The data comes from an ongoing study of blood samples sent to commercial laboratories across the United States.
By early December, an estimated 34% of Americans had antibodies that showed they had once been infected with the virus that causes Covid-19. By the end of February, following an avalanche of cases caused by the Omicron variant, this figure had risen to 58%.
“The highest jump in antibody detection was among children and adolescents,” said Dr. Kristie Clarke, a pediatrician who led the study for the CDC.
By February, about 75%, or 3 out of 4 children under the age of 18, had developed antibodies to Covid-19, according to the study. The lowest increase was among adults 65 years and older; The CDC estimates that 33% of seniors have been infected with Covid-19.
However, it is unclear what these test results mean for personal or societal protection against future infections.
“We still do not know how long infection-induced immunity will last, and we can again learn from the study whether all people who tested positive for SARS-CoV-2 antibodies still have protection against their previous infection,” Clarke said Tuesday.
For that reason, the CDC says it is still important for all Americans to stay up to date with their Covid-19 vaccines and get the recommended shots and boosters.
But, says Clarke, for people who have been infected within the past three months, “you may be able to wait for your second booster dose.”
The estimates are taken from random, anonymous tests of blood samples sent to commercial laboratories in the United States. The test measures antibodies made against parts of the virus that cause Covid-19; these antibodies are not generated by the vaccines, so the test is used to assess the percentage of the population that has been infected with the SARS-CoV-2 virus.
The CDC says the antibodies it measures for the study remain positive for at least two years after infection, so the study should capture infections that have occurred since the onset of the pandemic.
Clarke said the CDC does not recommend that people seek out an antibody test.
“This is not something we recommend on an individual level,” she said, “because it does not change our recommendation as to what you should do,” like staying up to date on vaccinations and discussing other preventative measures with your doctor.
The study comes as more infectious new variants, BA.2 and BA.2.12.1, dominate infection in the United States and cause cases and hospitalizations to rise again.
CDC Director Dr. Rochelle Walensky said Tuesday that the agency is very aware of Covid-19 cases in the Northeast.
“There are some areas of the country, especially in the Northeast, where we are seeing a higher number of cases and we are starting to see some admissions ticking up,” she said.
Walensky said health officials had not seen the numbers rise as much as we might have expected earlier in the pandemic, “thanks, I think, to a large amount of protection in society” and from vaccines.
“But it’s something we need to be aware of,” she said.