Direct impact of COVID-19 vaccination in Chile: averted cases, hospitalizations, ICU admissions, and deaths – BMC … – BMC Infectious Diseases

Covid-19 epidemic and vaccination campaign

At the end of the study period (July 2, 2022), 14,981,425 (95%) out of 15,740,549 Chileans aged 16 years or older had received at least one dose, 14,761,706 (94%) had received at least two doses, 13,510,471 (86%) had received at least three doses and 9,393,909 (60%) had received four doses. Vaccine doses were initially administered to healthcare workers starting on December 20, 2020, and the mass vaccination campaign began in early February 2021, prioritizing the elderly and individuals with comorbidities (Fig.2). At the end of August 2021, the first-booster (third-dose) vaccination campaign began, which continued to prioritize older individuals and vulnerable groups. The second-booster (fourth-dose) campaign commenced at the beginning of January, 2022, coinciding with the start of the Omicron wave in Chile.The second analysis modifies the assumptions by substituting the age pyramid from the 2021 census for the two-to-one combination of 2021 and 2022 census data used in the baseline scenario.

Cumulative proportion of the population vaccinated by age and each week in Chile

We estimated that 1,030,648 cases (95% ConfidenceInterval: 1,016,975-1,044,321), 268,784 (95% CI:264,524-273,045) hospitalizations, 85,830 (95% CI:83,466-88,194) ICU admissions, and 75,968 (95% CI:73,909-78,028) deaths related to COVID-19 were directly averted by vaccination among individuals aged 16 years or older between December 20, 2020 and July 2, 2022. It represents a reduction of 26% of cases, 66% of hospitalizations, 70% of ICU admissions and 67% of deaths with respect to a scenario without vaccination. Figure 3 shows a three-stage time series of the averted events. The first increase in the cumulative number of averted events between March and July 2021 corresponds to the administration of the second dose during the mass vaccination campaign and a wave of new cases due to the Lambda and Gamma variants. A second rise starting from October 2021 coincides with the booster-dose vaccination campaign and the Delta-variant wave while the third increase starting from January 2022 corresponds to the wave caused by the Omicron-variant.

Cumulative averted events in individuals 16 years of age and older in Chile. Each plot represents respectively the cumulative number of cases, hospital admissions, ICU admissions and deaths related to COVID-19 averted between December 20, 2020, and July 2, 2022, due to vaccination against COVID-19

Most preventions of severe outcomes were observed in individuals vaccinated with two doses: 125,472 (95% CI:123,453-127,491) hospitalizations, 43,113 (95% CI:41,955-44,272) ICU admissions and 40,036 (95% CI:38,806-41,267) deaths related to COVID-19 (Figure S1). Individuals 55 years old or older represented 30% of the Chilean population over 16 years old, but accounted for 42% of cases, 67% of hospitalizations, 73% of ICU admissions and 89% of deaths related to COVID-19 prevented (Table 1).

The first sensitivity analysis assumed a scenario in which the entire population remains susceptible to reinfection and severe outcomes over time (see counterfactual scenario A in Section 1.2 of the Supplementary Material 1 for more details). Under this assumption, we estimated that a total of 1,124,060 (95% CI:1,108,627-1,139,492) cases, 290,142 (95% CI:285,460-294,824) hospitalizations, 92,065 (95% CI:89,485-94,645) ICU admissions, and 80,979 (95% CI:78,745-83,214) deaths related to COVID-19 were prevented (Table S1). These estimates represent an increase of 9% for cases, 8% for hospitalizations, 7% for ICU admissions and 7% for deaths averted relative to the baseline scenario where infections detected over time were excluded. Thus, the results are fairly robust with respect to the population assumed to be susceptible.

The second analysis modifies the assumptions by substituting the age pyramid from the 2021 census for the two-to-one combination of 2021 and 2022 census data used in the baseline scenario. This adjustment yields a slightly different count of averted events spanning the study period (Figure S2), but the difference becomes much more pronounced with the onset of the Omicron wave in January 2022.

At the end of the study period, we estimate that 1,281,719 (95% CI:1,264,750-1,298,689) cases were averted, 329,941 (95 % CI:324,111-335,770) hospitalizations, 106,553 (95% CI:103,236-109,870) ICU admissions and 95,571 (95% CI:92,753-98,389) deaths were directly averted by vaccination. This connotes an importantincrease of 24% in cases, 23% in hospitalizations, 24% in ICU admissions and 26% in deaths averted with respect to the baseline scenario.

The substantial difference is due to the continuing reduction in the unvaccinated population as the vaccination campaign progresses. Consequently, the impact of the choice of census estimate used to compute this population becomes increasingly important over time.

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Direct impact of COVID-19 vaccination in Chile: averted cases, hospitalizations, ICU admissions, and deaths - BMC ... - BMC Infectious Diseases