Scientific advisers to the Centers for Disease Control and Prevention are meeting on Saturday to decide whether the benefits of Covid vaccines outweigh the risks for children under 5, the last Americans to qualify for the shots.
The meeting, which will be streamed live here, is scheduled to begin at 10 a.m. Eastern time. The advisers are all but guaranteed to vote yes, despite reservations about the paucity of data, especially regarding the efficacy of the Pfizer-BioNTech vaccine.
Earlier this week, another panel of experts advising the Food and Drug Administration unanimously backed the vaccines. The F.D.A. on Friday authorized the Moderna vaccine for children ages 6 months through 5 years, and the Pfizer vaccine for children ages 6 months through 4 years. (Pfizer-BioNTech’s vaccine has been available to children ages 5 and older since November.)
On Friday, the C.D.C.’s advisers heard evidence supporting the effectiveness of the vaccines in the youngest children. But the committee repeatedly pressed Pfizer on its estimates and noted that three doses of that vaccine would be needed to protect children, compared with two doses of the Moderna vaccine.
Both vaccines are safe, and both produced antibody levels similar to those seen in young adults. If the committee’s endorsement on Saturday is swiftly followed by a green light from the agency’s director, Dr. Rochelle Walensky, states are preparing to begin immunizing the children next week.
Among the tasks facing the C.D.C. panel on Saturday is the difficulty of recommending two very different vaccines for the same population.
“The implementation of these two rollouts is going to be incredibly challenging,” said Katelyn Jetelina, a public health expert and author of the widely read newsletter “Your Local Epidemiologist.”
“There’s going to have to be a lot of proactive communication about the difference between the two and the implications of taking one over the other,” she said.
In its clinical trials, Moderna found that two shots of its vaccine, each with one-fourth of the adult dose, produced antibody levels that were at least as high as those seen in young adults.
The company estimated the vaccine’s efficacy against symptomatic infection at about 51 percent among children ages 6 to 24 months, and 37 percent among children ages 2 through 5. The side effects were minor, although about one in five children experienced fevers.
Based on those data, the F.D.A. authorized two shots of the Moderna vaccine, spaced four weeks apart.
The Pfizer-BioNTech vaccine also produced a strong immune response, but only after three doses, company officials told the scientific advisers on Friday.
Two doses of the vaccine were inadequate, they said — justifying the F.D.A.’s decision in February to delay authorizing the vaccine until regulators had data regarding three doses. Two doses may not have been enough because the company gave the children just one-tenth of the adult dose in each shot, some advisers said.
The vaccine has an overall efficacy of 80 percent in children under 5, Pfizer’s scientists claimed on Friday. But that calculation was based on just three children in the vaccine group and seven who received a placebo, making it an unreliable metric, the C.D.C.’s advisers noted.
“We should just assume we don’t have efficacy data,” said Dr. Sarah Long, an infectious diseases expert at Drexel University College of Medicine. But Dr. Long said she was “comfortable enough” with other data supporting the vaccine’s potency.
Three doses of the Pfizer vaccine produced antibody levels comparable to those seen in young adults, suggesting that it is likely to be just as effective.
“The Pfizer is a three-dose series, but as a three-dose series, it’s quite effective,” said Dr. William Towner, who led vaccine trials for both Moderna and Pfizer at Kaiser Permanente in Southern California.
Either vaccine would be better than none, Dr. Towner added. He predicted that some parents may opt for Moderna because bringing children to a pediatrician for two shots is easier than arranging for them to receive three doses.
The Pfizer vaccine was authorized for children 5 to 11 in November, but fewer than 30 percent in that age group have received two shots. Parents of the youngest children may be more willing to opt for a Covid vaccine if it can be offered alongside other routine immunizations, Dr. Towner said.
“That’s the area that a lot of people are not sure of right now,” he said. “I’m hoping there’ll be some guidance offered around that.”