
A study from July 2020 to Nov 2022 shows previous infection effectiveness declined after Omicron's emergence, from 88.3% (Nov 2021) to 51.0% (Dec 2021). Primary vaccine series effectiveness dropped from 84.0% (April 2021) to 52.7% (Nov 2021), declining by about 1% every 5 days. Booster efficacy also declined, from 83.0% (Nov 2021) to 32.9% (Dec 2021). However, immunity against severe COVID-19 remained strong, with >80% effectiveness throughout.
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