Effectiveness of a fourth dose of mRNA-1273 against COVID-19 among older adults in the United States: Interim results from an observational cohort study.

We evaluated relative vaccine effectiveness (rVE) of 4- vs. 3-dose mRNA-1273 against SARS-CoV-2 infection, and COVID-19 hospitalization and death in immunocompetent adults aged ≥50 years at Kaiser Permanente Southern California. We included 178,492 individuals who received a fourth dose of mRNA-1273, and 178,492 randomly selected 3-dose recipients who were matched to 4-dose recipients by age, sex, race/ethnicity, and third dose date. Adjusted 4- vs. 3-dose rVE against SARS-CoV-2 infection, COVID-19 hospitalization, and COVID-19 hospitalization death were 25.9 % (23.5 %, 28.2 %), 67.3 % (58.7 %, 74.1 %), and 72.5 % (-35.9 %, 95.2 %), respectively. Adjusted rVE against SARS-CoV-2 infection ranged between 19.8 % and 39.1 % across subgroups. Adjusted rVE against SARS-CoV-2 infection and COVID-19 hospitalization decreased 2-4 months after the fourth dose. Four mRNA-1273 doses provided significant protection against COVID-19 outcomes compared with 3 doses, consistent in various subgroups of demographic and clinical characteristics, although rVE varied and waned over time.

Vaccine effectiveness of the mRNA-1273 3-dose primary series against COVID-19 in an immunocompromised population: A prospective observational cohort study.

BACKGROUND: Data on the effectiveness of the 3-dose mRNA-1273 primary series are limited, particularly in comparison to 2 doses. Given suboptimal COVID-19 vaccine uptake among immunocompromised populations, it is important to monitor the effectiveness of fewer than the recommended doses in this population. METHODS: We conducted a matched cohort study at Kaiser Permanente Southern California to evaluate the relative vaccine effectiveness (rVE) of the 3-dose series vs 2 doses of mRNA-1273 in preventing SARS-CoV-2 infection and severe COVID-19 outcomes among immunocompromised individuals. RESULTS: We included 21,942 3-dose recipients who were 1:1 matched with randomly selected 2-dose recipients (third doses accrued 08/12/2021-12/31/2021, with follow-up through 01/31/2022). Adjusted rVE of 3 vs 2 doses of mRNA-1273 against SARS-CoV-2 infection, COVID-19 hospitalization, and COVID-19 hospital death were 55.0 % (95 % CI: 50.8-58.9 %), 83.0 % (75.4-88.3 %), and 87.1 % (30.6-97.6 %), respectively. CONCLUSION: Three doses of mRNA-1273 were associated with a significantly higher rVE against SARS-CoV-2 infection and severe outcomes, compared to 2 doses. These findings were consistent across subgroups of demographic and clinical characteristics, and mostly consistent across subgroups of immunocompromising conditions. Our study highlights the importance of completing the 3-dose series for immunocompromised populations.

Effectiveness of mRNA-1273 vaccine booster against COVID-19 in immunocompetent adults.

