Researchers studied blood samples from more than 200,000 Americans and looked for virus-fighting antibodies derived from infections, not vaccines. They found that signs of past infection spiked between December and February, as a more contagious variant of the omicron spread across the US.
Among Americans of all ages, about 34% had evidence of a previous infection in December. In just two months, 58% did it.
“I really expected it to increase. I didn’t expect it to increase this much,” said Dr. Christy Clark, co-lead of the CDC team that tracks the spread of coronavirus infection.
The news comes when Pfizer requested permission to offer a booster dose to children aged 5 to 11, as people aged 12 and over can do.
In the CDC report, the most striking growth was in children. The percentage of people aged 17 and under with antibodies rose from about 45% in December to about 75% in February.
The older people were, the less they had signs of past infections. This may be because older people have higher vaccination rates and may be more likely to take other precautions against COVID-19, such as wearing masks and avoiding crowds, Clarke said.
Reported cases of COVID-19 had a huge spike in December and January and then fell almost as sharply as they increased. But in recent weeks, the number of daily cases is rising again.
The number of cases is thought to be an underestimate, but officials believe the recent rise reflects a real rise in infections. Many COVID-19 infections are mild enough that patients do not seek medical attention or have confirmatory laboratory tests. CDC officials say they plan to soon release a study that estimates there have been three infections for every reported case in recent months.
In another recent trend, U.S. health officials say there has been an increase in COVID-19 hospitalizations in two weeks, though the numbers remain relatively low. The number of hospital admissions is about 1,600 per day, up 9% from the previous week, according to the CDC.
The available data nonetheless give grounds for hope as to how the pandemic is progressing, officials suggested.
“We don’t expect more severe disease from some of these sub-options, but we’re actively investigating them,” CDC director Dr. Rochelle Walensky said Tuesday.
Tests that show how many people have previously had an infection can detect antibodies for one to two years after infection, and possibly longer. Studies have shown that a previous infection can protect some people from severe illness and hospitalization, but the CDC stressed that those previously infected should still receive COVID-19 vaccines.
The study looked for any detectable antibody level; he did not distinguish how many people had antibody levels that might be protective. Scientists are still trying to understand what role these types of antibodies play in protecting against future exposure to the virus.
Officials continue to urge Americans to get vaccines and boosters that provide additional protection against COVID-19 for everyone, including those who have been previously infected.
The U.S. currently offers a booster dose starting at age 12, but Pfizer and BioNTech on Tuesday asked the Food and Drug Administration to allow healthy young children to also get one dose—about six months after their last. vaccinations. The companies cited a small study showing that an extra shot for children aged 5 to 11 years caused the production of antibodies capable of fighting the highly contagious omicron variant. Pfizer children’s shots are one-third of the dose given to people aged 12 and over.
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