Inclusion Criteria Test Negative Design  KPSC patients 16 years or older who are admitted to the hospital (primary objective) with acute respiratory infection (ARI) after 14 December 2020 (date of first vaccinations at KPSC), and who receive a PCR test for SARS-CoV-2. For secondary objectives estimating VE against ED admission, the TND will include KPSC patients 16 years or older who present to the ED with ARI after 14 December 2020, and who receive a PCR test for SARS-CoV-2. Membership requirement of 6 months prior to index date, which is de-fined as the date of hospitalization or ED admission (allowing 31-day administrative gap), to facilitate accurate capture of comorbid conditions. Inclusion Criteria Cohort Design All KPSC members as of 14 December 2020 (date of first Pfizer vaccination at KPSC) who are age 16 and older. Patients must have at least 6 months of membership (allowing 31-day administrative gap) prior to 14 December 2020 (index date, date vaccinations first began at KPSC) to facilitate accurate capture of comorbid conditions.  Exclusion Criteria Test Negative Design • Patients who receive any other newly licensed or investigational SARS-CoV-2 vaccine or COVID-19 prophylactic agent other than Pfizer's COVID-19 vaccine prior to hospitalization (or ED, for secondary objective) will be excluded from the analysis. Patients will also be excluded if the index date is within certain time windows from vaccination date.  Exclusion Criteria Cohort Design  • There will be no exclusion criteria for the cohort design, however patients will be censored for receiving any other newly licensed or investigational SARS-CoV-2 vaccine or COVID-19 prophylactic agent other than Pfizer's COVID-19 vaccine. Patients will also be censored if the event (hospitalization, ED encounter, etc.) occurs within certain time windows from vaccination date.