SARS-CoV-2: serological evidence of circulation in Southern Italy in late 2019?

Background Italy has been one of the first countries hit by the COVID-19 pandemic. On 22 February, 2020, clusters of infection in Northern Italy were reported and transmission was assumed to be local. The presence of SARS-CoV-2 in wastewaters in Italy in late December 2019 suggested that the virus has been silently circulating before the first autochthonous case was identified. This study aimed to retrospectively assess the presence of antibodies against SARS-CoV-2 in a sample population from Southern Italy in the last months of 2019. Methods Human serum samples were collected from late September to the end of December 2019 in a local laboratory in the province of Bari, Apulia region, Southern Italy. A total of 455 samples were tested for SARS-CoV-2 IgA, IgM, and IgG by commercial ELISA. Samples with borderline or positive result for at least one antibody class were further tested by in house-ELISA for IgA, IgM, and IgG targeting the receptor-binding domain (RBD) of the spike protein of SARS-CoV-2 and by micro-neutralization test (MNT). Results After commercial ELISA testing, 11 samples were positive for IgG, 3 for IgM, and 3 and 2 were positive and borderline for IgA, respectively. 8 samples (1.8%, 95% CI 0.8-3.4) were positive when tested by in house-ELISA: 5 (1.1%, 95% CI 0.4-2.5) for IgG and 3 (0.7%, 95% CI 0.1-1.9) for IgM. No samples were positive for in house-ELISA IgA or by MNT. Conclusions The results of this study showed that some samples collected before the first autochthonous case was detected in Italy had antibodies reactive to the RBD of SARS-CoV-2. The Apulia region has been less severely affected by the first epidemic wave than the northern regions;however, these results suggest the spread of SARS-CoV-2 in the population in the late months of 2019. Since the majority of SARS-CoV-2 infections does not require hospitalization, it is possible that many of these have gone unnoticed. Key messages The spread of SARS-CoV-2 in the Italian population could have already occurred in late 2019. The extent of SARS-CoV-2 infections in the population could be far greater than reported.