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Background: SARS-CoV-2 antibody levels have been proposed as a correlate of protection (CoP) from infection. Yet, large-scale prospective studies of cost-effective scalable antibody measures as predictors of infection under real-world conditions are limited. We examined whether antibody levels measured using high-throughput variant-specific SARS-CoV-2 anti-spike immunoglobulin G (IgG) and ACE2-neutralization assays correlate with cell-based neutralizing antibody (NAb) measurements, and whether they can serve as a reasonable CoP from SARS-CoV-2 infection., Methods: We conducted a large, institutional cohort study between January 2022 and March 2023. Participants (n=2,513) provided dried blood spot (DBS) samples for assessment of anti-spike IgG and ACE2 inhibition levels using high-throughput assays. Comparison with authentic cell-based SARS-CoV-2 NAb assays was conducted using serum samples (n=105). Associations between antibody levels and risk of infection were evaluated., Findings: Correlation between serum and DBS sampling, and cell-based neutralizing and high-throughput antibody binding assays, was high for both anti-spike IgG and ACE2 neutralization, though the degree of correlation varied by variant. Longitudinal evaluation suggested both DBS-based IgG and ACE2 inhibition levels were anticorrelated with infection risk, with higher sensitivity noted for ACE2 inhibition and variant-matched measures. Both IgG and ACE2 inhibition levels decreased over time, with more durable responses observed in participants whose most recent priming event was infection versus vaccination., Interpretation: Findings suggest that variant-specific SARS-CoV-2 antibody levels may be a useful CoP for infection, which has important implications for vaccination recommendations and evaluating infection risk. High-throughput assays measured via DBS may have utility in timing of boosters, either at the population or an individual level., (© The Author(s) 2024. Published by Oxford University Press on behalf of Infectious Diseases Society of America. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

We isolated a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) BA.2 variant from a person with coronavirus disease 2019 recrudescence after nirmatrelvir/ritonavir treatment. Antiviral sensitivity and neutralizing antibody testing were performed with both parental SARS-CoV-2 and multiple variants of concern. We found that neither nirmatrelvir resistance nor absence of neutralizing immunity was a likely cause of the recrudescence., Competing Interests: Potential conflicts of interest. A. F. C. has received contract payments from Nurix Therapeutics and has options in Covicept Therapeutics. D. M. S. has served as a consultant for Bayer Healthcare, Kiadis Pharmaceuticals, and Signant Health, and has equity stake in Vx Biosciences, Model Medicines, Linear Therapies, and FluxErgy. All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed., (© The Author(s) 2022. Published by Oxford University Press on behalf of Infectious Diseases Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Surveillance for acute respiratory infection (ARI) and influenza-like illness (ILI) relies primarily on reports of medically attended illness. Community surveillance could mitigate delays in reporting, allow for timely collection of respiratory tract samples, and characterize cases of non–medically attended ILI representing substantial personal and economic burden. Text messaging could be utilized to perform longitudinal ILI surveillance in a community-based sample but has not been assessed. We recruited 161 households (789 people) in New York City for a study of mobile ARI/ILI surveillance, and selected reporters received text messages twice weekly inquiring whether anyone in the household was ill. Home visits were conducted to obtain nasal swabs from persons with ARI/ILI. Participants were primarily female, Latino, and publicly insured. During the 44-week period from December 2012 through September 2013, 11,282 text messages were sent. In responses to 8,250 (73.1%) messages, a household reported either that someone was ill or no one was ill; 88.9% of responses were received within 4 hours. Swabs were obtained for 361 of 363 reported ARI/ILI episodes. The median time from symptom onset to nasal swab was 2 days; 65.4% of samples were positive for a respiratory pathogen by reverse-transcriptase polymerase chain reaction. In summary, text messaging promoted rapid ARI/ILI reporting and specimen collection and could represent a promising approach to timely, community-based surveillance.