A recent survey of blood donations has found that more than 80% of Americans over the age of 16 have some level of immunity to COVID-19 — a figure that could have massive implications on the country’s public health policy moving forward.

The survey, conducted by researchers at the U.S. Centers for Disease Control and Prevention, also found that twice as many people have been infected by the pathogen than have been officially counted, CNN reported. As of Thursday, more than 39 million Americans have tested positive for the virus.

According to CNN, the CDC team, led by Dr. Jefferson Jones, embarked on the study to “determine how close the US might be to some kind of herd immunity.” Though the news outlet was sure to mention the researchers “do not claim to have any kind of handle on that yet.”

They accomplished the task by testing about 1.4 million blood samples provided by 17 different blood collection organizations from all 50 states.

Upon studying the samples, the team found that between July 2020 and May 2021, the percentage of blood samples that carried SARS-CoV-2 spike antibodies increased from 3.5% to 83.3% over that time period.

Based on a sample of blood donations in the United States from July 2020 through May 2021, estimated #SARSCoV2 sero… https://t.co/OyWffTBmRL

— JAMA (@JAMA)

The researchers highlighted the prominent role widespread vaccine efforts have played in raising the percentage. Though they also noted that prior infection provides similar protection against the virus.

“Several large studies have shown that among individuals who are seropositive from prior SARS-CoV-2 infection, COVID-19 incidence is reduced by 80% to 95%, similar to vaccine efficacy estimates,” they wrote.

The researchers acknowledged their study has “several limitations,” however, including that all of the testing occurred prior to the Delta variant’s full emergence in the U.S.

Of note, a recent study out of Israel found that unvaccinated individuals with prior infection were 13 times less likely to contract the Delta variant than vaccinated individuals without prior infection.

Likewise, the study focused only on the presence of antibodies in the blood samples and didn’t measure other immune responses, including those involving memory T-cells.

“Additional research is needed on the association between combined seroprevalence, protection, and herd immunity,” the researchers wrote.

“The study will continue until at least December 2021, and results will be made available on the CDC’s website,” they added.

This content was originally published here.