COVID-19 vaccination may have prevented more than 16,000 hospitalizations and 4,000 deaths in one Washington county during the first eight months of the vaccine rollout, according to a new modeling study published last week in BMC Public Health.
The study, conducted by researchers from Public Health–Seattle & King County, used local surveillance data to compare rates of COVID hospitalization and death among vaccinated and unvaccinated residents of King County ages 12 years and older from February through September 2021, a period that included the rise of the Alpha and Delta SARS-CoV-2 variants and the first COVID vaccinations.
During that time, King County recorded 3,086 hospitalizations and 591 deaths attributed to COVID. The researchers estimated that vaccination prevented an additional 16,607 hospitalizations and 4,307 deaths. Without vaccination, the researchers note, “the number of COVID-19 hospitalizations would have been over six times greater than was observed, and the number of COVID-19 deaths nearly eight times greater.”
Older adults experienced the greatest benefit
By the end of September 2021, 1.61 million King County residents (71% of the county’s total population) had completed a primary COVID vaccine series, compared with 51% of people nationwide. Coverage was particularly high among older adults, reaching 95% among those ages 65 and older.
The number of COVID-19 hospitalizations would have been over six times greater than was observed, and the number of COVID-19 deaths nearly eight times greater.
Older adults also experienced the greatest benefit. Vaccination was estimated to have prevented 8,314 hospitalizations and 3,125 deaths among people 65 and older, accounting for half of all averted hospitalizations and nearly three-quarters of prevented deaths.
“Our results suggest this level of vaccination prevented a substantial number of hospitalizations and deaths due to COVID-19 in King County residents aged 12 years and older,” write the researchers.
Sifting primary, incidental hospital cases
To bolster their findings, the researchers distinguished hospitalizations caused by COVID from those in which a patient tested positive for COVID but the infection wasn’t the reason for hospitalization. They found that 638 of the 3,791 hospitalizations were incidental. The ability to distinguish between primary and incidental COVID hospitalizations is a key strength of the study, argue the authors.
“Given that an estimated 32% of COVID-19 infections are asymptomatic and up to 26% of COVID-19 hospitalizations may be misclassified, distinguishing incidental hospitalizations from true COVID-19 related hospitalizations increases the robustness of the dataset used to ascertain COVID-19 outcomes averted,” they write.
One limitation of the study is the assumption that vaccinated people experienced severe outcomes at the same rate as unvaccinated people. That assumption can’t account for differences in health-seeking and infection-prevention behaviors such as masking and physical distancing between the two groups.
The findings help demonstrate the population-level impact of the initial COVID vaccine rollout, the authors conclude. They also note that an analysis using surveillance data could help other local health departments quantify the benefits of vaccination in their own communities.
