Incidence of SARS-CoV-2 Infection, Emergency Department Visits …

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Population-based rates of infection with SARS-CoV-2 (the virus that causes COVID-19) and related health care utilization help determine estimates of COVID-19 vaccine effectiveness and averted illnesses, especially since the SARS-CoV-2 B.1.617.2 (Delta) variant began circulating in June 2021. Among members aged ≥12 years of a large integrated health care delivery system in Oregon and Washington, incidence of laboratory-confirmed SARS-CoV-2 infection, emergency department (ED) visits, and hospitalizations were calculated by COVID-19 vaccination status, vaccine product, age, race, and ethnicity. Infection after full vaccination was defined as a positive SARS-CoV-2 molecular test result ≥14 days after completion of an authorized COVID-19 vaccination series.* During the July–September 2021 surveillance period, SARS-CoV-2 infection occurred among 4,146 of 137,616 unvaccinated persons (30.1 per 1,000 persons) and 3,009 of 344,848 fully vaccinated persons (8.7 per 1,000). Incidence was higher among unvaccinated persons than among vaccinated persons across all demographic strata. Unvaccinated persons with SARS-CoV-2 infection were more than twice as likely to receive ED care (18.5%) or to be hospitalized (9.0%) than were vaccinated persons with COVID-19 (8.1% and 3.9%, respectively). The crude mortality rate was also higher among unvaccinated patients (0.43 per 1,000) than in fully vaccinated patients (0.06 per 1,000). These data support CDC recommendations for COVID-19 vaccination, including additional and booster doses, to protect individual persons and communities against COVID-19, including illness and hospitalization caused by the Delta variant (1).

As of November 15, 2021, SARS-CoV-2 had infected approximately 46 million persons in the United States and caused approximately 759,000 deaths (2). A surge in cases, hospitalizations, and deaths began in June 2021 with the emergence of the Delta variant; after July 4, Delta became the predominant lineage in the U.S. Pacific Northwest (3). As of November 15, approximately 68% of the U.S. population had received ≥1 dose of an authorized COVID-19 vaccine, and approximately 59% of the population was fully vaccinated (4). To understand what percentage of authorized COVID-19 vaccine recipients developed infection resulting in ED visits or hospitalizations compared with unvaccinated persons, the incidence and characteristics of illness in vaccinated and unvaccinated persons with SARS-CoV-2 infection were evaluated.

Surveillance for SARS-CoV-2 infection was conducted within Kaiser Permanente Northwest (KPNW), an integrated health care system in Oregon and Washington. Persons aged ≥12 years with continuous health plan enrollment during the July 4–September 25, 2021 surveillance period were included. SARS-CoV-2 infections were identified from nucleic acid amplification test (NAAT) results among symptomatic or asymptomatic persons performed by a KPNW or an affiliated laboratory; rapid antigen tests were not available from KPNW and test results from other settings (e.g., home and school) were not included. Cases were identified through September 11 to permit 2 weeks of follow-up after testing to identify health care utilization.

Vaccination data were obtained from the KPNW electronic medical record, health insurance claims, and the Oregon state immunization information system. Fully vaccinated persons were defined as those with ≥2 doses of an mRNA vaccine product (Pfizer-BioNTech or Moderna) or 1 dose of the Janssen (Johnson & Johnson) vaccine completed ≥14 days before the NAAT. Persons with partial vaccination, defined as receipt of only 1 dose or <14 days since receipt of the second dose of Pfizer-BioNTech or Moderna vaccine, or <14 days since receipt of Janssen vaccine, were excluded (5). Unvaccinated persons were those who had no record of COVID-19 vaccination by September 25, 2021.

