Absence of neutralizing antibodies against the Omicron SARS-CoV-2 variant in convalescent sera from individuals infected with the ancestral SARS-CoV-2 virus or its Gamma variant. Clinics [Internet]. 2022 Jun. 16 [cited 2024 Jun. 18];77:100068. Available from: https://journals.usp.br/clinics/article/view/213419