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 people 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 company's move follows an announcement by U.S. health officials last week of plans to give COVID-19 booster shots to all Americans to shore up their protection amid the surging delta variant of the virus. Officials said it is “very clear” that the vaccines’ protection against COVID-19 infections wanes over time.

Pfizer's vaccine received full FDA approval earlier this week; it had been used since last December under an emergency use authorization.

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.

Earlier this month, U.S. regulators said transplant recipients or others with weakened immune systems can get an extra dose of Pfizer or Moderna vaccines.