The study, which was announced in October, had enrolled over 2,000 participants, a Pfizer spokeswoman said in an emailed statement.
Vaccines from Pfizer and Moderna Inc, which are being rolled out in the United States, are not yet available for use in children due to a lack of study data.
Children rarely experience severe COVID-19 symptoms but they could still spread the virus.
Last month, Moderna also began a study to test its vaccine, which uses a similar technology as the Pfizer vaccine, in adolescents aged 12 to less than 18.