The Centers for Disease Control and Prevention (CDC) said coronavirus vaccines may not initially be recommended for children when they become available.
Children, who rarely develop severe COVID-19 symptoms or require hospitalization for the disease, have not yet been tested for any experimental jab.
In the statement, posted on Wednesday, the CDC said that, up to this point, clinical trials have only included healthy, non-pregnant adults.
However, the federal agency noted the recommended groups could change in the future as clinical trials expand to recruit more people.
The CDC said most coronavirus vaccine trials have only included healthy, non-pregnant adults so far, but this could change as trials expand to include more people (file image)
'In early clinical trials for various COVID-19 vaccines, only non-pregnant adults participated,' the statement on the website reads.
'However, clinical trials continue to expand those recruited to participate. The groups recommended to receive the vaccines could change in the future.'
Children are often the last group to be tested during clinical trials because they are not merely little adults.
Their bodies and immune systems behave differently, meaning they might have different treatment needs.
What's more, children may need different doses or needle sizes depending on their height, weight and age - which is why most children are only vaccinated after safety has been well-document in the adult population.
On Tuesday, Pfizer Inc announced it will begin enrolling children, who are capable of passing on the virus to high-risk groups, as young as 12 in its large, late-stage COVID-19 vaccine trial.
A team at Cincinnati Children's