain (31.7%), Delta (15.0%) or Alpha (2.0%) periods.

Median time to reinfection and the variant period of previous infection, 17 jurisdictions. CDC figure.

What it means

Reinfections and their severe outcomes grew as more transmissible, immune-evading Omicron lineages emerged and first infections accumulated, reflecting an increasingly complex immune landscape. Younger adults' higher shares may stem from more first infections, later vaccine eligibility and lower coverage, more exposure, and less severe first infections. Reinfections were a smaller share of hospitalizations and deaths than of cases, consistent with evidence that immunity from past infection protects better against severe illness than against infection. Vaccination can reduce the risk of severe outcomes from reinfection, though this study did not measure vaccine effectiveness. CDC recommended staying up to date with COVID-19 vaccines and getting early antiviral treatment when eligible, and continued monitoring as some data sources changed or ended when the public health emergency expired on May 11, 2023.

Limitations: home testing increasingly hid cases in 2022, and reinfections were missed when infections were not lab-confirmed or could not be linked; reinfection trends before September 2021 could not be measured; reinfections within 90 days were excluded; only some jurisdictions reported severe outcomes, with varying definitions; and the analysis could not adjust for changes in immunity, behavior and the population at risk.

Sources

• Ma KC, Dorabawila V, León TM, et al. "Trends in Laboratory-Confirmed SARS-CoV-2 Reinfections and Associated Hospitalizations and Deaths Among Adults Aged ≥18 Years — 18 U.S. Jurisdictions, September 2021–December 2022." MMWR 2023;72(25).