Age, sex, self-reported race and ethnicity, health care utilization, and underlying medical conditions were obtained from the KPNW electronic medical record. Health care utilization included virtual telephone and video visits, outpatient clinic visits, ED visits, and hospitalizations during the period 3 days before through 14 days after a positive SARS-CoV-2 NAAT test result. Among persons who were hospitalized during the surveillance period, medical records were manually reviewed to ascertain whether the hospitalization was associated with COVID-19, determined by provider notes documenting diagnosis, symptoms, or treatment consistent with COVID-19 (5). Information about length of stay, intensive care unit (ICU) admission, and intubation and mechanical ventilation was also abstracted. All records of deaths were also manually reviewed.

Incidence of SARS-CoV-2 infection was calculated by dividing the number of persons with a positive test result by the number of fully vaccinated and unvaccinated persons. Rates were stratified by COVID-19 vaccination status, vaccine product, age, sex, race, and ethnicity, and 95% CIs were calculated assuming the Poisson distribution. Because race and ethnicity were unknown in >10% of the study population, multiple racial groups were combined into a non-White, non-Hispanic category for some analyses. Crude mortality rates were calculated by dividing the number of deaths among persons with a SARS-CoV-2 infection by the number of fully vaccinated and unvaccinated persons. To compare the risk for infection between vaccinated and unvaccinated persons, incidence rate ratios (IRRs) were estimated along with 95% CIs using Poisson regression models with log link function, overall and within demographic subgroups. All analyses were conducted using SAS (version 9.4; SAS Institute). This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy.^†

Among 482,464 eligible persons identified during the surveillance period, 137,616 (28.5%) were unvaccinated and 344,848 (71.5%) were fully vaccinated. Most (66.5%) vaccinated persons received Pfizer-BioNTech, 27.8% received Moderna, and 5.8% received Janssen.

A total of 7,155 laboratory-confirmed SARS-CoV-2 infections were identified, including 4,146 (57.9%) among unvaccinated and 3,009 (42.1%) among vaccinated persons (Table 1). Overall incidence was 30.1 per 1,000 unvaccinated persons and 8.7 per 1,000 vaccinated persons (IRR = 3.5). IRRs across most strata indicated that incidence was at least three times higher among unvaccinated than among vaccinated persons; IRRs were highest among unvaccinated multiple race persons (4.3), Black persons (4.2), Asian persons (4.1), and adolescents aged 12–17 years (8.9).

Within the vaccinated group, incidence varied by COVID-19 vaccine product received. The highest incidence occurred among Janssen vaccine recipients (15.3 per 1,000), followed by Pfizer-BioNTech (9.1); the lowest incidence was among Moderna recipients (6.5). Vaccinated Hispanic or Latino persons had a higher incidence of SARS-CoV-2 infection (13.4 per 1,000) than did non-Hispanic persons (8.7).

Among unvaccinated persons with SARS-CoV-2 infections, 18.5% had an ED encounter, and 9.0% were hospitalized, compared with 8.1% and 3.9%, respectively, of vaccinated patients (Table 2). Fifty-nine deaths occurred in unvaccinated patients, including 58 who were hospitalized; 22 deaths occurred among fully vaccinated patients, including 21 who were hospitalized. The crude mortality rate among unvaccinated persons (0.43 per 1,000) was sevenfold higher than that among fully vaccinated persons (0.06).

Among 492 hospitalizations, 100 of 117 (85%) that occurred in vaccinated persons and 348 of 375 (93%) in unvaccinated persons were determined to be COVID-19–related after medical record review (Table 3). COVID-19 hospitalizations were rare among fully vaccinated adolescents and young adults; 72% of hospitalizations among fully vaccinated persons occurred in persons aged ≥65 years, (median age = 72 years), 89% of vaccinated persons who were hospitalized because of COVID-19 had at least one underlying medical condition, 15% required ICU admission, and 21 (21%) patients died. In contrast, hospitalizations among unvaccinated persons were more evenly distributed across age groups: 33% were among persons aged ≥65 years (median age = 57 years), 63% had at least one underlying medical condition, 27% required ICU admission, and 58 (17%) died. The median age at death was 78 years (range = 54–94 years) among fully vaccinated and 68 years (range = 37–100 years) among unvaccinated hospitalized patients.