Okanogan County Public Health recently started showing different COVID-19 incidence rates for different populations. We’re calculating the COVID-19 incidence rate in the unvaccinated population, the vaccinated population, and the population as a whole.
Let’s break down what that means and how these calculations are done.
We’re tracking the number of new COVID-19 cases in the county and we’re tracking whether the person who tested positive for COVID-19 was vaccinated or unvaccinated. If a person tests positive for COVID-19 after being fully vaccinated (2 weeks after their second dose of Pfizer or Moderna, or 2 weeks after their J&J shot), then it is called a ‘breakthrough’ case.
By far, the large majority of new COVID-19 cases are occurring in people who are unvaccinated.
The incidence rate is a way to standardize the calculation so that comparisons can be made between groups with different size populations. So, we calculate the incidence rate based on a standard population size of 100,000 people.
- Based on the Office of Financial management (2020 estimate), the population of Okanogan County is 43,130
- As of July 12th, 2021: There were 34 active cases in the past 14 days (33 in unvaccinated individuals, 1 breakthrough case), and 47.1% of the population was fully vaccinated
- Calculation for incidence rate in unvaccinated individuals on July 12, 2021:
(0.471) * 43,130 = 20,314 fully vaccinated residents
43,130 – 20,314 = 22,816 unvaccinated residents
4. Incidence rate in unvaccinated population = 33 cases among 22,816 people = 145 cases/100,000 people/14 days
Incidence rate in vaccinated population = 1 case among 20,314 people = 5 cases/100,000 people/14 days
Incidence rate in total population = 34 cases among 43,310 people = 79 cases/100,000 people/14 days
There are limitations and assumptions in presenting the data this way:
We cannot say from the data that the vaccine is the only thing responsible for the difference in COVID-19 cases between groups. There are other differences between the people in each group that also affects the number of cases in each group.
We know that not everyone who has COVID-19 gets tested. This is true for vaccinated people and for unvaccinated people. There are probably more COVID-19 cases in each group than are counted here.
Why are we presenting the information this way?
In vaccinated individuals, there is currently low risk of contracting COVID-19 whereas the risk is higher among unvaccinated individuals. This is not clear when the incidence rate is shown for the total population as a whole.
The risk of COVID-19 infection remains high in our community for those who are unvaccinated. Because COVID-19 cases are occurring almost entirely in unvaccinated individuals, OCPH recommends that those who are unvaccinated should maintain COVID-19 safety protocols to protect themselves.