BACKGROUND: A prospective cohort study at Kaiser Permanente Southern California was conducted to evaluate the relative vaccine effectiveness (rVE) of a booster-dose vs. 2-dose primary series of mRNA-1273 in immunocompetent individuals during periods of Delta and Omicron predominance. METHODS: Immunocompetent adults who received a booster dose of mRNA-1273 from October through December 2021 were matched 1:1 to randomly selected 2-dose mRNA-1273 recipients by age, sex, race/ethnicity, and second dose date, and followed up through January 2022. Cox proportional hazards models were used to estimate adjusted hazard ratios (aHR) with 95% confidence intervals (CIs), comparing outcomes (SARS-CoV-2 infection, and COVID-19 hospitalization and hospital death) in the booster-dose and 2-dose groups. Adjusted rVE (%) was calculated as (1-aHR)x100. aHRs and rVEs were also estimated for SARS-CoV-2 infection by subgroups (age, sex, race/ethnicity, history of SARS-CoV-2 infection, pregnancy, chronic diseases), and for SARS-CoV-2 infection and severe COVID-19 disease by month of follow-up. RESULTS: The study included 431,328 booster-dose vaccinated adults matched to 431,328 2-dose vaccinated adults. rVE was 61.3% (95%CI: 60.5-62.2%) against SARS-CoV-2 infection, 89.0% (86.2-91.2%) against COVID-19 hospitalization, and 96.0% (68.0-99.5%) against COVID-19 hospital death. rVE against SARS-CoV-2 infection ranged from 55.6% to 66.7% across all subgroups. rVE against SARS-CoV-2 infection decreased from 67.1% (0-<1 month of follow-up) to 30.5% (2-<3 months). For COVID-19 hospitalization, rVE decreased from 91.2% (0-<1 month) to 78.7% (2-<3 months). CONCLUSIONS: Among immunocompetent adults, the mRNA-1273 booster conferred additional protection against SARS-CoV-2 infection and severe COVID-19 disease compared to the 2-dose mRNA-1273 primary series during periods of Delta and Omicron predominance.

Analysis of mRNA COVID-19 Vaccine Uptake Among Immunocompromised Individuals in a Large US Health System.

Importance: Immunocompromised individuals are at increased risk for severe outcomes due to SARS-CoV-2 infection. Given the varying and complex nature of COVID-19 vaccination recommendations, it is important to understand COVID-19 vaccine uptake in this vulnerable population. Objective: To assess mRNA COVID-19 vaccine uptake and factors associated with uptake among immunocompromised individuals from December 14, 2020, through August 6, 2022. Design, Setting, and Participants: This cohort study was conducted with patients of Kaiser Permanente Southern California (KPSC), an integrated health care system in the US. The study included patients aged 18 years or older who were immunocompromised (individuals with an immunocompromising condition or patients who received immunosuppressive medications in the year prior to December 14, 2020) and still met criteria for being immunocompromised 1 year later. Exposures: Age, sex, self-identified race and ethnicity, prior positive COVID-19 test result, immunocompromising condition, immunomodulating medication, comorbidities, health care utilization, and neighborhood median income. Main Outcomes and Measures: Outcomes were the number of doses of mRNA COVID-19 vaccine received and the factors associated with receipt of at least 4 doses, estimated by hazard ratios (HRs) and 95% Wald CIs via Cox proportional hazards regression. Statistical analyses were conducted between August 9 and 23, 2022. Results: Overall, 42 697 immunocompromised individuals met the study eligibility criteria. Among these, 18 789 (44.0%) were aged 65 years or older; 20 061 (47.0%) were women and 22 635 (53.0%) were men. With regard to race and ethnicity, 4295 participants (10.1%) identified as Asian or Pacific Islander, 5174 (12.1%) as Black, 14 289 (33.5%) as Hispanic, and 17 902 (41.9%) as White. As of the end of the study period and after accounting for participant censoring due to death or disenrollment from the KPSC health plan, 78.0% of immunocompromised individuals had received a third dose of mRNA COVID-19 vaccine. Only 41.0% had received a fourth dose, which corresponds to a primary series and a monovalent booster dose for immunocompromised individuals. Uptake of a fifth dose was only 0.9% following the US Centers for Disease Control and Prevention (CDC) recommendation to receive a second monovalent booster (ie, fifth dose). Adults aged 65 years or older (HR, 3.95 [95% CI, 3.70-4.22]) were more likely to receive at least 4 doses compared with those aged 18 to 44 years or 45 to 64 years (2.52 [2.36-2.69]). Hispanic and non-Hispanic Black adults (HR, 0.77 [95% CI, 0.74-0.80] and 0.82 [0.78-0.87], respectively, compared with non-Hispanic White adults), individuals with prior documented SARS-CoV-2 infection (0.71 [0.62-0.81] compared with those without), and individuals receiving high-dose corticosteroids (0.88 [0.81-0.95] compared with those who were not) were less likely to receive at least 4 doses. Conclusions and Relevance: These findings suggest that adherence to CDC mRNA monovalent COVID-19 booster dose recommendations among immunocompromised individuals was low. Given the increased risk for severe COVID-19 in this vulnerable population and the well-established additional protection afforded by booster doses, targeted and tailored efforts to ensure that immunocompromised individuals remain up to date with COVID-19 booster dose recommendations are warranted.

