A huge proportion of American adults and even more children were infected by the coronavirus between December and February, perhaps explaining why cases have risen but not skyrocketed since.
New government data, released Tuesday, looked at blood drawn for medical purposes during that timeframe and found antibodies to the virus that causes COVID-19 in nearly 60% of people, up from one-third just three months earlier.
In adults under 50, 37% had antibodies in December, compared with 64% in February; in those 50-64, antibody presence rose from 29% to 50%; and in adults 65-and-older it climbed from 19% to 33%.
The increase was highest among the youngest. In children, the antibody rate rose from about 45% to 75%.
A previous infection provides some protection against a subsequent one, though it’s not clear how long that protection lasts, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said in a phone call with media.
“We do believe there is a lot of protection in the communities both from vaccination as well as from boosting and from prior infections,” Walensky said.
Vaccination and boosting provide more reliable protection, she said. Age groups that were most vaccinated saw the smallest increase in infections over the winter, as measured by antibodies.
Protection from vaccination also fades over time for both children and adults.
Next week, the CDC plans to release another study showing that each omicron BA.1 infection led to roughly three more infections, the highest rate of transmission seen during the pandemic, said Dr. Kristie Clarke, who led the CDC’s research.
Overall, infections have fallen substantially since their January peak, though they are starting to climb again, with an increase of 25% compared with a week earlier, Walensky said.
The omicron variant is responsible for virtually all infections in the United States, she said, but the BA.1 version, which predominated until February, has now been largely replaced by BA.2, accounting for 68% of infections nationwide. A subvariant called BA.2.12.1 is now gaining steam, accounting for nearly 30% of cases nationwide and a majority of cases in New York state, where it is believed to have originated.
While the BA.1 variant is believed to cause less severe than earlier versions of the SARS-CoV-2 virus, it was still so common that it triggered a huge spike in hospitalizations and deaths, Walensky said.
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The CDC is collecting data from BA.2 and BA.2.12.1.
“We’re not anticipating seeing more disease from some of these subvariants, but we’re actively studying it,” she said.
The antibody test used in the study would likely detect whether someone was infected at any point over the last two years, Clarke said. But it cannot determine a person’s level of protection against a new infection, which fades over time, as it does with vaccination.
The antibodies tested are different than those created by vaccines, so it’s possible to distinguish between people who were infected and those who were vaccinated.
The new study had several limitations, including that the blood was collected during checkups and for medical purposes other than looking for COVID-19, so it probably underrepresents the healthy population.
Contact Karen Weintraub at kweintraub@usatoday.com.
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