On Aug. 12, the FDA updated the Emergency Use Authorizations (EUAs) for the COVID-19 vaccines made by Pfizer (NYSE:PFE) along with its partner, BioNTech (NASDAQ:BNTX), and Moderna (NASDAQ:MRNA) to include a third booster dose for immunocompromised individuals. One day later, the CDC changed its stance to recommend booster doses of the two messenger RNA (mRNA) vaccines for moderately to severely immunocompromised individuals.

With EUAs in hand for third booster doses, will the top COVID-19 vaccine makers rake in a lot more revenue now? Not really. Here's why Pfizer and Moderna won't make as much extra money from U.S. booster doses as you might think.

One key reason why Pfizer and Moderna won't make much more money is the limited scope of the EUA expansions. The FDA said that the broader authorizations will permit booster doses for "solid organ transplant recipients or those who are diagnosed with conditions that are considered to have an equivalent level of immunocompromise."

Acting FDA Commissioner Janet Woodcock stated, "After a thorough review of the available data, the FDA determined that this small, vulnerable group may benefit from a third dose of the Pfizer-BioNTech or Moderna Vaccines." Her wording clearly indicates that the target group for booster doses isn't very large.

Just how big is the opportunity for Pfizer and Moderna? Around 3% of U.S. adults are moderately to severely immunocompromised, according to the CDC. That's around 8 million people.

Using the $24 per dose price for the Pfizer-BioNTech COVID-19 vaccine, this market is in the ballpark of $190 million. And it will be divided between Pfizer and Moderna. Pfizer expects its COVID-19 vaccine will generate $33.5 billion in sales this year. Moderna projects around $20 billion in sales for its vaccine. The U.S. booster dose market under the current expanded EUAs represents under 1% of those totals.