Effectiveness of mRNA-1273 vaccination against SARS-CoV-2 omicron subvariants BA.1, BA.2, BA.2.12.1, BA.4, and BA.5.

Studies have reported reduced natural SARS-CoV-2 infection- and vaccine-induced neutralization against omicron BA.4/BA.5 compared with earlier omicron subvariants. This test-negative case-control study evaluates mRNA-1273 vaccine effectiveness (VE) against infection and hospitalization with omicron subvariants. The study includes 30,809 SARS-CoV-2 positive and 92,427 SARS-CoV-2 negative individuals aged ≥18 years tested during 1/1/2022-6/30/2022. While 3-dose VE against BA.1 infection is high and wanes slowly, VE against BA.2, BA.2.12.1, BA.4, and BA.5 infection is initially moderate to high (61.0%-90.6% 14-30 days post third dose) and wanes rapidly. The 4-dose VE against infection with BA.2, BA.2.12.1, and BA.4 ranges between 64.3%-75.7%, and is low (30.8%) against BA.5 14-30 days post fourth dose, disappearing beyond 90 days for all subvariants. The 3-dose VE against hospitalization for BA.1, BA.2, and BA.4/BA.5 is 97.5%, 82.0%, and 72.4%, respectively; 4-dose VE against hospitalization for BA.4/BA.5 is 88.5%. Evaluation of the updated bivalent booster is warranted.

A safety study evaluating non-COVID-19 mortality risk following COVID-19 vaccination.

BACKGROUND: The safety of COVID-19 vaccines plays an important role in addressing vaccine hesitancy. We conducted a large cohort study to evaluate the risk of non-COVID-19 mortality after COVID-19 vaccination while adjusting for confounders including individual-level demographics, clinical risk factors, health care utilization, and community-level socioeconomic risk factors. METHODS: The retrospective cohort study consisted of members from seven Vaccine Safety Datalink sites from December 14, 2020 through August 31, 2021. We conducted three separate analyses for each of the three COVID-19 vaccines used in the US. Crude non-COVID-19 mortality rates were reported by vaccine type, age, sex, and race/ethnicity. The counting process model for survival analyses was used to analyze non-COVID-19 mortality where a new observation period began when the vaccination status changed upon receipt of the first dose and the second dose. We used calendar time as the basic time scale in survival analyses to implicitly adjust for season and other temporal trend factors. A propensity score approach was used to adjust for the potential imbalance in confounders between the vaccinated and comparison groups. RESULTS: For each vaccine type and across age, sex, and race/ethnicity groups, crude non-COVID-19 mortality rates among COVID-19 vaccinees were lower than those among comparators. After adjusting for confounders with the propensity score approach, the adjusted hazard ratios (aHRs) were 0.46 (95% confidence interval [CI], 0.44-0.49) after dose 1 and 0.48 (95% CI, 0.46-0.50) after dose 2 of the BNT162b2 vaccine, 0.41 (95% CI, 0.39-0.44) after dose 1 and 0.38 (95% CI, 0.37-0.40) after dose 2 of the mRNA-1273 vaccine, and 0.55 (95% CI, 0.51-0.59) after receipt of Ad26.COV2.S. CONCLUSION: While residual confounding bias remained after adjusting for several individual-level and community-level risk factors, no increased risk was found for non-COVID-19 mortality among recipients of three COVID-19 vaccines used in the US.

