This month, data from several clinical trials will be released to determine whether “mixing and matching” different COVID-19 vaccines is safe and effective, or if it’s better to get the same booster as the one used in the primary series of shots.

There are two ways to think about the type of COVID-19 booster you can get: one is a homologous booster, which is the same shot you got in the first series. A heterologous booster, also known as a “mix-and-match” booster, allows you to receive a different vaccine based on availability or improved efficacy. This is an important scientific distinction that will most likely influence how COVID-19 booster shots are administered in the United States.

“There will be situations where a person may not be able to be boosted with the same product that they were originally vaccinated with for one reason or another,” Dr. Anthony Fauci, President Joe Biden’s chief medical adviser, told McClatchy last month.

However, analysts believe that mixing COVID-19 shots could reduce revenue projections for the vaccine market’s leaders.

“If ongoing mix-and-match trials show that the late entrants’ products are as effective at boosting as the original innovator vaccines themselves,” SVB Leerink analyst Geoffrey Porges told investors this week, “then these companies stand to gain potentially meaningful near-term revenues and will almost certainly take a significant share of whatever tail of future COVID vaccine revenue exists.”

Four COVID-19 vaccine manufacturers—the market leaders, AstraZeneca Johnson & Johnson Moderna Inc. and Pfizer Inc.—are expected to generate $46 billion in revenue for their COVID-19 shots in 2022.

The initial doses of their shots accounted for roughly $19 billion of the sales generated so far this year. In the United States, only recipients of the BioNTech SE and Pfizer vaccines are eligible for a booster shot (based on their health status, age, and line of work). Those who received Moderna or J&J shots are still waiting for regulators to decide whether they can receive a booster.

These are known as homologous boosters.

However, other studies are being conducted to determine whether it is safe to mix vaccines, and preliminary findings indicate that mixing vaccines is safe and may result in a stronger immune response in some cases. A National Institutes of Health study testing a Moderna booster with other types of vaccines is expected to make its findings public on Oct. 15 at a Food and Drug Administration advisory meeting evaluating J&J’s booster application.

A preprint from the United Kingdom published in June discovered that combining the BioNTech/Pfizer and AstraZeneca/University of Oxford vaccines resulted in better immunity than using the AstraZeneca shots alone. In June, German researchers stated that combining the same vaccines as the UK could “improve immunogenicity and mitigate potential intermittent supply shortages for individual vaccines.”

The outcome of the data could have a significant impact on the market share held by the leading COVID-19 vaccine manufacturers.

According to Porges, if clinical data shows that using a different type of COVID-19 booster is safe and effective, it could allow some of the smaller companies developing still-in-development shots to gain entry and a slice of market share.

“If mix-and-match boosting is found to be as effective as or better than three doses of an mRNA vaccine,” he wrote, “PFE and MRNA may capture less of the long tail of COVID vaccine revenues than is currently priced in, and companies like Sanofi, Novavax, Valneva, and Clover Biopharmaceuticals could participate in this market.”

Sanofi is currently in Phase 3 clinical trials with the experimental recombinant protein vaccine and booster it developed with GlaxoSmithKline.

The COVID-19 vaccine candidate from Novavax Inc. is expected to be submitted to the FDA for approval soon.

Valneva SE is a French biotech that is still in clinical trials with its experimental COVID-19 shot, while Clover Biopharmaceuticals is a privately held Chinese biotech with a vaccine candidate in Phase 2/3 trials.

So far this year, AstraZeneca shares in the United States have risen 18.8 percent, J&J shares have risen 0.4 percent, Moderna shares have risen 201.8 percent, and Pfizer shares have risen 13.5 percent. In 2021, the S&P 500 is up 15.7%.