The Pfizer/BioNTech coronavirus vaccine is "safe, well tolerated" and produces a "robust" antibody immune response in children aged five to 11, according to the findings of a US trial.
The researchers plan to submit the results to regulators to seek approval for vaccinating children younger than 12, the current age limit.
More than 2,000 children took part and were given two small doses of vaccine.
Results in children under five are expected soon.
The findings, distributed in a press release, are the first of their kind for children under 12 - a trial of the Moderna vaccine in six to 11-year-olds is ongoing.
Both jabs are already being offered to adolescents over 12 in countries including the UK, which is just beginning its rollout of jabs to healthy teenagers.
The young children in the Pfizer trial were given 10 microgram doses, rather than the 30 micrograms recommended for over 12s. Antibody responses and side effects were comparable. The under fives in the study are being given even smaller doses.
Dr Ugur Sahin, CEO and co-founder of BioNTech, said: "The safety profile and immunogenicity data in children aged five to 11 years vaccinated at a lower dose are consistent with those we have observed with our vaccine in other older populations at a higher dose.
"We are pleased to be able to submit data to regulatory authorities for this group of school-aged children before the start of the winter season."
Although Covid is rarely severe in children, they can still catch and spread it and some will get very sick.
The UK has this week begun offering one dose of the Pfizer vaccine to children in secondary schools.
Although this won't stop the virus circulating, it could help keep more pupils in class, say experts.
Pfizer chief executive Albert Bourla said: "We are eager to extend the protection afforded by the vaccine to this younger population, subject to regulatory authorization, especially as we track the spread of the Delta variant and the substantial threat it poses to children."
He added: "These trial results provide a strong foundation for seeking authorization of our vaccine for children five to 11 years old, and we plan to submit them to the FDA and other regulators with urgency."