Risk of Severe Coronavirus Disease 2019 Disease in Individuals With Down Syndrome: A Matched Cohort Study From a Large, Integrated Health Care System.

BACKGROUND: Down syndrome (DS) is associated with an increased risk of infections attributed to immune defects. Whether individuals with DS are at an increased risk of severe coronavirus disease 2019 (COVID-19) remains unclear. METHODS: In a matched cohort study, we evaluated the risk of COVID-19 infection and severe COVID-19 disease in individuals with DS and their matched counterparts in a pre-COVID-19 vaccination period at Kaiser Permanente Southern California. Multivariable Cox proportion hazard regression was used to investigate associations between DS and risk of COVID-19 infection and severe COVID-19 disease. RESULTS: Our cohort included 2541 individuals with DS and 10 164 without DS matched on age, sex, and race/ethnicity (51.6% female, 53.3% Hispanic, median age 25 years [interquartile range, 14-38]). Although the rate of COVID-19 infection in individuals with DS was 32% lower than their matched counterparts (adjusted hazard ratio [aHR], 0.68; 95% confidence interval [CI], .56-.83), the rate of severe COVID-19 disease was 6-fold higher (aHR, 6.14; 95% CI, 1.87-20.16). CONCLUSIONS: Although the risk of COVID-19 infection is lower, the risk of severe disease is higher in individuals with DS compared with their matched counterparts. Better infection monitoring, early treatment, and promotion of vaccine for COVID-19 are warranted for DS populations.

Durability of mRNA-1273 against COVID-19 in the time of Delta: Interim results from an observational cohort study.

BACKGROUND: We conducted a prospective cohort study at Kaiser Permanente Southern California to study the vaccine effectiveness (VE) of mRNA-1273 over time and during the emergence of the Delta variant. METHODS: The cohort for this planned interim analysis consisted of individuals aged ≥18 years receiving 2 doses of mRNA-1273 through June 2021, matched 1:1 to randomly selected unvaccinated individuals by age, sex, and race/ethnicity, with follow-up through September 2021. Outcomes were SARS-CoV-2 infection, and COVID-19 hospitalization and hospital death. Cox proportional hazards models were used to estimate adjusted hazard ratios (aHR) with 95% confidence intervals (CIs) comparing outcomes in the vaccinated and unvaccinated groups. Adjusted VE (%) was calculated as (1-aHR)x100. HRs and VEs were also estimated for SARS-CoV-2 infection by age, sex, race/ethnicity, and during the Delta period (June-September 2021). VE against SARS-CoV-2 infection and COVID-19 hospitalization was estimated at 0-<2, 2-<4, 4-<6, and 6-<8 months post-vaccination. RESULTS: 927,004 recipients of 2 doses of mRNA-1273 were matched to 927,004 unvaccinated individuals. VE (95% CI) was 82.8% (82.2-83.3%) against SARS-CoV-2 infection, 96.1% (95.5-96.6%) against COVID-19 hospitalization, and 97.2% (94.8-98.4%) against COVID-19 hospital death. VE against SARS-CoV-2 infection was similar by age, sex, and race/ethnicity, and was 86.5% (84.8-88.0%) during the Delta period. VE against SARS-CoV-2 infection decreased from 88.0% at 0-<2 months to 75.5% at 6-<8 months. CONCLUSIONS: These interim results provide continued evidence for protection of 2 doses of mRNA-1273 against SARS-CoV-2 infection over 8 months post-vaccination and during the Delta period, and against COVID-19 hospitalization and hospital death.

Recombinant Adjuvanted Zoster Vaccine and Reduced Risk of Coronavirus Disease 2019 Diagnosis and Hospitalization in Older Adults.

