This study estimates the effectiveness of the BNT162b2 XBB vaccine against COVID-19 outcomes among US adults 18 years and older. The research was conducted by Kaiser Permanente Southern California and Pfizer, focusing on the period from October 10, 2023, to December 10, 2023. The primary exposure was the receipt of the BNT162b2 XBB vaccine compared to not receiving any XBB vaccine, regardless of prior COVID-19 vaccination or infection history. The study compared cases (acute respiratory illness with a positive SARS-CoV-2 PCR test) and controls (acute respiratory illness with a negative SARS-CoV-2 PCR test) to estimate vaccine effectiveness. Key findings include: - The BNT162b2 XBB vaccine provided a median of 34 days of protection, with an adjusted effectiveness of 62% (95% CI, 32%-79%) against COVID-19 hospitalization and 58% (95% CI, 48%-67%) against ED/UC visits. - Older versions of COVID-19 vaccines did not show statistically significant reduced risk of COVID-19 outcomes compared to unvaccinated individuals. - The study reaffirms the importance of broad age-based use of annually updated COVID-19 vaccines, as older vaccines provided little to no additional protection. The study highlights the need for continued efforts to improve COVID-19 vaccine uptake, especially given the low vaccination rates and the ongoing circulation of SARS-CoV-2 strains.This study estimates the effectiveness of the BNT162b2 XBB vaccine against COVID-19 outcomes among US adults 18 years and older. The research was conducted by Kaiser Permanente Southern California and Pfizer, focusing on the period from October 10, 2023, to December 10, 2023. The primary exposure was the receipt of the BNT162b2 XBB vaccine compared to not receiving any XBB vaccine, regardless of prior COVID-19 vaccination or infection history. The study compared cases (acute respiratory illness with a positive SARS-CoV-2 PCR test) and controls (acute respiratory illness with a negative SARS-CoV-2 PCR test) to estimate vaccine effectiveness. Key findings include: - The BNT162b2 XBB vaccine provided a median of 34 days of protection, with an adjusted effectiveness of 62% (95% CI, 32%-79%) against COVID-19 hospitalization and 58% (95% CI, 48%-67%) against ED/UC visits. - Older versions of COVID-19 vaccines did not show statistically significant reduced risk of COVID-19 outcomes compared to unvaccinated individuals. - The study reaffirms the importance of broad age-based use of annually updated COVID-19 vaccines, as older vaccines provided little to no additional protection. The study highlights the need for continued efforts to improve COVID-19 vaccine uptake, especially given the low vaccination rates and the ongoing circulation of SARS-CoV-2 strains.