About 83 percent of the U.S. population ages 16 and up has some degree of immunity to the coronavirus from vaccination or prior infection, a new Centers for Disease Control and Prevention (CDC) study estimates.
Based on antibody levels in about 1.4 million patients’ blood samples, CDC researchers estimated that 20 percent of Americans have immunity from prior infection – though the number varies by age and other demographics. The other 60 per cent have received a shot which confers similar immunity.
Experts previously cited 70 to 80 percent protection as a goal for herd immunity, but now many say we need more vaccinations and boosters to protect against the super-contagious Indian ‘Delta’ variant.
Still, the study indicates that the U.S. has had a lot more Covid cases than those that have been officially reported – possibly more than double the official count.
Over 80 percent of Americans over age 16 have some degree of protection against Covid, a new study estimates. Pictured: Vaccination at a high school in Central Falls, Rhode Island, February 2021
As of May 2021, 20 percent of Americans over age 16 have protection from a prior infection, while an additional 60 percent are protected by vaccination, the researchers estimate
As of September 2.175 million Americans are now fully vaccinated – meaning they’re protected against severe illness from Covid.
But millions of others are also protected to some extent, because they recovered from a previous Covid case.
Upon fighting off the coronavirus, a body’s immune system will remember how to react to this invader – and will be more prepared for future Covid encounters.
The CDC counts about 39.5 million Americans who have gotten sick with Covid – or, 39.5 million people with some potential degree of immunity.
The true number is likely much higher, however, because many people with mild cases or no cases didn’t know to get Covid tested, and thus were never recorded.
A new CDC study – published Thursday in the journal JAMA – suggests that this true number accounts for about 20 percent of Americans over age 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 survey, the CDC utilized blood donation specimens from all 50 states, Washington D.C., and Puerto Rico.
In total, about 1.4 million blood samples were surveyed. These samples were collected between July 2020 and May 2021.
The researchers included higher numbers of samples from blood donor regions with larger racial/ethnic minority populations, and made other analytical adjustments, attempting to match their final estimates to the demographic makeup of the U.S.
They acknowledged, however, that a perfect matchup wasn’t possible – and that other differences between the blood donors included in the study and the overall population likely weren’t unaccounted for.
The final estimate: in May 2021, about 20 percent of Americans over age 16 had a prior Covid infection.
That number jumped significantly from July 2020 (3.5 percent) and December 2020 (11.5 percent), indicating that millions of Americans were infected – and gained immunity – during the winter surge.
Combining these estimates with vaccination numbers, the CDC researchers estimated that 83.3 percent of Americans over age 16 had some degree of protection against Covid in May 2021.
By May 2021, the Midwest and South had higher Covid protection from prior infection (left map), while all parts of the country had significant protection from vaccination (right map)
Protection varies by demographic group. In May 2021, the Midwest and South had higher rates of Covid antibodies from prior infection, compared to other regions – 23.5 percent and 22.2 percent, respectively.
The Midwest and South likely had higher prior infection rates in May because these regions were hit harder in the summer and winter surges.
The Northeast had the highest overall immunity (prior infection plus vaccination), though – 87.2 percent total. The South had the lowest – 80.2 percent.
In a similar pattern, teenagers and young adults (ages 16 to 29) had the highest rates of Covid antibodies from infection – 27 percent in May 2021.
But this group had the lowest overall immunity, a rate of 76.1 percent. Teens and young adults are more likely to have mild Covid cases than their elders – but they’re also less likely to get vaccinated.
Seniors (over age 65) had the highest combined immunity rate, at 92 percent. According to CDC data, 82 percent of U.S. seniors are fully vaccinated.
In some cases, the groups with the highest protection from prior Covid cases were also the groups with lower vaccination rates – leading to lower overall protection
Black and Hispanic/Latino Americans had higher rates of Covid antibodies from infection than other racial and ethnic groups – 30 percent and 21.1 percent, respectively.
Combined immunity rates (prior infection plus vaccination) were highest among Asian and white Americans – 91 percent and 83.8 percent, respectively.
This combined immunity rate was 79.2 percent for black Americans and 81.5 percent for Hispanic/Latino 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 publishing data.
With over 80 percent of Americans potentially protected against Covid, it’s easy to jump to the conclusion that the country has attained herd immunity.
After all, President Biden made a 70 percent vaccination rate his national goal.
The Delta variant has changed the game, however. Now, researchers say that 95 percent or more Americans must be protected in order to stall outbreaks.
The CDC researchers note that ‘additional research is needed’ on the connections between a prior infection and herd immunity.
‘Infection-induced protection might wane more quickly than vaccine-induced protection,’ they write.
For this reason, experts recommend that Americans get vaccinated even if they had a prior Covid infection.
Still, the study indicates that we have had a lot more Covid cases in the U.S. than those that have been officially reported.
The CDC researchers estimate that, in July 2020, for every Covid case that was reported – two more could be detected by blood tests later.
In May 2021, that number dropped to one additional case for every case that was officially reported.
The number may be higher again now, however, as many vaccinated people are not seeking out tests for mild colds and other symptoms that may indicate a breakthrough Covid case.
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