The Centers for Disease Control and Prevention reports that the coronavirus has infected more than half of Americans

Federal health officials reported Tuesday that sixty percent of Americans, including 75 percent of children, had contracted the coronavirus by February — another milestone in the pandemic that continues to confound expectations.

The highly contagious Omicron variant was responsible for much of the loss. In December 2021, when the variant began to spread, only half of the people had antibodies indicating a previous infection, according to new research from the Centers for Disease Control and Prevention.

While the numbers came as a shock to many Americans, some scientists said they expected the numbers to be even higher, given the infectious variables that have swept across the country over the past two years.

Some experts said there may be good news in the data. Acquisition of immunity at the population level may provide at least partial protection against future waves. This trend may explain why the boom now spreading in China and many countries in Europe has been silenced in the United States.

A high rate of previous infections may also mean that there are now fewer cases of life-threatening or fatal illnesses than infections. “We’re going to see less severe disease, more and more shift toward clinically mild disease,” said Florian Kramer, an immunologist at the Icahn School of Medicine in Mount Sinai, New York.

“It will be more and more difficult for the virus to cause massive damage,” he added.

Administration officials also believe that the data portends a new phase of the pandemic in which infections may sometimes be common but cause less damage.

At a press briefing on Tuesday, Dr. Ashish Jha, the new coordinator for Covid at the White House, said stopping infection “wasn’t even a political goal. The goal of our policy should be: Obviously, to reduce infection as much as possible, but to make sure people don’t get infected.” seriously ill.”

The average number of new confirmed cases per day in the United States — more than 49,000 as of Monday, according to the New York Times database — is comparable to levels last seen in late July, even as cases have risen more than 50 percent over the past two weeks, he attributed Infectious disease experts trend to new sub variants of Omicron.

Dr. Jha and other officials cautioned against complacency, urging Americans to continue receiving vaccinations and booster shots, saying that antibodies from previous infections do not guarantee protection against the virus.

During the Omicron wave, infections rose sharply among children and teens, according to new research. Previous infections increased less among adults 65 years of age or older, who have the highest vaccination rates and may be more likely to take precautions.

“Evidence for previous Covid-19 infections increased significantly among each age group,” Dr. Kristi Clark, the agency researcher who led the new study, said at a press briefing Tuesday.

The spread of infection raises a disturbing possibility: a possible increase in protracted Covid cases, a poorly understood constellation of chronic symptoms.

Up to 30 percent of people infected with the coronavirus may have persistent symptoms, including worrying changes in the brain and heart. Vaccination is thought to reduce the odds of prolonged Covid, although it is unclear how much.

“The long-term effects on health care are not clear, but they certainly deserve to be taken seriously, as some of the people will struggle for a long time to face the consequences,” said Bill Hanage, an epidemiologist at Harvard T.H. Chan School of Public Health. health.

Even a very small percentage of infected or vaccinated people who develop long-term COVID disease will translate into millions of people nationwide.

While the focus is often on preventing the health care system from collapsing in the light of the boom, “we should also be concerned that our health care system will be overwhelmed by the ongoing health care needs of the long-Covid population,” he said. Zoe McClaren, a health policy expert at the University of Maryland, Baltimore County.

There are still tens of millions of Americans who do not have immunity to the virus, and are still vulnerable to the short and long-term consequences of infection, said Dr. Tom Inglesby, director of the Center for Health Security at Johns Hopkins Bloomberg. College of Public Health.

“Betting that 60 percent is a big gamble,” he said. “For anyone who hasn’t been vaccinated and boosted, I would take this new data as a direct message to get this done or predict that the virus is likely to catch you if it hasn’t already.”

Although cases are back on the rise again, particularly in the Northeast, the rise in hospitalizations has been minimal, and deaths are still falling. According to the agency’s latest standards, more than 98 percent of Americans live in low- or moderate-risk communities.

Even among those hospitalized, CDC Director Dr. Rochelle Walinsky said, “We’re seeing lower oxygen use, fewer intensive care unit stays, and we’ve, fortunately, seen no increase in associated deaths.” “We hope the positive trends will continue.”

The country has recorded a nearly fivefold decline in PCR testing for the virus since Omicron’s heyday, so tracking new cases has become difficult. Dr. Walinsky said the reported number is much lower, about 70-fold, reflecting a “true and reliable decline in our overall cases.”

New sub-variants of Omicron, called BA.2 and BA.2.12.1, have replaced the previous iteration, BA.1, which began circulating in the country in late November and sent cases to record levels within weeks.

“Of course, more are infected now, because BA.2 will infect some of those who have avoided it until now,” said Dr. Hanage.

By February, three out of four children and teens had already contracted the virus, compared to a third of older adults, according to the new study.

Having so many children carrying the antibodies may provide relief for parents of those aged 5 and under, who are not eligible for vaccination, as many of them may have acquired at least some immunity through infection.

But Dr Clark urged parents to vaccinate eligible children once regulators have approved a vaccine for them, regardless of their previous infection. She noted that of children hospitalized with the virus, up to 30 percent may need intensive care.

Although many of these children also have other medical conditions, about 70 percent of cases of multisystem inflammatory disease, a rare result of Covid-19 infection, occur in otherwise healthy children.

“As a pediatrician and parents, I would fully support children receiving vaccinations, even if they become infected,” said Dr. Clark.

Some experts said they are concerned about the long-term consequences, even in children with mild symptoms.

“Given the very high rate of infection in children and adults that occurred earlier this year, I am concerned that prolonged Covid cases will rise as a result,” said Akiko Iwasaki, an immunologist at Yale University who studies the case.

To measure the percentage of the population infected with the virus, the study relied on the presence of antibodies produced in response to infection.

CDC researchers began assessing people’s antibody levels at 10 sites early in the pandemic, and have since expanded the effort to include all 50 states, the District of Columbia and Puerto Rico. The investigators used a test sensitive enough to identify previously infected people for at least one to two years after exposure.

The researchers analyzed blood samples collected from September 2021 to February 2022 for antibodies to the virus, then analyzed the data by age, gender and geographic location. The researchers specifically looked for a type of antibody that is produced after infection but not after vaccination.

Between September and December 2021, antibody prevalence in samples increased steadily by 1 to 2 percentage points every four weeks. But it jumped sharply after December, increasing by about 25 points by February 2022.

The percentage of samples containing antibodies increased from about 45 percent among children aged 11 and younger, and among adolescents aged 12-17, to about 75 percent in both age groups.

By February 2022, nearly 64 percent of adults ages 18 to 49, about 50 percent of those ages 50 to 64 and about 33 percent of older adults were infected, according to the study.

Despite the record high cases during the omicron surge, the reported statistics may not have captured all infections, because some people have few or no symptoms, may not have opted for testing or may have tested themselves at home.

According to an upcoming study by the Centers for Disease Control and Prevention, there may be more than three infections per reported case, Dr. Clark said.

Noah Welland Contributed to reporting from Washington.

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