Evaluation of Immunogenicity by Pseudovirus Neutralization Assays for SARS-CoV-2 Variants after Primary and Booster Immunization

Objectives: To determine the status of immune responses after primary and booster immunization for SARS-CoV-2 variants and evaluate the differences in disease resistance based upon titers of neutralizing antibodies (NAbs) against the variants. Methods: Participants aged 18-59 years received 2 doses of inactivated COVID-19 vaccine, 14 days apart, and a booster dose after 12 months. Blood samples were collected before vaccination (baseline), 1 and 6 months after primary immunization, and at multiple instances within 21 days of the booster dose. NAbs against the spike protein of Wuhan-Hu-1 and 3 variants were measured using pseudovirus neutralization assays. Results: Of 400 enrolled participants, 387 completed visits scheduled within 6 months of the second dose and 346 participants received the booster dose in the follow-up research. After 1 month of primary immunization, geometric mean titers (GMTs) of NAbs peaked for Wuhan-Hu-1, whereas GMTs of other variants were < 30. After 6 months of primary immunization, GMTs of NAbs against all strains were < 30. After 3 days of booster immunization, GMTs were unaltered, seroconversion rates reached approximately 50% after 7 days, and GMTs of NAbs against all strains peaked at 14 days. Conclusion: Two-dose of inactivated COVID-19 vaccine induced the formation of NAbs and memory-associated immune responses, and high titers of NAbs against the variants obtained after booster im-munization may further improve the effectiveness of the vaccine. (c) 2022 The Authors. Published by Elsevier Ltd on behalf of International Society for Infectious Diseases. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

Automated summary

This study investigated the immune responses post primary and booster immunization with two doses of inactivated COVID-19 vaccine, highlighting an increase in neutralizing antibodies against variants after booster dose which may enhance vaccine effectiveness.