The Biden administration's top health officials announced Wednesday that beginning the week of Sept. 20, Americans who received a Pfizer or Moderna vaccine will be offered a booster shot eight months after their second dose.

Why it matters: The decision reflects a desire by the administration to get ahead of declining vaccine effectiveness, as the highly contagious Delta variant drives a new surge in infections across the country.

• Officials said they expect to recommend booster shots for people who received the single-dose J&J vaccine, but are waiting for more data in the next few weeks before laying out a plan.
• The plan is still subject to approval by the FDA and a CDC advisory committee. The new round of inoculations will prioritize individuals vaccinated earliest in the pandemic, including health care workers and seniors.

What they're saying: "The COVID-19 vaccines authorized in the United States continue to be remarkably effective in reducing risk of severe disease, hospitalization, and death, even against the widely circulating Delta variant," top officials from the Department of Health and Human Services said in a statement.

• But they added: "Based on our latest assessment, the current protection against severe disease, hospitalization, and death could diminish in the months ahead, especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout."
• "For that reason, we conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability."

Between the lines: Giving Americans third doses of the COVID-19 vaccines poses ethical concerns, given the vaccine shortages that plague many countries around the world.

• Uneven global distribution of the vaccines helped lead to the mutation of more dangerous variants in the first place, experts say.

The big picture: The FDA has already authorized a third vaccine dose for certain immunocompromised people, including "solid organ transplant recipients or those who are diagnosed with conditions that are considered to have an equivalent level of immunocompromise."

• 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 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.