BackgroundEarly in the COVID-19 pandemic, peak viral loads coincided with symptom onset. We hypothesized that in a highly immune population, symptom onset might occur earlier in infection, coinciding with lower viral loads.MethodsWe assessed SARS-CoV-2 and influenza A viral loads relative to symptom duration in recently-tested adults. Symptomatic participants ≥16y presenting to testing sites in Georgia (4/2022– 4/2023; Omicron variant predominant) provided symptom duration. Nasal swab samples were tested by the Xpert Xpress SARS-CoV-2/Flu/RSV assay and Ct values recorded. Nucleoprotein concentrations in SARS-CoV-2 PCR-positive samples were measured by Single Molecule Array. To estimate hypothetical antigen rapid diagnostic test (Ag RDT) sensitivity on each day after symptom onset, percentages of individuals with Ct value ≤30 or ≤25 were calculated.ResultsOf 621 SARS-CoV-2 PCR-positive individuals (64.1% women, median 40.9y), 556/621 (89.5%) had a history of vaccination, natural infection, or both. By both Ct value and antigen concentration measurements, median viral loads rose from the day of symptom onset and peaked on the fourth day. Ag RDT sensitivity estimates were 35.7–71.4% on the first day, 63.9–78.7% on the third day, and 78.6–90.6% on the fourth day of symptoms.In 74 influenza A PCR-positive individuals (55.4% women; median 35.0y), median influenza viral loads peaked on the second day of symptoms.ConclusionsIn a highly immune adult population, median SARS-CoV-2 viral loads peaked on the fourth day of symptoms. Influenza A viral loads peaked soon after symptom onset. These findings have implications for ongoing use of Ag RDTs for COVID-19 and influenza.