Impact of Previous Infection or Vaccination on Clinical Symptoms and Viral Load of Acute SARS-CoV-2 Infection

Background: SARS-CoV-2 vaccination or previous infection can induce protective immunity. This study aims to evaluate the impact on clinical symptoms and viral load during a subsequent infection. Methods: Adults with symptoms of SARS-CoV-2 infection were prospectively enrolled into a cohort study between December 2021 and May 2023 (Omicron variant). Nasal swabs were collected for quantitative reverse transcription polymerase chain reaction (RT-qPCR) testing. Descriptive statistics, t-tests, linear regression, logistic regression, and ANOVA were used to investigate the associations between symptoms and vaccination or infection histories. Results: Among 1,140 participants tested, 352 (30.9%) participants tested positive by RT-qPCR for SARS-CoV-2 infection. Among positives, mean (SD) age was 42.3 (15.2) years, 182 (51.7%) of them were female, and mean (SD) number of symptoms was 6.8 (2.4). 89.5% were vaccinated and 16.2% reported having had a prior infection. Prior infection was associated with higher relative risk of shortness of breath, smell loss, nasal congestion, and vomiting during the current reinfection. 2 doses of vaccine was associated with lower risk of nausea and diarrhea, and 3 doses of vaccine was associated with lower risk of chills, fever and vomiting. A higher cycle threshold (CT) value on a positive test was associated with fewer reported symptoms (15.6 increase in CT value per 1 fewer reported symptoms), even after adjustment for time since last vaccination. CT value was approximately 0.1 units lower for each month duration since last vaccination. No statistically significant difference in CT value was observed across different vaccination or infection histories. Conclusions: Vaccine dosage and recency were associated with fewer symptoms among people testing positive for SARS-CoV-2. Vaccine dosage was further associated with fewer presentation of certain symptoms, while prior infection was associated with more presentation of certain symptoms. Prior infection or vaccination mildly impact viral load (as measured by test CT value) only when certain symptoms present. SARS-CoV-2 vaccination and infection may prime the immune system and thus modify symptoms of COVID-19.