The Centers for Disease Control and Prevention released two studies on Thursday that underscored the importance of vaccinating children against the coronavirus.
One study found that serious problems among children 5 to 11 who had received the Pfizer-BioNTech vaccine were extremely rare. The other, which looked at hundreds of pediatric hospitalizations in six cities last summer, found that nearly all of the children who became seriously ill had not been fully vaccinated.
More than eight million doses of the Pfizer vaccine have been given to children 5 to 11 in the United States so far. But concerns about the unknowns of a new vaccine caused some parents to hesitate in allowing their children to be inoculated, including those who said they preferred to wait for the broader rollout to bring any rare problems to the surface.
By Dec. 19, roughly six weeks into the campaign to vaccinate 5- to 11-year-olds, the C.D.C. said that it had received very few reports of serious problems. The agency evaluated reports received from doctors and members of the public, as well as survey responses from the parents or guardians of roughly 43,000 children in that age group.
Many of the surveyed children reported pain at the site of the shot, fatigue, or a headache, especially after the second dose. Roughly 13 percent of those surveyed reported a fever after the second shot.
But reports of myocarditis, an inflammation of the heart muscle that has been linked in rare cases to coronavirus vaccines, remained scarce. The C.D.C. said there were 11 verified reports that had come in from doctors, vaccine manufacturers or other members of the public. Of those, seven children had recovered and four were recovering at the time of the report, the C.D.C. said.
The C.D.C. said that reporting rates for vaccine-related myocarditis appeared highest among boys and men aged 12 to 29.
A number of parents or doctors also reported instances of 5- to 11-year-olds receiving the incorrect, larger vaccine dose meant for older children and adults. The C.D.C. said that those problems were “not unexpected,” and that most such reports mentioned that the children experienced no problems afterward.
The C.D.C. detailed two reports of deaths, in girls ages 5 and 6, who the agency said had chronic medical conditions and were in “fragile health” before their shots. “On initial review, no data were found that would suggest a causal association between death and vaccination,” the agency said.
The C.D.C.’s separate report on pediatric hospitalizations provided additional evidence about the importance of vaccinating all eligible children. The study, which looked at more than 700 children under 18 who were admitted to hospitals with Covid-19 last summer, found that 0.4 percent of those children who were eligible for the shots had been fully vaccinated.
The study also found that two-thirds of all the hospitalized children had a comorbidity, most often obesity, and that about one-third of children 5 and older were sick with more than one viral infection.
Overall, nearly one-third of the children were so sick they had to be treated in intensive care units, and almost 15 percent needed medical ventilation. Among all those hospitalized, 1.5 percent of the children died, the study found. The six hospitals were in Arkansas, Florida, Illinois, Louisiana, Texas and Washington, D.C.
“This study demonstrates that unvaccinated children hospitalized for Covid-19 could experience severe disease and reinforces the importance of vaccination of all eligible children to provide individual protection and to protect those who are not yet eligible to be vaccinated,” the authors of the study wrote.