The Food and Drug Administration will update its emergency use authorization for the Pfizer and Moderna coronavirus vaccines as early as Thursday to allow immunocompromised people to get a third dose, a source familiar with the matter told Axios.

Why it matters: Data suggest that people with weakened immune systems don't generate strong enough levels of protection agains the virus with just two doses, but a third dose could significantly help.

The big picture: Scientists have debated who should receive booster shots and when, as the highly contagious Delta variant drives up the number of new cases across the country.

• About 2.7% of U.S. adults are immunocompromised, a group that encompasses people that are undergoing cancer treatment, living with HIV, or are organ transplant recipients, among others, according to the Centers for Disease Control and Prevention (CDC).
• More than 1 million people in the U.S. have received unauthorized booster shots of the Moderna or Pfizer vaccines according to an internal CDC briefing document obtained by ABC News.

State of play: In July, the Advisory Committee on Immunization Practices (ACIP) to the Centers for Disease Control and Prevention urged action on additional doses for immunocompromised people, per NBC News.

• In July, Israel became the first country to offer booster shots for immunocompromised people and seniors.
• Germany and the U.K. have also announced they plan to offer boosters starting September.
• Yes, but: The World Health Organization has called for a moratorium on booster shots through at least September to allow for poorer countries to have access to doses.

What to watch: The ACIP will meet Friday to make recommendations on booster shots of the immunocompromised.

Go deeper:The CDC's booster messaging mess