BACKGROUND: Some vaccines elicit nonspecific immune responses that may protect against heterologous infections. We evaluated the association between recombinant adjuvanted zoster vaccine (RZV) and coronavirus disease 2019 (COVID-19) outcomes at Kaiser Permanente Southern California. METHODS: In a cohort design, adults aged ≥50 years who received ≥1 RZV dose before 1 March 2020 were matched 1:2 to unvaccinated individuals and followed until 31 December 2020. Adjusted hazard ratios (aHRs) and 95% confidence intervals (CIs) for COVID-19 outcomes were estimated using Cox proportional hazards regression. In a test-negative design, cases had a positive severe acute respiratory syndrome coronavirus 2 test and controls had only negative tests, during 1 March-31 December 2020. Adjusted odds ratios (aORs) and 95% CIs for RZV receipt were estimated using logistic regression. RESULTS: In the cohort design, 149 244 RZV recipients were matched to 298 488 unvaccinated individuals. The aHRs for COVID-19 diagnosis and hospitalization were 0.84 (95% CI, .81-.87) and 0.68 (95% CI, .64-.74), respectively. In the test-negative design, 8.4% of 75 726 test-positive cases and 13.1% of 340 898 test-negative controls had received ≥1 RZV dose (aOR, 0.84 [95% CI, .81-.86]). CONCLUSIONS: RZV vaccination was associated with a 16% lower risk of COVID-19 diagnosis and 32% lower risk of hospitalization. Further study of vaccine-induced nonspecific immunity for potential attenuation of future pandemics is warranted.

Effectiveness of mRNA-1273 against SARS-CoV-2 Omicron and Delta variants.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron (B.1.1.529) variant is highly transmissible with potential immune escape. We conducted a test-negative case-control study to evaluate mRNA-1273 vaccine effectiveness (VE) against infection and hospitalization with Omicron or Delta. The large, diverse study population included 26,683 SARS-CoV-2 test-positive cases with variants determined by S gene target failure status (16% Delta and 84% Omicron). The two-dose VE against Omicron infection at 14-90 days was 44.0% (95% confidence interval, 35.1-51.6%) but declined quickly. The three-dose VE was 93.7% (92.2-94.9%) and 86.0% (78.1-91.1%) against Delta infection and 71.6% (69.7-73.4%) and 47.4% (40.5-53.5%) against Omicron infection at 14-60 days and >60 days, respectively. The three-dose VE was 29.4% (0.3-50.0%) against Omicron infection in immunocompromised individuals. The three-dose VE against hospitalization with Delta or Omicron was >99% across the entire study population. Our findings demonstrate high, durable three-dose VE against Delta infection but lower effectiveness against Omicron infection, particularly among immunocompromised people. However, three-dose VE of mRNA-1273 was high against hospitalization with Delta and Omicron variants.

Real-world effectiveness of the mRNA-1273 vaccine against COVID-19: Interim results from a prospective observational cohort study.

BACKGROUND: Phase 3 trials found mRNA-1273 was highly effective in preventing COVID-19. We conducted a prospective cohort study at Kaiser Permanente Southern California (KPSC) to determine the real-world vaccine effectiveness (VE) of mRNA-1273 in preventing COVID-19 infection and severe disease. METHODS: For this planned interim analysis, individuals aged ≥18 years receiving 2 doses of mRNA-1273 ≥24 days apart (18/12/2020-31/03/2021) were 1:1 matched to randomly selected unvaccinated individuals by age, sex, and race/ethnicity, with follow-up through 30/06/2021. Outcomes were COVID-19 infection (SARS-CoV-2 positive molecular test or COVID-19 diagnosis code) or severe disease (COVID-19 hospitalization and COVID-19 hospital death). Adjusted hazard ratios (aHR) and confidence intervals (CI) for COVID-19 outcomes comparing vaccinated and unvaccinated individuals were estimated by Cox proportional hazards models accounting for multiple comparisons. Adjusted VE was calculated as (1-aHR)x100. Whole genome sequencing was performed on SARS-CoV-2 positive specimens from the KPSC population. FINDINGS: This analysis included 352,878 recipients of 2 doses of mRNA-1273 matched to 352,878 unvaccinated individuals. VE (99·3% CI) against COVID-19 infection was 87·4% (84·8-89·6%). VE against COVID-19 hospitalization and hospital death was 95·8% (90·7-98·1%) and 97·9% (66·9-99·9%), respectively. VE was higher against symptomatic (88·3% [98·3% CI: 86·1-90·2%]) than asymptomatic COVID-19 (72·7% [53·4-84·0%]), but was generally similar across age, sex, and racial/ethnic subgroups. VE among individuals with history of COVID-19 ranged from 8·2-33·6%. The most prevalent variants were Alpha (41·6%), Epsilon (17·5%), Delta (11·5%), and Gamma (9·1%), with Delta increasing to 54·0% of variants by June 2021. INTERPRETATION: These interim results provide reassuring evidence of the VE of 2 doses of mRNA-1273 across age, sex, and racial/ethnic subgroups, and against asymptomatic and symptomatic COVID-19, and severe COVID-19 outcomes. Among individuals with history of COVID-19, mRNA-1273 vaccination may offer added protection beyond immunity acquired from prior infection. Longer follow-up is needed to fully evaluate VE of mRNA-1273 against emerging SARS-CoV-2 variants. FUNDING: Moderna Inc.

