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Background: The recently emerged SARS-CoV-2 Omicron variant exhibits several mutations on the spike protein, enabling it to escape the immunity elicited by natural infection or vaccines. Avidity is the strength of binding between an antibody and its specific epitope. The SARS-CoV-2 spike protein binds to its cellular receptor with high affinity and is the primary target of neutralizing antibodies. Therefore, protective antibodies should show high avidity. This study aimed at investigating the avidity of receptor-binding domain (RBD) binding antibodies and their neutralizing activity against the Omicron variant in SARS-CoV-2 infected patients and vaccinees.Methods: Samples were collected from 42 SARS-CoV-2 infected patients during the first pandemic wave, 50 subjects who received 2 doses of mRNA vaccine before the Omicron wave, 44 subjects who received 3 doses of mRNA vaccine, and 35 subjects who received heterologous vaccination (2 doses of adenovirus-based vaccine plus mRNA vaccine) during the Omicron wave. Samples were tested for the avidity of RBD-binding IgG and neutralizing antibodies against the wild-type SARS-CoV-2 virus and the Omicron variant.Results: In patients, RBD-binding IgG titers against the wild-type virus increased with time, but remained low. High neutralizing titers against the wild-type virus were not matched by high avidity or neutralizing activity against the Omicron variant. Vaccinees showed higher avidity than patients. Two vaccine doses elicited the production of neutralizing antibodies, but low avidity for the wild-type virus; antibody levels against the Omicron variant were even lower. Conversely, 3 doses of vaccine elicited high avidity and high neutralizing antibodies against both the wild-type virus and the Omicron variant.Conclusions: Repeated vaccination increases antibody avidity against the spike protein of the Omicron variant, suggesting that antibodies with high avidity and high neutralizing potential increase cross-protection against variants that carry several mutations on the RBD. [ABSTRACT FROM AUTHOR]

Since the first detection of a novel Coronavirus (SARS-CoV-2) in December 2019 in Wuhan (China), it has become crucial to assess and quantize the human humoral immune response after SARS-CoV-2 natural infection and/or vaccination. Having well standardized and reliable serological assays able to accurately measure the total IgG antibodies response as well as the neutralization dynamics, play a pivotal role for the evaluation of "second" and "third" vaccines generation and in monitoring the effect in case of reinfection in the human population caused by the original strains or new SARS-CoV-2 variants. In the present study we reported that both symptomatic convalescent and vaccinated donors showed the presence of different levels of neutralizing antibodies. In addition, vaccinated subjects presented high levels of anti-S antibodies, whereas the complete absence of anti-N antibodies, whereas convalescent patients presented high levels of both anti-S and anti-N antibodies. The evaluation of the correlation between SARS-CoV-2 neutralizing and binding antibodies in convalescent and vaccinated subjects revealed that the IgG anti-S log-values were significantly higher in the vaccinated group respect to convalescent subjects. In addition, the level of binding antibodies recognizing the S protein shows a positive linear regression when compared to neutralizing titres in both the two groups evaluated. [ABSTRACT FROM AUTHOR]