In a recent study published in the American Journal of Public Health, researchers assessed the effectiveness of vaccine-conferred immune protection against novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections, any-cause visits to hospitals and emergency departments (ED), and deaths in Indiana.
Studies have reported that coronavirus disease 2019 (COVID-19) vaccinations and natural SARS-CoV-2 infections induce immune responses against SARS-CoV-2 to prevent reinfections and COVID-19 severity outcomes. However, mixed immune responses have been reported regarding the immune protection provided by SARS-CoV-2 vaccines compared to COVID-19. Moreover, data on the age-stratified durability of protective immune responses induced by previous SARS-CoV-2 infections and COVID-19 vaccination against breakthrough infections and any-cause hospital admissions and deaths are limited.
About the study
In the present observational study, researchers examined the incidence of COVID-19, any-cause ED visits, hospitalizations, and deaths among previously vaccinated individuals compared to previously infected individuals in Indiana.
The team leveraged population health vaccination data and electronic medical records from the INPC (Indiana network for patient care) and the Indiana vaccination registry to match individuals who had been administered ≥1.0 dose of COVID-19 vaccines with individuals having prior COVID-19 history. The study participants were matched for index dates, sex, age, ethnicity/race, postal codes, clinical diagnosis, and comorbidities identified by the CDC (Centers for Disease Control and Prevention) to be a suggestive or conclusive risk factor for severe SARS-CoV-2 infections.
The cumulative incidence rates for SARS-CoV-2 infections (among vaccinees), reinfections (among previously infected individuals), any cause of emergency department (ED) visits, hospital admissions, and mortality were comparatively assessed. All outcome events in the study were extracted from INPC data, and mortality data were retrieved from the Indiana state mortality records. Individuals included individuals residing in Indiana aged ≥12 years, with ≥1.0 previously reported healthcare encounters with INPC between 1 January 2016 and 9 February 2022.
Follow-up of SARS-CoV-2-infected individuals was initiated after 30.0 days of infection up to six months or COVID-19 vaccination, the event that occurred first. Likewise, COVID-19 vaccinees were followed up for 30 days post-infection up to six months or SARS-CoV-2 infection, the event that occurred first. The index dates for vaccinees were described as 30.0 days post-initial COVID-19 vaccination, and for previously infected individuals, the dates 30 days post-initial SARS-CoV-2 infection were considered the index dates.
A total of 736,193 previously infected individuals and 2,798,709 previously vaccinated individuals were identified from the INPC between 29 November 2020 and 9 February 2022, from which 267,847 matched individual pairs were obtained. After six months of index dates, COVID-19 incidence was significantly greater among vaccinees (7.0%) than individuals with prior COVID-19 history (3.0%). Any-cause deaths among vaccinees, however, were 37.0% lesser than those observed among individuals with prior COVID-19 history.
The rates of any-cause emergency department visits and hospital admissions were 24.0% and 37.0% lower among the COVID-19 vaccinees compared to those with prior COVID-19 history. At six months, seven percent of individuals with prior COVID-19 history and five percent of previously vaccinated individuals had documented ED visits. After 6.0 months of the index dates, two percent of individuals with prior COVID-19 history and one percent of COVID-19 vaccinees were hospitalized. Six months post-index date, the death rates were 0.5% and 0.3% among individuals with prior COVID-19 history and COVID-19 vaccinees, respectively.
The six-month COVID-19 incidence rates among children were 8.0% and 5.0% among COVID-19 vaccinees and SARS-CoV-2-infected, respectively. Of note, despite more SARS-CoV-2 infections among vaccinees, the incidence rate of any-cause emergency department visits was lower among vaccinated (5.0%) compared to priorly infected children (7.0%). Additionally, the incidence rate of any-cause hospital admission was lower among the vaccinated children (0.3%) compared to children with prior COVID-19 history (0.6%). Death rates were very low among vaccinated children and those with previous SARS-CoV-2 infections, with non-significant differences.
Among adult individuals aged between 20 and 39 years, at six months, the incidence rates of SARS-CoV-2 infections were greater among vaccinees (8.0%) than previously infected individuals (3.0%) at six months. However, the incidence rate of any-cause ED visits was greater among previously infected individuals (8.0%) compared to previously vaccinated individuals (5.0%). Likewise, at 6.0 months, the incidence rates of hospital admission and deaths were significantly greater among individuals with prior COVID-19 history than among vaccinees.
Among adults between 40.0 years and 59.0 years, COVID-19 incidence was greater among previously vaccinated individuals (6.0%) compared to those previously exposed to SARS-CoV-2 (2.0%) at six months. However, the incidence rates for ED visits, hospitalizations, and deaths were significantly lower among vaccinees than individuals with prior COVID-19 history.
Among adults aged between 60.0 years and 79.0 years, COVID-19 incidence was greater among vaccinees (3.0%) compared to individuals with prior COVID-19 (2.0%) at six months. However, incidence rates for any-cause emergency department visits were lower among vaccinees (5.0%) compared to individuals having prior COVID-19 history(6.0%). Likewise, the incidence rates of any-cause hospitalizations and deaths were lower among vaccinees compared to individuals with prior SARS-CoV-2 exposure. Similar findings were obtained for individuals aged ≥80 years.
Overall, the study findings showed a significantly lower rate of any-cause ED visits, hospital admissions, and deaths among COVID-19 vaccinees than previously infected individuals across ages.
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