Effectiveness of mRNA-1273 against delta, mu, and other emerging variants of SARS-CoV-2: test negative case-control study.

OBJECTIVES: To evaluate the effectiveness of the mRNA-1273 vaccine against SARS-CoV-2 variants and assess its effectiveness against the delta variant by time since vaccination. DESIGN: Test negative case-control study. SETTING: Kaiser Permanente Southern California (KPSC), an integrated healthcare system. PARTICIPANTS: Adult KPSC members with a SARS-CoV-2 positive test sent for whole genome sequencing or a negative test from 1 March 2021 to 27 July 2021. INTERVENTIONS: Two dose or one dose vaccination with mRNA-1273 (Moderna covid-19 vaccine) ≥14 days before specimen collection versus no covid-19 vaccination. MAIN OUTCOME MEASURES: Outcomes included infection with SARS-CoV-2 and hospital admission with covid-19. In pre-specified analyses for each variant type, test positive cases were matched 1:5 to test negative controls on age, sex, race/ethnicity, and specimen collection date. Conditional logistic regression was used to compare odds of vaccination among cases versus controls, with adjustment for confounders. Vaccine effectiveness was calculated as (1-odds ratio)×100%. RESULTS: The study included 8153 cases and their matched controls. Two dose vaccine effectiveness was 86.7% (95% confidence interval 84.3% to 88.7%) against infection with the delta variant, 98.4% (96.9% to 99.1%) against alpha, 90.4% (73.9% to 96.5%) against mu, 96-98% against other identified variants, and 79.9% (76.9% to 82.5%) against unidentified variants (that is, specimens that failed sequencing). Vaccine effectiveness against hospital admission with the delta variant was 97.5% (92.7% to 99.2%). Vaccine effectiveness against infection with the delta variant declined from 94.1% (90.5% to 96.3%) 14-60 days after vaccination to 80.0% (70.2% to 86.6%) 151-180 days after vaccination. Waning was less pronounced for non-delta variants. Vaccine effectiveness against delta infection was lower among people aged ≥65 years (75.2%, 59.6% to 84.8%) than those aged 18-64 years (87.9%, 85.5% to 89.9%). One dose vaccine effectiveness was 77.0% (60.7% to 86.5%) against infection with delta. CONCLUSIONS: Two doses of mRNA-1273 were highly effective against all SARS-CoV-2 variants, especially against hospital admission with covid-19. However, vaccine effectiveness against infection with the delta variant moderately declined with increasing time since vaccination.

COVID-19 Vaccination and Non-COVID-19 Mortality Risk - Seven Integrated Health Care Organizations, United States, December 14, 2020-July 31, 2021.

