Monitoring Incidence of COVID-19 Cases, Hospitalizations, and Deaths, by Vaccination Status

13 U.S. Jurisdictions, April 4-July 17, 2021

Heather M. Scobie, PhD; Amelia G. Johnson, DrPH; Amitabh B. Suthar, PharmD; Rachel Severson, MS; Nisha B. Alden, MPH; Sharon Balter, MD; Daniel Bertolino, MPH; David Blythe, MD; Shane Brady, MPH; Betsy Cadwell, MSPH; Iris Cheng, MS; Sherri Davidson, PhD; Janelle Delgadillo; Katelynn Devinney, MPH; Jeff Duchin, MD; Monique Duwell, MD; Rebecca Fisher, MPH; Aaron Fleischauer, PhD; Ashley Grant, MPH; Jennifer Griffin, PhD; Meredith Haddix, MPH; Julie Hand, MSPH; Matt Hanson, MD; Eric Hawkins, MS; Rachel K. Herlihy, MD; Liam Hicks, MPH; Corinne Holtzman, MPH; Mikhail Hoskins, MPH; Judie Hyun, MHS; Ramandeep Kaur, PhD; Meagan Kay, DVM; Holly Kidrowski, MPH; Curi Kim, MSPH; Kenneth Komatsu, MPH; Kiersten Kugeler, PhD; Melissa Lewis, MPH; B. Casey Lyons, MPH; Shelby Lyons, MPH; Ruth Lynfield, MD; Keegan McCaffrey; Chelsea McMullen, MS; Lauren Milroy, MPH; Stephanie Meyer, MPH; Leisha Nolen, MD; Monita R. Patel, PhD; Sargis Pogosjans, MPH; Heather E. Reese, PhD; Amy Saupe, MPH; Jessica Sell, MPH; Theresa Sokol, MPH; Daniel Sosin, MD; Emma Stanislawski, MPH; Kelly Stevens, MS; Hailey Vest, MPH; Kelly White, MPH; Erica Wilson, MD; Adam MacNeil, PhD; Matthew D. Ritchey; Benjamin J. Silk, PhD

Disclosures

Morbidity and Mortality Weekly Report. 2021;70(37):1284-1290. 

In This Article

Discussion

In 13 U.S. jurisdictions, rates of COVID-19 cases, hospitalizations, and deaths were substantially higher in persons not fully vaccinated compared with those in fully vaccinated persons, similar to findings in other reports.[2,3] After the week of June 20, 2021, when the SARS-CoV-2 Delta variant became predominant, the percentage of fully vaccinated persons among cases increased more than expected for the given vaccination coverage and a constant VE. The IRR for cases among persons not fully vaccinated versus fully vaccinated decreased substantially; IRRs for hospitalizations and deaths changed less overall, but moderately among adults aged ≥65 years. Findings from this crude analysis of surveillance data are consistent with recent studies reporting decreased VE against confirmed infection but not hospitalization or death, during a period of Delta variant predominance and potential waning of vaccine-induced population immunity†††.[4–6]

The findings in this report are subject to at least five limitations. First, combining unvaccinated and partially vaccinated persons resulted in lower IRR and VE estimates. Second, variable linkage of case surveillance, vaccination, hospitalization, and mortality data might have resulted in misclassifications that could influence IRR estimates; no substantial differences in ascertainment of outcomes by vaccination status were noted in jurisdictions that were able to assess this. Lags in reporting of deaths might have affected the second period differentially. Third, this was an ecological study in which IRRs lacked multivariable adjustments and causality could not be assessed (i.e., possible differences in testing or behaviors in vaccinated and unvaccinated persons). VE is being assessed through ongoing controlled studies. Fourth, the period when the SARS-CoV-2 Delta variant reached ≥50% overall prevalence was assumed to be the first week when most cases were infected with the Delta variant, but the week varied by jurisdiction. Finally, the data assessed from 13 jurisdictions accounted for 25% of the U.S. population, and therefore might not be generalizable.

Monitoring COVID-19 outcomes in populations over time by vaccination status is facilitated through reliable linkage of COVID-19 case surveillance and vaccination data. However, interpreting state-level variation by week might be challenging, especially for severe outcomes with small numbers. The framework used in this analysis allows for comparisons of observed IRRs and percentages of vaccinated cases, hospitalizations, and deaths to expected values. The data might be helpful in communicating the real-time impact of vaccines (e.g., persons not fully vaccinated having >10 times higher COVID-19 mortality risk) and guiding prevention strategies, such as vaccination and nonpharmacologic interventions.

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