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LOS ANGELES - On Tuesday, Pfizer and BioNTech announced they have submitted a request for emergency use authorization from the U.S. Food and Drug Administration for a COVID-19 vaccine booster for all U.S. adults aged 18 and older.

Older Americans and other groups particularly vulnerable to the virus have had access to a third dose of the Pfizer and BioNTech vaccine since September. But the Food and Drug Administration has said it would move quickly to expand boosters to younger ages if warranted.

Currently, a booster dose of both the Pfizer and Moderna vaccines is authorized by U.S. regulators for individuals 65 years of age and older as well as adults with high risk of severe COVID-19. A booster shot of the one-dose Johnson & Johnson vaccine is authorized for anyone who received a first dose.

Pfizer said its request to broaden access to its booster dose among adults is based on results from a randomized Phase 3 trial with more than 10,000 participants in which researchers found boosters to provide more than 95% effectiveness against symptomatic COVID-19, compared with the original two doses administered amid the prevalence of the highly transmissible delta variant.

A median of 11 months after their last Pfizer vaccination, trial participants were given either a third dose or a dummy shot. Researchers tracked any infections that occurred at least a week later, and so far have counted five cases of symptomatic COVID-19 among booster recipients compared to 109 cases among people who got dummy shots. Side effects were similar to those seen with the company’s first two shots.

While experts say all three vaccines used in the U.S. continue to offer strong protection against severe COVID-19 illness and death, the shots’ effectiveness against milder infection can wane over time.

A study published this month in the journal "Science" provided new insight into just how much protection from the vaccines diminishes as time passes.

After collecting data from 780,225 veterans. nearly 500,000 of whom were inoculated, researchers found that initial efficacy from all three vaccines rendered high levels of protection against the virus. In March, Moderna’s protection measured at 89%, Pfizer’s registered at 87% and J&J came in at 86%.

But six months later, Moderna’s jab had dropped to just 58%, Pfizer was 45% and J&J fell to a mere 13%.

The study’s authors said their findings do not directly address the benefits and risk of booster shots. But they said the data should be interpreted in the context of the ongoing public debate surrounding additional doses.

The Biden administration had originally envisioned boosters for all adults, but faced a stinging setback in September when the FDA’s scientific advisers rejected extra Pfizer doses for everyone. The panel at that time wasn’t convinced that young healthy people needed another dose, particularly when most of the world’s population remained unvaccinated.

Still, under today’s policies, about 2 of every 3 vaccinated adults is expected to qualify within the next few months, and many who don’t can nonetheless score an extra shot because most vaccine providers don’t check qualifications.

Currently, there are still restrictions on who qualifies and when for a booster. Starting six months past their last Pfizer or Moderna vaccination, people are urged to get a booster if they're 65 or older, nursing home residents, or at least 50 and at increased risk of severe disease because of health problems. Boosters also were allowed, but not urged, for adults of any age at increased risk of infection because of health problems or their jobs or living conditions. That includes health care workers, teachers and people in jails or homeless shelters.

And regardless of the initial vaccination, the U.S. has cleared getting a booster of a different company’s vaccine, what’s called mixing and matching.

If the FDA authorizes Pfizer boosters for all adults, the Centers for Disease Control and Prevention then will make recommendations for how to use them.

Walgreens, Walmart, Sam’s Club and Rite Aid announced in October that they’ll have the Pfizer, Moderna or J&J COVID-19 booster vaccines for eligible patients. CVS will carry Pfizer and Moderna’s booster shot.

Globally, access to boosters is a hodge-podge. Some countries restrict them to older or medically fragile people while others have few restrictions. Israel, for example, has authorized Pfizer boosters for anyone 12 and older.

This story is developing. Please check back for updates. The Associated Press, Jordan Smith and Chris Williams contributed.