The data come from an ongoing study of blood samples sent to commercial laboratories across the US.
At the beginning of December, an estimated 34% of Americans had antibodies showing that they had once been infected with the virus that causes Covid-19. By the end of February, after an avalanche of cases caused by the Omicron variant, that number had jumped 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, roughly 75%, or 3 in 4 children under the age of 18, had developed antibodies to Covid-19, according to the study. The lowest increase was among adults 65 and older; the CDC estimates that 33% of seniors have been infected with Covid-19.
It’s unclear what these test results mean for personal or community protection against future infections, however.
“We still do not know how long infection-induced immunity will last, and we cannot know from the study, again, whether all people who tested positive for SARS-CoV-2 antibodies continue to have protection from their prior infection,” Clarke said Tuesday.
For that reason, the CDC says it is still important for all Americans to stay up to date on their Covid-19 vaccines, getting the recommended shots and boosters.
However, Clarke says, for people who have been infected within the past three months, “you may be able to wait on your second booster dose.”
The estimates are pulled from random, anonymous tests of blood samples sent to commercial laboratories in the US. The testing measures antibodies made against pieces of the virus that causes Covid-19; these antibodies are not generated by the vaccines, so the testing is used to estimate what percentage of the population has been infected by the SARS-CoV-2 virus.
The CDC says the antibodies it measures for the study stay positive for at least two years after infection, so the study should be capturing infections that have happened since the beginning of the pandemic.
Clarke said that 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 doesn’t change our recommendation in terms of what you should do,” like staying up to date on vaccinations and discussing other preventive measures with your doctor.
The study comes as more contagious new variants, BA.2 and BA.2.12.1, are dominating transmission in the US and causing cases and hospitalizations to rise once again.
CDC Director Dr. Rochelle Walensky said Tuesday that the agency is paying close attention to Covid-19 cases in the Northeast.
“There are some areas of the country, particularly in the Northeast, where we’re seeing a higher number of cases and we’re starting to see some hospitalizations tick up,” she said.
Walensky said health officials had not seen the numbers tick up as much as we might have expected earlier in the pandemic, “thanks to, I believe, a large amount of protection in the community” and from vaccines.
“But this is something that we need to watch carefully,” she said.