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Next week can’t come early enough for Jacqueline Almeida.

He watched his friends roll their eyes when he wanted to meet outside. He tries to persuade his sister to vaccinate his son, but fails. On Twitter, strangers told her that putting her daughter in a mask was tantamount to child abuse.

Yet vaccines for the youngest Americans have faced imminent delay. “It was very disappointing to see things go hard every month,” said Ms. Almeida, 33, who lives in Franklin, Tenn.

But now there’s some good news: The vaccines for her 6-month-old son and 2-year-old daughter will be ready in days. Scientific advisors from the Food and Drug Administration on Wednesday recommended the Pfizer-BioNTech vaccine for children 6 months to 4 years old and the Moderna vaccine for children 6 months to 5 years old.

The agency itself authorized vaccines on Friday, and the Centers for Disease Control and Prevention will likely follow suit on Saturday. If all goes as planned, around 18 million children in this age group will qualify for the first time to be vaccinated against the coronavirus, the latest part of the country’s vaccination strategy.

However, after a series of delays by regulators, only one in five parents plan to vaccinate their young children immediately. one last poll.

In a letter to FDA officials in April, nearly 70 scientists submitted their assessments: The delay could have been avoided. Its arguments are technical, but with broad implications.

The scientists said the agency and manufacturers chose to evaluate the vaccines by monitoring blood levels of antibodies. But if regulators had taken into account other parts of the immune system, it may have become clear earlier that vaccines could prevent serious, if not infections, disease in young children.

In particular, the scientists argued that vaccine manufacturers should measure T cells that can kill infected cells and expel the virus from the body. “This would probably have allowed us to make a different decision about allowing the vaccine to advance sooner,” said John Wherry, director of the University of Pennsylvania Institute of Immunology and one of the signatories to the letter.

“If we don’t measure T cells, we’re missing a lot of what’s going on,” he added. “God, we’ve been in this business for 18 months, at this point we can put some energy into things like this.”

The FDA declined to comment on the letter, but Dr. Wherry said agency officials called scientists about a month ago to discuss their ideas.

Vaccine manufacturers conducted large trials to measure the effectiveness of vaccines in preventing symptomatic infection in adults. But in experiments on children, researchers looked at blood levels of antibodies after vaccination, comparing them to levels seen in young adults.

The FDA used this method, called immunobridging, to authorize the Pfizer-BioNTech vaccine for children ages 5 to 11 and adolescents ages 12 to 15. But in December, the companies reported that two doses of their vaccine did not produce high levels of antibodies in the United States. Children from 2 to 4 years old.

The companies decided to evaluate whether a third dose improved the performance of the vaccine. Then, over the winter, some young children in the clinical trial were infected with the Omicron variant.

Based on preliminary data from these infections, the FDA said it will consider allowing two doses of the vaccine as companies continue to test the third vaccine. provoked mixed reactions from parents and experts.

But the number of infections among children increased, and the accumulating data did not support the FDA’s decision and prompted the agency to take action. cancel scheduled review. back and forth left parents confused and his children remained vulnerable as variants of Omicron spread throughout the nation.

The CDC reported in April: 75 percent of children Many may have been infected with the coronavirus by March, mostly during the Omicron surge. A record number of children have been hospitalized, but still in far lower numbers than adults.

Experts said in interviews that more knowledge about the T-cell immunity that vaccines generate could help helpless parents get vaccinated sooner and prevent at least some of these hospitalizations.

Antibodies are necessary to neutralize the virus upon entry and prevent infection and can be easily measured in just a drop or two of blood. But while there is dozens of quick tests For antibody levels, at least a few milliliters of blood is required for evaluation of T cells and at least one day to test only a few samples.

MD, an infectious disease physician at Massachusetts General Hospital and scientific advisor to the CDC. Camille Kotton said that including T cells in the vaccine analysis would “really increase the complexity and cost of the study.”

“It’s never as easy as antibodies, but it will definitely help,” he said.

And not everyone is convinced that T cells are an important measure of immunity. An immunologist at the University of New South Wales in Sydney, Dr. Miles Davenport said low antibody levels alone may be enough to prevent serious illness.

Dr. “There are no studies that show that the level of vaccine-induced T cells predicts the risk of infection or serious disease,” Davenport said.

Yet epidemiological data show that hospitalizations and deaths in vaccinated people remain relatively low, even as antibody levels drop and infections rise sharply. Dr. This shows that something other than antibody levels protects people from serious illness, Wherry said.

“The death rate increases only in the very elderly population or those who are immunocompromised, where we are deficient or have weak T-cell responses,” he added. “There’s a lot of good conditional data out there, but we’re really missing a smoking gun.”

A new initiative at the University of Pennsylvania may provide some answers. Called Immune Health ProjectThe study will simultaneously evaluate antibody and T cell responses in immunocompromised patients after vaccination.

Unanswered questions about immunity in children may explain why roughly 40 percent of parents of young children are reluctant to get the vaccine: Less than 30 percent of children ages 5 to 11 have received two doses, and demand for youngest children may be even lower. .

Monica Lo, 35, a vice principal at a school in Seattle, is among those who hesitate. “The Covid vaccine is getting so fast and so fast that we wanted to give it a little more time,” she said.

Ms. Lo and her husband are both fully vaccinated, but their 7-year-old son, Gian, only received one vaccination in January before a planned trip to Hawaii. Ms. Lo said they decided to delay her second dose because of data suggesting that it could produce better immunity.

The couple also has a 2-year-old daughter, and Ms. Lo is pregnant with their third child, who could be born in July. However, Lo said that they are not planning to vaccinate their daughters yet. “We won’t be the first in line,” said Lo.