CDC study estimates more than 80% of Americans are now protected from COVID-19

About 83% of the U.S. population aged 16 and over have some degree of immunity to the coronavirus following vaccination or a previous infection, according to a new study from the Centers for Disease Control and Prevention (CDC).

Based on the antibody levels in about 1.4 million patient blood samples, CDC researchers estimated that 20% of Americans are immune to a previous infection – although the number varies by age and other demographics. The remaining 60% received an injection which confers similar immunity.

Experts previously cited 70-80% protection as a goal for herd immunity, but now many are saying we need more vaccinations and boosters to protect ourselves against the super-contagious Indian “Delta” variant.

Still, the study says the United States has had many more cases of Covid than those officially reported – perhaps more than double the official number.

As of September, 2.175 million Americans were now fully vaccinated, meaning they are protected against serious illnesses from Covid.

But millions more are also protected to some extent, as they have recovered from a previous case of Covid.

By fighting the coronavirus, a body’s immune system will remember how to respond to this invader – and will be better prepared for future encounters with Covid.

The CDC has an estimated 39.5 million Americans who have fallen ill with Covid – that’s 39.5 million people with some potential degree of immunity.

The actual number is likely much higher, however, as many people with mild or no cases were unaware of getting tested for Covid, and therefore were never recorded.

A new CDC study – published Thursday in the journal JAMA – suggests that actual number represents about 20% of Americans over 16.

The researchers determined this estimate with a seroprevalence survey – estimating the number of people with antibodies in their blood indicating a past immune response to Covid.

For the investigation, the CDC used donated blood samples from all 50 states, Washington DC and Puerto Rico.

In total, approximately 1.4 million blood samples were analyzed. These samples were collected between July 2020 and May 2021.

The researchers included a larger number of samples from blood donor regions with larger racial / ethnic minority populations and made other analytical adjustments, trying to match their final estimates to the demographic makeup of the United States.

They recognized, however, that a perfect match was not possible – and that other differences between the blood donors included in the study and the general population were likely not ignored.

The final estimate: As of May 2021, around 20% of Americans over 16 had already had a Covid infection.

That number increased significantly between July 2020 (3.5%) and December 2020 (11.5%), indicating that millions of Americans were infected – and gained immunity – during the winter wave.

Combining these estimates with vaccination figures, CDC researchers estimated that 83.3% of Americans over 16 had some degree of protection against Covid in May 2021.

Protection varies by demographic group. In May 2021, the Midwest and South had higher rates of Covid antibodies from a previous infection, compared to other regions – 23.5% and 22.2%, respectively.

The Midwest and South likely had earlier higher infection rates in May, as those areas were hit hardest during summer and winter outbreaks.

The Northeast had the highest overall immunity (previous infection plus vaccination), although -87.2% overall. The South had the lowest – 80.2 percent.

In a similar pattern, adolescents and young adults (aged 16-29) had the highest levels of Covid antibodies due to infection – 27% as of May 2021.

But this group had the lowest overall immunity, a rate of 76.1%. Teens and young adults are more likely to have mild cases of Covid than their elders – but they are also less likely to get the vaccine.

The elderly (over 65) had the highest combined immunity rate at 92%. According to CDC data, 82% of American seniors are fully immunized.

Blacks and Hispanics / Latin Americans had higher levels of Covid antibodies due to the infection than other racial and ethnic groups – 30% and 21.1%, respectively.

Combined immunity rates (previous infection plus vaccination) were highest among Asian and White Americans – 91% and 83.8%, respectively.

This combined immunity rate was 79.2% for Black Americans and 81.5% for Hispanics / Latin Americans, reflecting lower vaccination rates in these groups.

The CDC’s blood sampling study will continue until at least December 2021, and the researchers say they intend to continue releasing data.

With over 80% of Americans potentially protected from Covid, it’s easy to conclude that the country has achieved collective immunity.

After all, President Biden has made a 70% vaccination rate his national goal.

The Delta variant was a game-changer, however. Now, researchers say 95% or more of Americans need to be protected in order to block outbreaks.

CDC researchers note that “more research is needed” on the links between a previous infection and herd immunity.

“The protection induced by the infection could decrease more rapidly than the protection induced by the vaccine,” they write.

For this reason, experts recommend that Americans get vaccinated even if they have had a Covid infection in the past.

Yet the study indicates that we have had many more cases of Covid in the United States than those that were officially reported.

CDC researchers estimate that in July 2020, for every reported Covid case, two more could be detected by blood tests later.

In May 2021, that number dropped to one more case for each officially reported case.

The number may be higher again now, however, as many vaccinated people do not seek tests for mild colds and other symptoms that may indicate a breakthrough in the case of Covid.