Dynamics of viral shedding during ancestral or Omicron BA.1 SARS-CoV-2 infection and enhancement of pre-existing immunity during breakthrough infections

Omicron variant is circulating in the presence of a globally acquired immunity unlike the ancestral SARS-CoV-2 isolate. Herein, we investigated the normalized viral load dynamics and viral culture status in 44 fully vaccinated healthcare workers (HCWs) infected with the Omicron BA.1 variant. Viral load dynamics of 38 unvaccinated HCWs infected with the 20A variant during the first pandemic wave was also studied. We then explored the impact of Omicron infection on pre-existing immunity assessing anti-RBD IgG levels, neutralising antibody titres against 19A, Delta and Omicron isolates, as well as IFN-γ release following cell stimulation with SARS-CoV-2 peptides. We reported that, throughout the follow-up, Omicron viral load decreased faster than that of 20A. We found that Omicron breakthrough infections led to an overall enhancement of vaccine-induced humoral and cellular immunity as soon as a median [interquartile range] of 8 [7-9] days post symptom onset. Among samples with similar high viral loads, non-culturable samples exhibited higher neutralising antibody titres and anti-RBD IgG levels than culturable samples. Additionally, Omicron infection led to an enhancement of antibodies neutralisation capacity against other SARS-CoV-2 isolates. Taken together, the results suggest that Omicron BA.1 vaccine breakthrough infection is associated with a faster viral clearance than that of the ancestral SARS-CoV-2, in addition this new variant leads to a rapid enhancement of the humoral response against multiple SARS-CoV-2 variants, and of the cellular response.