Overall decrease of SARS-CoV-2 viral load and reduction of clinical burden: the experience of a Northern Italy hospital

Piubelli, Chiara; Deiana, Michela; Pomari, Elena; Silva, Ronaldo; Bisoffi, Zeno; Formenti, Fabio; Perandin, Francesca; Gobbi, Federico; Buonfrate, Dora

OBJECTIVES: In Italy the burden of patients with Coronavirus Disease 2019 (Covid-19) gradually decreased from March to the end of May. In this work, we aimed at evaluating a possible association between the severity of clinical manifestations and viral load over time, during the epidemiological transition from high to low transmission setting. METHODS: We reviewed the cases of Covid-19 diagnosed at the Emergency Room of our hospital, retrieving the proportion of patients admitted to the intensive care unit. A raw estimation of the viral load was done evaluating the Ct (cycle threshold) trend obtained from our diagnostic Reverse Transcriptase real-time PCR test. RESULTS: The proportion of patients requiring intensive care significantly reduced from 6.7% (19/281) in March, to 1.1% (1/86) in April, and to none in May (Fisher’s test p-value=0.0067). As for viral load, we observed a trend of Ct increasing from a median value of 24 (IQR 19-29) to 34 (IQR 29-37) between March and May, with a statistically significant difference between March and April (Pairwise Wilcoxon test with Stepdown Bonferroni Adjustment for multiple testing, p=0.0003). CONCLUSIONS: We observed a reduction over time of the proportion of patients with Covid-19 requiring intensive care, along with decreasing median values of viral load. As the epidemiological context changes from high to low transmission setting, people are presumably exposed to a lower viral load, which has been previously associated to less severe clinical manifestations.