Pfizer Inc. and Moderna Inc. may need to wait for a second round of COVID-19 booster shots before they see profits from the U.S. government's push for additional vaccinations, analysts have said.

Under plans announced by the Biden administration Aug. 19, booster doses of Pfizer's or Moderna's COVID-19 vaccines will be available to people from Sept. 20, to combat waning immunity provided by the initial immunizations.

The eight-month gap between administering the second and third dose, known as a booster, means the U.S. already has the anticipated supply in hand.

An estimated 600 million doses of mRNA vaccines have been delivered to the U.S. government so far, Jefferies analyst Michael Yee said in an Aug. 18 note. Of those doses, about 260 million remain on shelves, and White House representatives said they expect about 100 million people to seek out the booster.

Pfizer forecast $33.5 billion in COVID-19 vaccine revenue for 2021 based on 2.1 billion doses contracted worldwide through mid-July. Analysts at the time of the pharmaceutical giant's July 28 earnings call said these estimates were conservative and that booster doses of the vaccine, developed with Germany's BioNTech SE, remained a long-term swing factor beyond 2021.

Similarly, Moderna expects to bring in $20 billion in 2021 revenue from its own COVID-19 vaccine, the company's only authorized product. Moderna stopped taking orders for 2021 but has signed advance purchase agreements out to 2023 for boosters in Israel and Switzerland.

The U.S. Food and Drug Administration is reviewing data for full approval of both vaccines, with the green light for Pfizer-BioNTech's shot expected in September.

Vaccines available in the U.S. at local providers, health departments and hospitals in refrigeration units are likely to be sufficient for the first supply of booster shots, North Carolina State University Department of Industrial and Systems Engineering head Julie Swann said in a panel discussion hosted by Newswise on Aug. 18. But the demand for the third shot is still unknown.

"I anticipate that if everyone who received the two doses needs a third, there would be additional supply that would be shipped from the manufacturers and distribution centers, just as we saw before," Swann said. "I don't anticipate that we're going to see major problems with the manufacturing at this point in the process after they've worked out a lot of those early troubles that were stopping them."

The booster shot would be offered only to those over the age of 18, U.S. Surgeon General Vivek Murthy said in an Aug. 18 announcement. The plan is conditional upon an independent evaluation of safety and effectiveness from the FDA.

Both Pfizer and Moderna plan to finish submitting data from vaccine booster studies in the next few weeks. Booster trials for Johnson & Johnson's COVID-19 vaccine — the only other authorized immunization in the U.S. — are also being conducted, but a plan has not been solidified as those shots were not rolled out until March.

A week before the Aug. 18 announcement, regulators cleared booster shots for people who are immunocompromised.

The need for boosters arose from a combination of waning immunity and the strength of the widespread delta variant of the virus, Murthy said.

"Even highly effective vaccines become less effective over time," Murthy added. "Our goal is then to determine when that time might come for the COVID-19 vaccines so we can make a plan to take proactive steps to extend and enhance the protection the vaccines are giving us."

The president's chief medical adviser, Anthony Fauci, explained that protective antibody levels decline over time after the initial vaccination and that higher levels of antibody may be required to protect against the delta variant. The booster shot has been shown to increase those antibody levels by at least 10-fold, Fauci said.

"All of this supports the use of a third booster mRNA immunization to increase the overall level of protection," Fauci said.

Equity, access again in the spotlight

Just as in the initial rollout of the COVID-19 vaccines, equity of distribution and access to booster shots is a crucial part of the vaccination process, Swann said during the panel discussion.

"As we move into this booster phase, I think it's even more important that we continue to focus on uptake across all of the different subgroups in the population to ensure that we're bringing up that level of vaccination in ways that are appropriate," Swann said.

"Making it easy for people to get it, making sure that we have multiple languages, making transportation easy for those who might not be mobile," Swann added. "And we should also do this for vaccines rolling out to children — it's really important that we're vaccinating all the segments of our communities."

The plan to authorize a booster shot in the U.S. has also raised questions of global equity when much of the world has not received an initial vaccination. White House representatives pointed out that the U.S. has already shipped 115 million doses, which is more than all other countries combined, and has a goal to donate 600 million doses overall.