Pfizer and BioNTech plan to study a three-dose regimen of their COVID-19 vaccine in young children 6 months to 4 years old, a move that may extend the wait for anxious parents as fast-spreading variants complicate clinical trials.

When tested in children ages 2 to 4, the two-shot regimen failed to meet a laboratory standard for immune response set in an older age group, according to a statement. The vaccine partners said they amended their clinical trial protocol following a request by an external, independent data monitoring committee. No safety concerns were identified among the young children.

If three doses prove successful, Pfizer and BioNTech will approach the U.S. Food and Drug Administration for an emergency-use authorization for the regimen in that age group in the first half of 2022.

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The abrupt alteration in the trial is a sign of the unexpected power of the coronavirus against vaccine immunity, making it look more like three-shot regimens will become the standard fare of the future. Pfizer and BioNTech will also evaluate a third dose among other age groups as the omicron variant weakens the potency of their vaccine.

“Control of omicron may only be possible with three or more doses,” said Kathrin Jansen, Pfizer’s head of vaccine research and development, in an investor conference call on Friday.

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The announcement may come as a disappointment to parents who had hoped to get their young children vaccinated before the end of the year. Pfizer had said it aimed to seek an emergency-use authorization for 2- to 4-year-olds toward the end of 2021, or at the outset of 2022.

The change “reflects the companies’ commitment to carefully select the right dose to maximize the risk-benefit profile,” they said in a statement. The additional shot will be a 3-microgram dose.

The companies plan to test a third, 10-microgram dose in children 5 to under 12 years of age, they said in the statement. They’re also launching a study of a third dose, either at 10 or 30 micrograms, among 600 adolescents ages 12 to 17.

Pfizer fell 2.4% at 10:20 a.m. in New York, while BioNTech’s American depositary receipts fell 0.9%.

Both the FDA and the European Medicines Agency have endorsed the clinical trial amendments.

Pfizer also raised its forecast for 2022 vaccine sales to $31 billion, based on contracts for 1.9 billion doses signed through mid-November. The guidance, given on the conference call, is an increase from the company’s earlier view of $26 billion in sales for next year.

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