Model-Based Analysis of mRNA COVID-19 Vaccination Effectiveness in Washington and Oregon
Mia Moore, PhD, presents findings showing 74% reduction in hospitalizations and key insights into primary vaccination and booster impact on population immunity in Washington and Oregon.
A study evaluating mRNA COVID-19 vaccination effectiveness in Washington and Oregon found a 74% reduction in hospitalizations from January 2020 to December 2022. Primary vaccination was particularly effective during the Alpha and Delta waves, while booster doses contributed 15% to the reduction.
Researchers used a model-based counterfactual approach, accounting for immunity buildup, variant emergence, and social distancing changes. Vaccination averted 90% of hospitalizations during the Alpha wave and 87% during the Delta wave. Younger adults, aged 18-49, played a crucial role by indirectly preventing 42.4% of hospitalizations among those aged 65+.
At CROI 2025, Mia Moore, PhD, Staff Scientist at Fred Hutchinson Cancer Research Center presented these findings, she said,"We aimed to track individuals' immunity status over time, considering both vaccination and infection-induced immunity. Our model allows us to understand what might have happened in a vaccine-free scenario and how the vaccination campaign averted hospitalizations."
The study found that vaccination reduced hospitalizations by 74%, with booster doses alone contributing 15% of that reduction. The benefits of primary vaccination were most notable in 2021, with hospitalizations reduced by 90% during the Alpha wave and by 87% during the Delta wave. Notably, vaccination had a particularly significant impact in older populations, which exhibited higher vaccination uptake.
"During the Alpha and Delta waves, vaccination had a tremendous impact on hospitalizations," Moore explained. "Without the vaccine, these waves would have caused much greater hospitalization rates, similar to what we saw in the early days of the pandemic."
The analysis also revealed the importance of vaccinating younger adults, particularly those aged 18-49. Vaccinating this age group indirectly prevented 42.4% of hospitalizations among individuals aged 65 and older, suggesting that younger adults played a key role in reducing transmission and protecting older, more vulnerable populations.
These results suggest that the COVID-19 vaccination campaign was highly effective, particularly during the Alpha and Delta waves. While booster uptake was limited, booster doses still contributed to a significant reduction in hospitalizations during the Omicron wave.
"Booster doses, even with limited uptake, helped reduce hospitalizations during the Omicron wave," Moore said. "It shows that while vaccine effectiveness wanes over time, boosters still offer significant benefits in reducing severe outcomes, especially among older populations."
The study also highlighted the importance of vaccinating individuals aged 18-49, emphasizing that vaccination in younger adults is just as crucial as vaccinating those aged 50 and older due to the indirect benefits in reducing transmission to older individuals.
Moore also shared her broader concerns about vaccine equity, particularly for marginalized populations. "Vaccine equity remains a critical issue," she noted. "Communities of color were disproportionately affected by COVID-19, as many individuals worked in essential roles where social distancing wasn't possible. Vaccines became an essential tool for protecting these groups."
Although, Moore pointed out the challenges posed by a lack of data on race and ethnicity in vaccine distribution. "We were unable to break down the impacts by race and ethnicity due to a lack of comprehensive data. This gap underscores the importance of open and federally available data to inform future public health efforts."
Moore said that the findings underscore the need for ongoing vaccination efforts, particularly among younger populations, to continue reducing COVID-19 hospitalizations. "The critical importance of vaccination for both at-risk individuals and the broader population cannot be overstated," she concluded. "It’s essential to protect not just the most vulnerable but the entire community, particularly in light of emerging variants."
The study utilized data from 16 regions across Washington and Oregon from January 2020 to December 2022, simulating infection and hospitalization scenarios in a counterfactual vaccine-free scenario. The team calibrated their model based on over 100 papers to ensure accuracy in tracking immunity status and the evolution of variants throughout the study period. The findings offer a detailed analysis of how vaccination campaigns can significantly reduce hospitalizations and how indirect effects play a role in population immunity.
Moore concluded, "It’s clear that vaccination, especially among younger adults, has been a pivotal strategy in mitigating hospitalizations and protecting vulnerable populations."
