A COVID-19 booster shot (or third shot) will be offered to Americans starting this upcoming fall, United States officials just announced.

The shots, which should be administered 8 months after an individual's second dose, will be made available beginning the week of September 20, officials say.

The move requires a final sign off from the FDA and the CDC, but both organizations are expected to approve the measure as they were included in today's announcement.

Today's announcement, however, affects the Pfizer/BioNTech and Moderna vaccines only.

"We are prepared to offer booster shots for all Americans beginning the week of September 20 and starting 8 months after an individual's second dose," US health official said in a joint statement issued Wednesday morning.

Health officials say that the COVID-19 vaccines have proven to be remarkably effective in reducing the risk of severe disease, hospitalization and death (even against the highly contagious Delta variant).

However they say it's clear that the protection against the coronavirus begins to "decrease over time."

"We conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability," officials said on Wednesday.

The exact details about the rollout of a third dose of the vaccine will likely be ironed out in the coming days, but it's likely involve existing providers (including CVS and Walgreens).

Because the rollout of the single dose Johnson & Johnson vaccine began a little later (in March) there's less data available on that vaccine's efficacy over time. As a result, that data is still be studied by scientists. So they anticipate that further decision on any Johnson & Johnson vaccine boosters would be made soon.

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