COVID-19: how the risk changes for vaccinated people compared to unvaccinated people

Dr. Jeff Duchin, King County Health Officer

By Jeffrey Duchin, MD, Health Officer, 

Public Health - Seattle and King County 

Prof. Medicine/Infectious Diseases, 

University of Washington

We hear/read a lot about "breakthrough" infections and the % of people getting or hospitalized or dying with CoV-19 that are vaccinated. But this tells us nothing about what is most important: how the risk changes for vaccinated people compared to unvaccinated people

To determine the risk, you need to know how frequently these outcomes occur in vaccinated and unvaccinated groups (the rates of disease in each group). To compare most accurately, we should compare vaccinated and unvaccinated people of similar ages (adjusting for age).

Age-adjusted CoV-19 rates for past 30 days in King County, WA:

Compared to fully-vaccinated people, those not fully vaccinated are:

• 6 times more likely to test positive for COVID-19
• 37 times more likely to be hospitalized for COVID-19
• 67 times more likely to die due to COVID-19 related illness

Dark line shows cases among the unvaccinated or not fully vaccinated
Light line shows "breakthrough" cases among the fully vaccinated

Not fully vaccinated:

In the recent Delta surge, the age-adjusted rate of CASES among people who are not fully vaccinated increased from 7 daily cases/100,000 people at the beginning of July to 77 daily cases/100,000 people as of August 17, an increase of 70 new cases per day per 100,000 people.

Fully vaccinated:

During this same period, the age-adjusted rate of CASES among fully vaccinated people increased from 1 daily case/100,000 people to 10 daily cases/100,000 people, an increase of 9 new cases per day/100,000 people.

King county hospitalization rates
Not vaccinated (dark line) vs vaccinated (light line)

Among not-fully-vaccinated people, age-adjusted HOSPITALIZATIONS  increased from 1/100,000 people/day to 7/100,000 people/day vs. an increase from 0.04/100,000/day to 0.2/100,000 people/day for the fully vaccinated.

Bottom line: CoV-19 vaccines are doing a terrific job protecting the vaccinated from CoV-19, especially for hospitalizations and death.