Anti-SARS-CoV-2 Antibody Levels Associated with COVID-19 Protection in Outpatients Tested for SARS-CoV-2, US Flu VE Network, October 2021-June 2022.

Background: We assessed the association between antibody concentration ≤5 days of symptom onset and COVID-19 illness among patients enrolled in a test-negative study.
Methods: From October 2021-June 2022, study sites in seven states enrolled and tested respiratory specimens from patients of all ages presenting with acute respiratory illness for SARS-CoV-2 infection using rRT-PCR. In blood specimens, we measured concentration of anti-SARS-CoV-2 antibodies against the ancestral strain spike protein receptor binding domain (RBD) and nucleocapsid (N) antigens in standardized binding antibody units (BAU/mL). Percent reduction in odds of symptomatic COVID-19 by anti-RBD antibody was estimated using logistic regression modeled as (1-adjusted odds ratio of COVID-19)×100, adjusting for COVID-19 vaccination status, age, site, and high-risk exposure.
Results: A total of 662 (33%) of 2,018 symptomatic patients tested positive for acute SARS-CoV-2 infection. During the Omicron-predominant period, geometric mean anti-RBD binding antibody concentrations measured 823 BAU/mL (95%CI:690-981) among COVID-19 case-patients versus 1,189 BAU/mL (95%CI:1,050-1,347) among SARS-CoV-2 test-negative patients. In the adjusted logistic regression, increasing levels of anti-RBD antibodies were associated with reduced odds of COVID-19 for both Delta and Omicron infections.
Conclusion: Higher anti-RBD antibodies in patients were associated with protection against symptomatic COVID-19 during emergence of SARS-CoV-2 Delta and Omicron variants.
Competing Interests: CONFLICTS OF INTEREST Dr. Zimmerman reports grants from CDC, during the conduct of the study, and grants from Sanofi Pasteur, outside the submitted work. Dr. Grijalva reports other from CDC, grants from NIH, other from FDA, grants and other from AHRQ, other from Merck, and other from Syneos Health, outside the submitted work. Dr. Talbot reports grants from CDC, during the conduct of the study. All other authors report not conflicts of interest.