Pfizer is seeking U.S. approval of a booster dose of its two-shot COVID-19 vaccine.

The drugmaker said Wednesday that it has started the application process for a third dose of its vaccine for everyone ages 16 and older. The company said it will complete the application with the Food and Drug Administration by the end of this week.

The FDA earlier this month said transplant recipients or others with weakened immune systems can get an extra dose of Pfizer or Moderna vaccines. Then last week, U.S. health officials announced plans to give COVID-19 booster shots to all Americans to shore up their protection amid the surging delta variant of the coronavirus.

That plan drew criticism from scientists at the World Health Organization, who noted that poor countries are not getting enough vaccine for their initial rounds of shots.

Pfizer's vaccine, jointly developed with Germany's BioNTech, received full FDA approval earlier this week, also for those ages 16 and older. It had been given to that age group under an emergency use authorization, which is still the case for those ages 12 to 15.

More than 200 million doses of the Pfizer vaccine have been administered in the U.S. — and hundreds of millions more worldwide — since it got the green light in December.

The three vaccines used in the U.S. made by Pfizer, Moderna and Johnson & Johnson are still preventing hospitalizations and deaths from COVID-19. But the vaccines don't appear quite as strong against the highly contagious delta variant as they were against earlier versions of the virus.

The U.S. booster plan calls for an extra dose eight months after people get their second shot of the Pfizer or Moderna vaccine.

Also on Wednesday, J&J said it plans to talk to regulators about a booster dose for its single-shot vaccine. The drugmaker said it also saw a big jump in virus-fighting antibodies in people who received the extra shot.

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