Young people have been less likely to suffer severe disease and more likely to have asymptomatic infection, allowing them to unwittingly transmit COVID-19 to others.
The vaccine was 100% effective in protecting against symptomatic disease, said the companies, who tested the drug on more than 2,260 children during phase three trial.
Researchers also didn’t find any safety concerns, Pfizer and BioNTech said.
The risk of infection fell 90% by two weeks after the second shot, the study of just under 4,000 vaccinated U.S. healthcare personnel and first responders found.
The Hang Seng Index fell 2.03 percent, or 579.24 points, to 27,918.14 -- putting it in a correction after falling more than 10 percent from its recent high seen in February.
US Treasury yields were lower as investors sought out the safe haven -- yields go in the opposite direction to prices -- providing respite from a recent increase that had led to worries about a hike in interest rates from the record lows that have been a key pillar of support to a year-long equity rally.
The decision came two days before Spain was to resume use of the AstraZeneca vaccine after the EU's drug regulator deemed the jab "safe and effective".
The EU has so far approved three vaccines -- Pfizer-BioNTech, Moderna and AstraZeneca-Oxford. Three other vaccines are under "rolling review" by the Amsterdam-based EMA -- Novavax, CureVac and Russia's Sputnik.