Lower nasopharyngeal viral load during the latest phase of COVID-19 pandemic in a Northern Italy University Hospital

Objectives: A milder clinical course of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has been anecdotally reported over the latest phase of COVID-19 pandemic in Italy. Several factors may contribute to this observation, including the effect of lockdown, social distancing, lower humidity, lower air pollution, and potential changes in the intrinsic pathogenicity of the virus. In this regard, the clinical severity of COVID-19 could be attenuated by mutations in SARS-CoV-2 genome that decrease its virulence, as well as by lower virus inocula. Methods: In this pilot study, we compared the reverse transcription polymerase chain reaction (RT-PCR) amplification profile of 100 nasopharyngeal swabs consecutively collected in April, during the peak of SARS-CoV-2 epidemic, to that of 100 swabs collected using the same procedure in May. Results: The mean Ct value of positive samples collected in May was significantly higher than that of samples collected in the previous period (ORF 1a/b gene: 31.85 ± 0.32 vs. 28.37 ± 0.5, p<0.001; E gene: 33.76 ± 0.38 vs. 29.79 ± 0.63, p<0.001), suggesting a lower viral load at the time of sampling. No significant differences were observed between male and females in the two periods, whilst higher viral loads were found in (i) patients over 60-years old, and (ii) patients that experienced severe COVID-19 during the early stages of the pandemic. Conclusions: This pilot study prompts further investigation on the correlation between SARS-CoV-2 load and different clinical manifestation of COVID-19 during different phases of the pandemic. Laboratories should consider reporting quantitative viral load data in the molecular diagnosis of SARS-CoV-2 infection.