The effect of COVID-19 vaccination on viral load among health care workers: a retrospective cohort study

The current novel coronavirus disease pandemic was declared by the World Health Organization in 2020. Considering the difference in the infection and mortality rates in healthcare workers in Iran compared to other countries, as well as the diversity of vaccines used and their effectiveness, this study was conducted to investigate the effect of coronavirus disease 2019 (COVID-19) vaccination on the viral loads and symptoms of healthcare workers.

In this retrospective cohort study, viral loads of first and breakthrough COVID-19 infections of Imam Reza and Mohammad Kermanshahi Hospital personnel were measured before and after full dose COVID-19 vaccination, through measuring delta cycle threshold (ΔCt), an inverse measure of viral load, from 2020 to 2021 in Kermanshah, Iran.

Polymerase chain reaction test results of 110 cases with an average age of 35.15±8.04 were compared. Female participants were 71 (64.5%), and the rest were male. Breakthrough viral loads were significantly lower than first infection (mean ΔCt, 22.04±4.59 vs. 26.61±3, p<0.001). Cough, fever, chills, and myalgia frequencies were significantly decreased in breakthrough infections (p<0.05), except for dyspnea. Gastrointestinal symptoms remained the same, and anosmia frequency increased in breakthrough infections. Viral loads based on vaccine type and clinical symptoms were significantly reduced (p<0.05).

Based on our findings, 2 doses of COVID-19 vaccination could be effective in reducing the viral loads and frequency of clinical symptoms in Iranian healthcare workers.