By September 21, 2021, an estimated 182 million persons in the United States were fully vaccinated against COVID-19.* Clinical trials indicate that Pfizer-BioNTech (BNT162b2), Moderna (mRNA-1273), and Janssen (Johnson & Johnson; Ad.26.COV2.S) vaccines are effective and generally well tolerated (1-3). However, daily vaccination rates have declined approximately 78% since April 13, 2021†; vaccine safety concerns have contributed to vaccine hesitancy (4). A cohort study of 19,625 nursing home residents found that those who received an mRNA vaccine (Pfizer-BioNTech or Moderna) had lower all-cause mortality than did unvaccinated residents (5), but no studies comparing mortality rates within the general population of vaccinated and unvaccinated persons have been conducted. To assess mortality not associated with COVID-19 (non-COVID-19 mortality) after COVID-19 vaccination in a general population setting, a cohort study was conducted during December 2020-July 2021 among approximately 11 million persons enrolled in seven Vaccine Safety Datalink (VSD) sites.§ After standardizing mortality rates by age and sex, this study found that COVID-19 vaccine recipients had lower non-COVID-19 mortality than did unvaccinated persons. After adjusting for demographic characteristics and VSD site, this study found that adjusted relative risk (aRR) of non-COVID-19 mortality for the Pfizer-BioNTech vaccine was 0.41 (95% confidence interval [CI] = 0.38-0.44) after dose 1 and 0.34 (95% CI = 0.33-0.36) after dose 2. The aRRs of non-COVID-19 mortality for the Moderna vaccine were 0.34 (95% CI = 0.32-0.37) after dose 1 and 0.31 (95% CI = 0.30-0.33) after dose 2. The aRR after receipt of the Janssen vaccine was 0.54 (95% CI = 0.49-0.59). There is no increased risk for mortality among COVID-19 vaccine recipients. This finding reinforces the safety profile of currently approved COVID-19 vaccines in the United States.

Distribution of SARS-CoV-2 Variants in a Large Integrated Health Care System - California, March-July 2021.

Data from observational studies demonstrate that variants of SARS-CoV-2, the virus that causes COVID-19, have evolved rapidly across many countries (1,2). The SARS-CoV-2 B.1.617.2 (Delta) variant of concern is more transmissible than previously identified variants,* and as of September 2021, is the predominant variant in the United States.† Studies characterizing the distribution and severity of illness caused by SARS-CoV-2 variants, particularly the Delta variant, are limited in the United States (3), and are subject to limitations related to study setting, specimen collection, study population, or study period (4-7). This study used whole genome sequencing (WGS) data on SARS-CoV-2-positive specimens collected across Kaiser Permanente Southern California (KPSC), a large integrated health care system, to describe the distribution and risk of hospitalization associated with SARS-CoV-2 variants during March 4-July 21, 2021, by patient vaccination status. Among 13,039 SARS-CoV-2-positive specimens identified from KPSC patients during this period, 6,798 (52%) were sequenced and included in this report. Of these, 5,994 (88%) were collected from unvaccinated persons, 648 (10%) from fully vaccinated persons, and 156 (2%) from partially vaccinated persons. Among all sequenced specimens, the weekly percentage of B.1.1.7 (Alpha) variant infections increased from 20% to 67% during March 4-May 19, 2021. During April 15-July 21, 2021, the weekly percentage of Delta variant infections increased from 0% to 95%. During March 4-July 21, 2021, the weekly percentage of variants was similar among fully vaccinated and unvaccinated persons, but the Delta variant was more commonly identified among vaccinated persons then unvaccinated persons overall, relative to other variants. The Delta variant was more prevalent among younger persons, with the highest percentage (55%) identified among persons aged 18-44 years. Infections attributed to the Delta variant were also more commonly identified among non-Hispanic Black persons, relative to other variants. These findings reinforce the importance of continued monitoring of SARS-CoV-2 variants and implementing multiple COVID-19 prevention strategies, particularly during the current period in which Delta is the predominant variant circulating in the United States.