Assessing vaccine effectiveness against severe COVID-19 disease caused by omicron variant. Report from a meeting of the World Health Organization

Daniel R. Feikin*, Laith J. Abu-Raddad, Nick Andrews, Mary Ann Davies, Melissa M. Higdon, Walter A. Orenstein, Minal K. Patel

*Corresponding author for this work

Research output: Contribution to journalComment/debate

58 Citations (Scopus)


Vaccine effectiveness is lower and wanes faster against infection and symptomatic disease caused by the omicron variant of SARS-CoV-2 than was observed with previous variants. Vaccine effectiveness against severe omicron disease, on average, is higher, but has shown variability, including rapid apparent waning, in some studies. Assessing vaccine effectiveness against omicron severe disease using hospital admission as a measure of severe disease has become more challenging because of omicron's attenuated intrinsic severity and its high prevalence of infection. Many hospital admissions likely occur among people with incidental omicron infection or among those with infection-induced exacerbation of chronic medical conditions. To address this challenge, the World Health Organization held a virtual meeting on March 15, 2022, to review evidence from several studies that assessed Covid-19 vaccine effectiveness against severe omicron disease using several outcome definitions. Data was shown from studies in South Africa, the United States, the United Kingdom and Qatar. Several approaches were proposed that better characterize vaccine protection against severe Covid-19 disease caused by the omicron variant than using hospitalization of omicron-infected persons to define severe disease. Using more specific definitions for severe respiratory Covid-19 disease, such as indicators of respiratory distress (e.g. oxygen requirement, mechanical ventilation, and ICU admission), showed higher vaccine effectiveness than against hospital admission. Second, vaccine effectiveness against progression from omicron infection to hospitalization, or severe disease, also showed higher vaccine protection. These approaches might better characterize vaccine performance against severe Covid-19 disease caused by omicron, as well as future variants that evade humoral immunity, than using hospitalization with omicron infection as an indicator of severe disease.

Original languageEnglish
Pages (from-to)3516-3527
Number of pages12
Issue number26
Publication statusPublished - 9 Jun 2022

Bibliographical note

Funding Information:
We want to acknowledge Manish Patel, U.S. Centers for Disease Control and Prevention, for sharing data from the IVY network. We acknowledge Anurima Baidya and Karoline Walter, International Vaccine Access Center, for their work in extracting data for studies for the ongoing systematic review.

Publisher Copyright:
© 2022 Elsevier Ltd


  • COVID-19
  • Omicron variant
  • Vaccine effectiveness


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