Humoral and cellular response induced by a second booster of an inactivated SARS-CoV-2 vaccine in adults

CoronaVac03CL Study Group

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5 Scopus citations


BACKGROUND: The Omicron variant has challenged the control of the COVID-19 pandemic due to its immuno-evasive properties. The administration of a booster dose of a SARS-CoV-2 vaccine showed positive effects in the immunogenicity against SARS-CoV-2, effect that is even enhanced after the administration of a second booster.

METHODS: During a phase-3 clinical trial, we evaluated the effect of a second booster of CoronaVac®, an inactivated vaccine administered 6 months after the first booster, in the neutralization of SARS-CoV-2 (n = 87). In parallel, cellular immunity (n = 45) was analyzed in stimulated peripheral mononuclear cells by flow cytometry and ELISPOT.

FINDINGS: Although a 2.5-fold increase in neutralization of the ancestral SARS-CoV-2 was observed after the second booster when compared with prior its administration (Geometric mean units p < 0.0001; Geometric mean titer p = 0.0002), a poor neutralization against the Omicron variant was detected. Additionally, the activation of specific CD4 + T lymphocytes remained stable after the second booster and, importantly, equivalent activation of CD4 + T lymphocytes against the Omicron variant and the ancestral SARS-CoV-2 were found.

INTERPRETATION: Although the neutralizing response against the Omicron variant after the second booster of CoronaVac® was slightly increased, these levels are far from those observed against the ancestral SARS-CoV-2 and could most likely fail to neutralize the virus. In contrast, a robust CD4 +T cell response may confer protection against the Omicron variant.

FUNDING: The Ministry of Health, Government of Chile, the Confederation of Production and Commerce, Chile and SINOVAC Biotech.NIHNIAID. The Millennium Institute on Immunology and Immunotherapy.

Original languageEnglish
Article number104563
Pages (from-to)104563
Early online date24 Apr 2023
StatePublished - May 2023

Bibliographical note

Copyright © 2023 The Author(s). Published by Elsevier B.V. All rights reserved.


  • Cellular immunity
  • CoronaVac®
  • Humoral immunity
  • Omicron variant
  • SARS-CoV-2
  • Second booster dose


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