Vaccine effectiveness in symptom and viral load mitigation in COVID-19 breakthrough infections in South Korea.

Objectives: 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.

Methods: 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).

Results: 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).

Conclusion: 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.