Over the past 10 months, as millions of children and teenagers received the Pfizer-BioNTech vaccine, the companies sat on the sidelines, its shot limited to adults.
Moderna may be poised for a comeback at a critical juncture in the nation's vaccine campaign. The company is expected to send initial data this week on how well the vaccine works for children.
Only 18 million children under the age of 5 are eligible for vaccinations in the US. Many parents are still waiting for the chance to protect their babies, toddlers and preschoolers, even though the rate for older children has been slow.
Moderna is trying to find a vaccine that is strong enough to offer lasting protection, but not so, in order to compete with Pfizer-BioNTech.
Moderna has decided to use two doses at a quarter the strength of its adult dose for children under 6. A three-shot regimen for children under 5 is expected to be approved by Pfizer next month.
The next age group that Moderna is expected to release data from is children aged 6 to 11.
New data from both Moderna and BioNTech will offer critical insight into the effectiveness of their shots for children. She said researchers are watching to see if Moderna's stronger doses result in more robust immune responses than Pfizer's shots have elicited in young children.
A series of new studies raising questions about how long Pfizer's lower doses protect elementary-school-age children has caught the attention of federal scientists and vaccine experts.
Dr. Levy, a member of the Food and Drug Administration's independent vaccine advisory committee, said he thought Pfizer might have chosen too low a dose for 5- to 11-year-olds.
Pfizer has been the only player in the vaccine market for younger Americans, winning authorization to give shots to 12 to 15 year olds last May and 5 to 11 year olds in October. Adults can use its authorization for 16- and 17-year-olds.
The F.D.A. delayed considering Moderna's request because of concerns about the risk of myocarditis, a condition involving inflammation of the heart that has been tied to both Moderna and the Pfizer shots.
More than 22 million people in the United States under the age of 18 have been fully immunized with the Pfizer vaccine. Even though shots have been offered to that group for more than four months, only one in four children are fully vaccine free.
There is still a demand to protect the youngest children as more of the country is revealed, more parents return to work and the summer travel season approaches.
There is no question that in children the benefit of an effective vaccine is less, because fewer get really sick, said Dr. Eric Rubin, an infectious disease expert at the Harvard T.H. Chan School of Public Health.
He said that it would benefit some individuals.
Last month, regulators pressed Pfizer-BioNTech to submit preliminary results from its three-dose trial, which led to hopes that the youngest children would soon be covered. The F.D.A. wanted to start the campaign with two doses while waiting for the final results.
The effort collapsed when new data from Pfizer showed that two doses of Omicron didn't protect against infections.
More detailed results from Pfizer's and Moderna's trials are coming out at the same time. Both companies say their research shows they are safe, even though neither knows if their vaccines will work for the youngest age group.
Dr. Paul Burton, Moderna's chief medical officer, said in an interview that they feel confident in the safety profile.
Pfizer-BioNTech chose the safest and most tolerable dose for young children, according to Amy Rose, Pfizer's spokeswoman. The companies are hopeful that a three-shot regimen will provide strong protection for children under the age of 5. Pfizer is testing a 10-microgram dose for children under the age of 5, a third of the adult and teenage dose, and 3 micrograms for children under the age of 5.
The full adult dose for those ages 12 to 17 and 50 for children 6 to 11 would be substantially higher than Pfizer's proposal. Regulators are likely to review the firm's data for all three age groups at the same time.
Dr. Burton said that they really stood behind those doses. Some studies of adults suggest that Moderna's protection holds up longer than federal officials say.
He said initial results showed a robust immune response in children after a 50-microgram Moderna dose. Researchers hope that the trial will show how well the vaccine prevents illness, not just how high it is.
The F.D.A. spent a lot of time last year worrying about the safety of Moderna's vaccine for those under 18 because of some studies showing a higher risk of myocarditis.
The question is always: What is the dose that gives rise to an immune response that is likely to be protective?
The C.D.C. encouraged some people to wait eight weeks between their first and second Pfizer or Moderna shots because of concerns about myocarditis. The studies show that teenage boys and young men are most at risk of developing the side effect.
Recent British data shows myocarditis is very rare and typically mild in both Pfizer and Moderna recipients, according to Dr. Burton.
Dr. Walid F. Gellad, a drug safety expert at the University of Pittsburgh, said it was not certain whether Moderna's higher dose might increase the risk of myocarditis in young children. He and other experts said that the company's studies are too small to identify the risk of the side effect.
Several studies raised questions about the protection that two doses of Pfizer-BioNTech's vaccine provide for children. The health department in New York State found that the protection against infections from two doses faded quickly.
The effectiveness of the Pfizer shot against moderate forms of the disease in children ages 5 to 11 dropped significantly during the Omicron wave, according to C.D.C. researchers.
Some vaccine experts are debating whether a stronger dose would have been better or if children need a third dose. Everyone 12 and up can now get a booster shot. Pfizer expects the results of its study of a three-dose regimen for the younger children next month.
The Pfizer dose may have been too weak, but scientists are not sure. He said he expected regulators to be careful in considering higher doses for young children because of the low risks of them catching Covid now and getting seriously ill.
It is clear that the back and forth over which vaccine will be better for young children could discourage people from taking the vaccine.
Alison M. Buttenheim is a behavioral health expert at the University of Pennsylvania. She said that it would pave the way to say, "I'm going to hold off."
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