(Reuters) — The U.S. Centers for Disease Control and Prevention (CDC) said on Wednesday that COVID-19 vaccines may not be initially recommended for children when they become available.

Children, who rarely have severe COVID-19 symptoms, have not yet been tested for any experimental coronavirus vaccine. The CDC said so far early clinical trials have only included non-pregnant adults, noting the recommended groups could change in the future as clinical trials expand to recruit more people.

Pfizer Inc has said it will enroll 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, while AstraZeneca has said a sub-group of patients in a large trial will test children between five to 12.

There is no vaccine for COVID-19 yet, but a handful of companies such as Pfizer and Moderna Inc are in final-stage trials of their experimental vaccines.

The CDC also said on Wednesday that any coronavirus vaccine would, at least at first, be used under the Food and Drug Administration’s emergency use authorization, and that there could be a limited supply of vaccines before the end of 2020.

In case of limited supply, some groups may be recommended to get a COVID-19 vaccine first, the CDC said.

Coronavirus vaccines should be rolled out in four phases, with initial supply going to front-line health workers and first responders, an independent expert panel tapped by top U.S. health officials recommended earlier this month.

(Reporting by Manas Mishra in Bengaluru; Editing by Shinjini Ganguli)