Impact of full vaccination with booster doses during periods of different SARS-CoV-2 variant emergence and dominance in the United States

By Nidhi Saha, BDSReviewed by Danielle Ellis, B.Sc.Jan 27 2022

Previous reports indicate that the immunity induced by coronavirus disease 2019 (COVID-19) vaccination against severe COVID-19 illness faded in some individuals, and more so, after the emergence of the B.1.617.2 (Delta) variant of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Between August and November 2021, the Centers for Disease Control and Prevention (CDC) proposed additional COVID-19 vaccine doses for immunocompromised individuals and booster doses for adults ≥18 years.

Study: COVID-19 Incidence and Death Rates Among Unvaccinated and Fully Vaccinated Adults with and Without Booster Doses During Periods of Delta and Omicron Variant Emergence — 25 U.S. Jurisdictions, April 4–December 25, 2021. Image Credit: eamesBot/Shutterstock

The study

A recent study published in Morbidity And Mortality Weekly Report estimated the case and death rates and incidence rate ratios (IRRs) among unvaccinated and fully vaccinated (with booster doses) adults during pre-Delta (April–May 2021); Delta emergence (June 2021); Delta predominance (July–November 2021); and Omicron emergence (December 2021) periods in the United States. This study aimed to assess the impact of full vaccination with additional and booster doses.

Findings

In December 2021, B.1.1.529 (Omicron) variant emerged in the United States, which accounted for 72% of the identified cases by the end of the month. The average weekly, age-standardized case incidence rate ratios (IRRs) decreased among unvaccinated individuals during the same year. The reductions were – 13.9 pre-Delta to 8.7 with the emergence of Delta and 5.1 during the Delta predominance. While the case IRRs declined to 4.9 among fully vaccinated individuals with booster doses and 2.8 for those without booster doses with the emergence of the Omicron variant.

The risks for infection and COVID-19-associated death among unvaccinated individuals were 13.9 and 53.2 times compared with fully vaccinated individuals who received booster doses, and 4.0 and 12.7 times with respect to fully vaccinated individuals who were not given booster doses during the months October-November’21. The highest impact of booster doses was evident in individuals aged 50-64 and ≥65 years.

The study considered weekly COVID-19 cases reported and associated deaths by vaccination status—from April-December, 2021—including additional and booster doses commencing in October, from 25 state and local health departments – that periodically link case surveillance to vaccination data.

It could be predicted that ≥5% of each age group and jurisdiction would always be unvaccinated – rendering a fully vaccinated coverage of ≤95%.

Overall, 6,812,040 COVID-19 cases among unvaccinated individuals and 2,866,517 cases among fully vaccinated individuals were reported among adults aged ≥18 years in 25 U.S. jurisdictions, between April 4–December 25, 2021. Meanwhile, COVID-19 accounted for 94,640 and 22,567 fatalities by December 4 – in unvaccinated and fully vaccinated individuals, respectively. Average weekly, age-standardized rates of cases and deaths were higher during the Delta predominance and Omicron emergence compared to the pre-Delta and Delta emergence periods. These measures were consistently higher in all periods among unvaccinated individuals.

The age-standardized IRR for cases in unvaccinated versus fully vaccinated individuals progressively declined during June’21, July-November’21, and December’21 relative to April-May’21. These findings suggest corresponding changes in crude vaccine effectiveness (VE) for infection during the periods of Delta emergence, Delta predominance, and Omicron emergence. At the same time, the age-standardized IRRs for deaths among unvaccinated versus fully vaccinated individuals remained consistent.

Moreover, the rates of COVID-19 cases and fatalities were lowest among fully vaccinated individuals with a booster dose. Age-standardized case IRRs among unvaccinated individuals versus vaccinated individuals with a booster dose represented potential decreases in crude VE for infection from 93% to 80%. Corresponding reductions for crude VE against death were from 98% to 92%,

Age-standardized case IRRs among unvaccinated individuals versus fully vaccinated individuals without a booster dose represented decreases in VE from 75% to 64%. Notably, protection improved among individuals who received a booster dose, regardless of primary series vaccine product type. While, booster doses conferred the most protective benefits among the elderly aged ≥65 years, followed by in individuals aged 50-64 years compared with younger adults.

However, peaks in age-standardized case and death rates were recorded during August’21 (corresponding to the Delta period) and in case rates during the two latter weeks in December’21 (corresponding to the Omicron period). Although the differences in case rates between fully vaccinated individuals with and without a booster dose reduced over time, booster doses afforded more protection – even during Omicron emergence.

The findings confirmed that COVID-19 vaccines effectively reduced risks for SARS-CoV-2 infection and COVID-19-associated death during the Delta variant predominance and infection risk during the Omicron variant emergence. Vaccine effectiveness was the highest among those fully vaccinated and who had received a booster dose. Despite vaccination coverage, the case rates escalated during the Omicron predominance period among unvaccinated and vaccinated individuals.