• Three out of every 5 people in the U.S. now have antibodies from a previous Covid-19 infection, according to a new CDC analysis.
• The proportion is even higher among children, demonstrating how widespread the virus was during the winter omicron surge.
• CDC officials told reporters on a call Tuesday that the study did not measure whether people with prior infections had high enough antibody levels to protect against reinfection and severe illness.
• However, CDC Director Dr. Rochelle Walensky said health officials believe there is a lot of protection against the virus in communities from vaccination, boosting and infection taken together.

Three out of every 5 people in the U.S. now have antibodies from a previous Covid-19 infection with the proportion even higher among children, demonstrating how widespread the virus was during the winter omicron surge, according to data from the Centers for Disease Control and Prevention.

The proportion of people with natural Covid antibodies increased substantially from about 34% of the population in December to about 58% in February during the unprecedent wave of infection driven by the highly contagious omicron variant. The CDC's analysis didn't factor in people who had antibodies from vaccination.

The CDC published the data in its Morbidity and Mortality Weekly Report on Tuesday.

The increase in antibody prevalence was most pronounced among children, indicating a high rate of infection among kids during the winter omicron wave. About 75% of children and teenagers now have antibodies from past Covid infections, up from about 45% in December.

The high rate of infection among children is likely due to lower vaccination rates than adults. Only 28% of children 5- to 11-years-old and 59% of teens 12- to 17-years-old were fully vaccinated as of April. Children under 5-years-old are not yet eligible for vaccination.

About 33% of people ages 65 and older, the group with the highest vaccination rate, had antibodies from infection. Roughly 64% of adults ages 18 to 49 and 50% of people 50 to 64 had the antibodies.

The CDC analyzed about 74,000 blood samples every month from September through January from a national commercial lab network. The sample size decreased to about 46,000 in February. The CDC tested the samples for a specific type of antibody that is produced in response to Covid infection, not from vaccination.

CDC officials told reporters on a call Tuesday that the study did not measure whether people with prior infections had high enough antibody levels to protect against reinfection and severe illness. However, CDC Director Dr. Rochelle Walensky said health officials believe there is a lot of protection in communities across the country from vaccination, boosting and infection taken together, while cautioning that vaccination is the safest strategy to protect yourself against the virus.

"Those who have detectable antibody from prior infection, we still continue to encourage them to get vaccinated," Walensky told reporters during the call. "We don't know when that infection was. We don't know whether that protection has waned. We don't know as much about that level of protection than we do about the protection we get from both vaccines and boosters."

Scientists in Qatar affiliated with Cornell University found that natural infection provides about 73% protection against hospitalization if a person is reinfected with BA.2. However, three doses of Pfizer's vaccine provided much higher protection against hospitalization at 98%. The study, published in March, has not undergone peer-review.

About 66% of the U.S. population is fully vaccinated and 77% have received at least one dose, according to data from the CDC.

Infections and hospitalizations have dropped more than 90% from the peak of the omicron wave in January when infections in the U.S. soared to an average of more than 800,000 a day. New cases are rising again due to the BA.2 subvariant. Another subvariant, BA.2.12.1, is now gaining ground in the U.S., representing about 29% of new infections, according to CDC data. Walensky said the public health agency believes BA.2.12.1 spreads about 25% faster than BA.2. However, she said the CDC does not expect to see more severe disease from BA.2.12.1, though studies are ongoing.

More than 98% of the U.S. population lives in areas where they do not need to wear masks indoors under CDC guidance due to low Covid community levels, which takes into account both infections and hospitalizations. A U.S. district judge last week struck down the CDC's mask mandate for public transportation, though the Justice Department has filed an appeal. Walensky said the CDC continues to recommend that people wear masks on public transportation.