Reposted from the Cliff Mass Weather Blog

**I think I found something important.**

But I will let you be the judge. And I would love to hear from any epidemiologists that read my blog.

The number of COVID cases is increasing in Oregon and Washington, while most of the other states are seeing declining or stable COVID caseloads. The increases are sufficiently worrying that Governor Inslee may reverse the opening of the State.

There is a lot of talk about the need to get herd immunity, in which enough of the population will be vaccinated or have had COVID so that the virus lacks sufficient susceptible people for spread. **But no one seems to be sure about what percentage of immune individuals is needed. 60%, 80% or 90%. **Some scientific papers and media accounts suggest we will never get to herd immunity. But what is the truth?

**I am not an epidemiologist,** but it seems to me that there should be sufficient information to get some real answers to the above questions. And so I sat down with a cup of coffee (actually several) and the data from the Centers for Disease Control and cranked through the numbers.

**And I found something interesting. **

For each state, I estimated what percentage of the population is immune to COVID-19 and then I compared that to the change in the number of new cases over the past few weeks.

I found a significant relationship between the two. And I may have discovered the magic herd immunity percentage.

**Estimation of the Percent of the Population Immune to COVID-19**

First, I started with the percentage of each state’s population that has been reported to have had COVID-19, which ranges from roughly 4 (Oregon) to 14 (Rhode Island) percent.

There is an extensive literature suggesting the actual numbers of infected people are far higher than the official case numbers, with several research papers suggesting that a factor of three is reasonable. So I started by taking the CDC state case percentages and multiplying by 3.

Then I secured the vaccine data for each state, and based on the CDC numbers of single and double vaccinations, I assigned 60% of the total vaccinations as the number of people that have gotten at least a single shot. I then added this number to the 3 times case percentages noted above, to get a final fraction of immune individuals for each state, with numbers running from .56 (Oregon) to .936 (Rhode Island). This will undoubtedly be a bit of an overestimation because there will be some people that have had COVID that are vaccinated, but that would only shift my estimates of herd immunity by roughly 5-10% at most. In any case, the approach is consistent for all states.

**That Change in COVID Cases for April 15-April 28th**

My hypothesis is that increasing percentages of antibodies for COVID should be related to declines in COVID case numbers. Sot I next turned to the CDC COVID trends pages.

Here is the one for Oregon. Not good. Cases have been rising quite a bit (blue are daily values) and red are 7-day running means).