RESUMO
The SARS-CoV-2 Omicron variant has increased infectivity and immune escape compared with previous variants, and caused massive COVID-19 waves globally. Despite a vast majority ([~]90%) of the population of Santa Fe city, Argentina, had been vaccinated and/or had been infected by SARS-CoV-2 by the arrival of Omicron, the epidemic wave that followed was by far the largest one experienced in the city. A serosurvey conducted prior to the arrival of Omicron allowed to assess the acquired humoural defences preceding the wave and to evaluate associations with infection risk and severity during the wave. A very large proportion of the 1455 sampled individuals had immunological memory against COVID-19 at the arrival of Omicron (almost 90%), and about half (48.9%) had high anti-Spike IgG levels (>200 UI/ml). The antibody titres were strongly associated with the number of vaccine shots and the vaccine platform received, and also depended markedly on prior COVID-19 diagnosis and the days elapsed since last antigen exposure (vaccine shot or natural infection). Following-up 514 participants provided real-world data linking antibody levels with protection against COVID-19 during a period of high risk of exposure to an immune-escaping highly transmissible variant. Pre-wave antibody titres were strongly associated with COVID-19 incidence and severity of symptoms during the wave. Also, receiving a vaccine shot during the follow-up period reduced the COVID-19 risk drastically (15-fold). These results highlight the importance of maintaining high defences through vaccination anticipating or during a period of high risk of exposure to immune-escaping variants.
