As soon as Tuesday, the U.S. Food and Drug Administration could authorize COVID-19 booster shots for Americans over 50 years old, two officials familiar with the matter told ABC News, though the fourth shots are likely to be only offered and not formally recommended.
The officials stressed that the details are still under discussion and could change in the next few days.
After FDA's expected authorization early this week, the Centers for Disease Control and Prevention will give guidance on how to implement it in pharmacies and doctors offices around the country, as the process has gone throughout the pandemic.
The language from CDC Director Rochelle Walensky is expected to be that people over 50 may get a second booster shot, rather than should get a second booster shot, officials said.
In other words, the shots would be available for people to make individual decisions based on their health, risk tolerance and age. In the past, the CDC has used similar language to open up booster shots first to the most vulnerable and then to the general population.
FDA's panel of experts will convene on April 6 to discuss the broader population and what population will need booster shots next, as well as the need for a variant-specific booster.
Officials weighing the decision are also considering that anyone who gets a booster this spring would likely get boosted again when they are recommended for the broader public later this year, potentially in the fall, according to another person familiar with the matter.
Pfizer and Moderna asked the FDA last week to authorize another booster dose -- especially for elderly Americans, a group that tends to have weaker immune protection.
Pfizer asked the FDA to authorize fourth doses for people older than 65, while Moderna asked for authorization for everyone 18 and older.
ABC News' Sony Salzman contributed to this report.