Effectiveness of a Third Dose of mRNA Vaccines Against COVID-19–Associated Emergency Department and Urgent Care Encounters and Hospitalizations Among Adults During Periods of Delta and Omicron Variant Predominance — VISION Network, 10 States, August 2021–January 2022

SARS-CoV-2 Infection and Hospitalization Among Adults Aged ≥18 Years, by Vaccination Status, Before and During SARS-CoV-2 B.1.1.529 (Omicron) Variant Predominance — Los Angeles County, California, November 7, 2021–January 8, 2022

“…These findings align with those from recent studies, indicating that COVID-19 vaccination protects against severe COVID-19 caused by SARS-CoV-2 variants, including Omicron (7,8). *** Efforts to promote COVID-19 vaccination and boosters are critical to preventing COVID-19-associated hospitalizations and severe outcomes.…”

Clinical Characteristics and Outcomes Among Adults Hospitalized with Laboratory-Confirmed SARS-CoV-2 Infection During Periods of B.1.617.2 (Delta) and B.1.1.529 (Omicron) Variant Predominance — One Hospital, California, July 15–September 23, 2021, and December 21, 2021–January 27, 2022

“…Increases in infections among vaccinated persons during the period of Omicron predominance were likely driven both by waning vaccine-derived immunity over time and by relative resistance to vaccine neutralization in the Omicron variant compared with the Delta variant (2,4). This is consistent with the observed decline in effectiveness *** https://covid19.ca.gov/vaccination-progress-data/ of 2-dose vaccination against COVID-19 hospitalization during the Omicron period (5). A previous study also found that, compared with the period of Delta predominance, the period of Omicron predominance in Los Angeles County was associated with a decrease in the degree of protection against COVID-19 and hospitalization (6).…”

Increased risk of infection with SARS-CoV-2 Omicron BA.1 compared with Delta in vaccinated and previously infected individuals, the Netherlands, 22 November 2021 to 19 January 2022

“…After booster vaccination, the vaccine effectiveness against infection with the Omicron variant increased but was still lower than against Delta [14][15][16][17][18]. Studies have shown that vaccine effectiveness against severe COVID-19 with the Omicron variant is very high after booster vaccination, although lower than against severe COVID-19 with Delta [19,20].…”

“…The effectiveness of primary vaccination against hospitalisation with Omicron infection was reported to be around 50% at 25 weeks or more after vaccination, and the effectiveness increased to ca 90% within 2-9 weeks after booster vaccination [19,20]. Even with lower disease severity for Omicron compared with Delta, the higher transmissibility will probably lead to more hospital admissions if no additional measures are taken.…”

“…Several studies suggest that infection with the Omicron variant causes less severe disease, with lower rates of hospitalisations and intensive care admission than for Delta infection, also when considering the effect of vaccination and previous infection [ 12 , 25 , 26 ]. The effectiveness of primary vaccination against hospitalisation with Omicron infection was reported to be around 50% at 25 weeks or more after vaccination, and the effectiveness increased to ca 90% within 2–9 weeks after booster vaccination [ 19 , 20 ]. Even with lower disease severity for Omicron compared with Delta, the higher transmissibility will probably lead to more hospital admissions if no additional measures are taken.…”