The Food and Drug Administration is expected to authorize booster shots of the Pfizer-BioNTech coronavirus vaccine for children ages 5 to 11 as soon as Tuesday, making an extra dose available to protect school-aged children as a descendant of the omicron variant is becoming dominant and cases tick upward. 

Outside experts who counsel the Centers for Disease Control and Prevention, the Advisory Committee on Immunization Practices, are scheduled to meet Thursday and are expected to recommend boosters for that age group. CDC director Rochelle Walensky is expected to concur shortly afterward.

Experts have not predicted a summer surge, but an offshoot of omicron, called BA.2.12.1, is rapidly becoming dominant in the United States. This version of omicron has an edge over even its highly contagious predecessors, as it may be more transmissible and better able to evade the immune defenses generated by vaccines and previous infections.

The Pfizer-BioNTech shot is the only coronavirus vaccine available as a primary series or a booster in individuals younger than 18, although that may change in the next several weeks as the FDA hastens consideration of pediatric vaccines.

A month ago, Pfizer and its German partner, BioNTech, announced that data from a small study showed their vaccine booster increased immune defenses for elementary school-aged children, especially against omicron. The children received the third dose six months after being fully vaccinated with two shots.

The study did not directly test how well-protected children were against illness but measured the effect a booster shot had on their immune defenses.

Blood tests showed that among children in the study, antibody levels capable of blocking the original version of the virus were six times higher after the booster shot, the companies said in a news release in April. In a subset of the children, the booster sparked a 36-fold increase in antibodies that neutralize omicron, the firms said. The data were not peer-reviewed.

At the time, Pfizer and BioNTech said the results reinforced "the potential function of a third dose of the vaccine in maintaining high levels of protection against the virus in this age group."

The expected authorization as soon as Tuesday by the FDA was first reported by the New York Times.

The FDA authorized the primary two-dose series of the vaccine for 5-to- 11-year-olds last fall. Each dose is 10 micrograms, as is the booster - one-third the dose for adults. The companies said in April the dose for school-age children was "carefully selected based on safety, tolerability and immunogenicity data."

Whether parents will rush to get their children booster shots is far from certain. While some parents are eager to bolster protection for their children, others have proved less than enthusiastic. CDC has said only 28% of children 5 to 11 have been fully vaccinated.

Health officials have recommended booster shots for everyone 12 and older and said people 50 and older, or those with impaired immune systems, can consider getting a second booster.

More vaccine activity involving children is expected soon. Those younger than 5 remain the only group without access to any coronavirus vaccine, a sore spot for many frustrated parents. They remain unshielded from COVID-19 even as many adults have ditched their masks and are attempting to return to a pre-pandemic lifestyle.

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Lena H. Sun contributed to this report.