| Summary: | <h4>Objectives</h4> Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine effectiveness in coronavirus disease (COVID-19) patients with breakthrough infections has not been established in South Korea. To address this, we assessed the impact of vaccination on symptom occurrence and viral load. <h4>Methods</h4> We performed a retrospective cohort study of 9,030 COVID-19 patients enrolled between February and November 2021. The impact of vaccination on the incidence of symptoms and viral load as indicated by cycle threshold (Ct) values of RdRp and E genes was evaluated using relative risks (RRs) and 95% confidence intervals (95% CIs). <h4>Results</h4> Compared with unvaccinated patients, fully vaccinated patients were associated with a reduced symptom onset of cough, sputum, and myalgia in COVID-19 patients (RR (95% CI) = 0.86 (0.75–0.99) for cough; RR (95% CI) = 0.74 (0.56–0.98) for sputum; RR (95% CI) = 0.65 (0.53–0.79) for myalgia, respectively). Additionally, lower risk of high viral load, Ct value of RdRp gene <15 or Ct value of E gene <15, was observed especially in fully vaccinated patients younger than 40 years ((RR (95% CI) = 0.69 (0.49–0.96) for RdRp gene; (RR (95% CI) = 0.71 (0.53–0.95) for E gene). <h4>Conclusion</h4> SARS-CoV-2 vaccination was associated with a reduced risk of COVID-19 symptoms as well as decreased viral load, especially in patients younger than 40 years.
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