Emerging in November 2021, the SARS-CoV-2 Omicron variant of concern exhibited marked immune evasion resulting in reduced
vaccine effectiveness against
SARS-CoV-2 infection and symptomatic disease. Most
vaccine effectiveness data on Omicron are derived from the first Omicron subvariant,
BA.1, which caused large waves of
infection in many parts of the world within a short period of time.
BA.1, however, was replaced by
BA.2 within months, and later by BA.4 and BA.5 (BA.4/5). These later Omicron subvariants exhibited additional mutations in the spike
protein of the virus, leading to speculation that they might result in even lower
vaccine effectiveness. To address this question, the World Health Organization hosted a virtual meeting on December 6, 2022, to review available evidence for
vaccine effectiveness against the major Omicron subvariants up to that date. Data were presented from South Africa, the United Kingdom, the United States, and Canada, as well as the results of a review and meta-regression of studies that evaluated the duration of the
vaccine effectiveness for multiple Omicron subvariants. Despite heterogeneity of results and wide confidence intervals in some studies, the majority of studies showed
vaccine effectiveness tended to be lower against
BA.2 and especially against BA.4/5, compared to
BA.1, with perhaps faster waning against severe disease caused by BA.4/5 after a booster dose. The interpretation of these results was discussed and both
immunological factors (i.e., more immune escape with BA.4/5) and methodological issues (e.g., biases related to differences in the timing of subvariant circulation) were possible explanations for the findings.
COVID-19 vaccines still provide some protection against
infection and symptomatic disease from all Omicron subvariants for at least several months, with greater and more durable protection against severe disease.