Take a guess how many companies market a flu vaccine in the U.S.

If your answer was six, pat yourself on the back. There are nine different flu vaccines approved for the U.S. market. But some drugmakers market more than one flu vaccine, so six companies currently market flu vaccines in the U.S.

Now for a much bigger number: As of now, there are 28 companies actively involved in developing COVID-19 vaccines, according to biotechnology trade group BIO. Of course, some of those companies are partnering together -- for example, Pfizer (NYSE:PFE) and BioNTech. One biotech on the list, Emergent BioSolutions, is manufacturing vaccines for others rather than developing its own COVID-19 vaccine. However, BIO's list also omits some drugmakers in the COVID-19 vaccine race such as AstraZeneca (NYSE:AZN), which has teamed up with the University of Oxford.

It's great that there are a lot of companies working on COVID-19 vaccines, because that improves the chances of a safe and effective vaccine becoming available sooner rather than later. But it also raises the question: Is a COVID-19 vaccine supply glut on the way?

Vaccine vials

Image source: Getty Images.

Premature? Yes, but...

You might think that it's way too early to pose this question. No COVID-19 vaccine has yet advanced to the point where it's on the verge of being approved for marketing. But development is progressing at a rapid pace.

AstraZeneca and the University of Oxford began a phase 2/3 clinical trial of COVID-19 vaccine candidate AZD1222 on May 22. It could take up to six months to determine whether the vaccine is working. However, AstraZeneca is preparing to be able to ship doses as early as September of this year.

Moderna (NASDAQ:MRNA) kicked off a phase 2 study of its COVID-19 vaccine mRNA-1273 last week. The company hopes to advance its messenger RNA (mRNA) vaccine to phase 3 testing in July. Moderna CEO Stephane Bancel has stated publicly that mRNA-1273 could possibly be available for emergency use (such as for healthcare professionals) by the fall of 2020.

Other drugmakers aren't too far behind. Inovio Pharmaceuticals (NASDAQ:INO) expects to announce interim results from the phase 1 study of its COVID-19 vaccine candidate INO-4800 in June. Novavax (NASDAQ:NVAX) anticipates reporting preliminary results from its phase 1 study of COVID-19 vaccine candidate NVX‑CoV2373 in July. And there are more study results on the way over the next few months as well.

A billion here, a billion there

The number of drugmakers working on COVID-19 vaccines doesn't necessarily mean that a supply glut could be on the way. But what about how many doses these companies are planning to produce?

AstraZeneca said recently that it has the capacity to make 1 billion doses so far. And the British drugmaker is expanding its production capacity. Novavax's acquisition of Praha Vaccines will allow the biotech to produce over 1 billion doses of NVX‑CoV2373 beginning in 2021.

In May, Moderna announced a collaboration with Lonza Group to ramp up production capacity for up to 1 billion doses per year of mRNA-1273. Johnson & Johnson (NYSE:JNJ) teamed up with Emergent BioSolutions to manufacture more than 1 billion doses per year of its experimental COVID-19 vaccine; J&J plans to begin a phase 1 study of its vaccine in September.

Gloved hands holding vials labeled Coronavirus Vaccine

Image source: Getty Images.

GlaxoSmithKline (NYSE:GSK) isn't developing a COVID-19 vaccine of its own. However, the big pharmaceutical company is supplying its vaccine adjuvant to several other drugmakers, including Sanofi. Last week, GlaxoSmithKline announced its plans to make 1 billion doses of its adjuvant for use in COVID-19 vaccines in 2021.

Pfizer expects to produce "millions of vaccine doses in 2020, increasing to hundreds of millions in 2021" for the mRNA vaccine that it's partnering with BioNTech to develop. Inovio is working with other companies to produce 1 million doses of its experimental COVID-19 vaccine by the end of this year, and ramping up production significantly in 2021.

Glut-free

In case you weren't counting, the projected production capacity for COVID-19 vaccines for just the companies mentioned above totals close to 6 billion annually. Only around half of Americans get the flu vaccine each year. If that rate is applicable to the world for the COVID-19 vaccine, roughly 4 billion people will be vaccinated against the novel coronavirus when vaccines become available.

Those numbers might make it seem like we could be headed for a COVID-19 vaccine supply glut. But there are a few important things to consider.

First, vaccines often require more than one dose. For example, Moderna's mRNA vaccine includes two doses spaced 28 days apart.

Second, the number of people who would opt to receive a COVID-19 vaccine could and probably will be higher than 50%. A poll conducted by The Washington Post and ABC found that seven out of 10 Americans would either definitely or probably get a COVID-19 vaccine.

Third, and most important, it's unlikely that all of the COVID-19 vaccines will prove to be safe and effective in clinical trials. Sure, the large number of drugmakers developing COVID-19 vaccines increases the chances that multiple candidates will be successful. But the odds are still heavily against all of them working.

Bill Gates estimates that at least 7 billion doses of a COVID-19 vaccine will be needed each year, and perhaps as many as 14 billion doses. Granted, Gates isn't an epidemiologist or a physician, but he's been actively involved in funding vaccine research for various viral diseases for years. And the high end of Gates' estimated range is close to the 15 billion vaccine doses per year that Thomas Cueni, director of the International Federation of Pharmaceutical Manufacturers and Associations (IFPMA), thinks might be needed.

The bottom line is that a supply glut of COVID-19 vaccines almost certainly isn't on the way, despite the tremendous efforts underway to develop and manufacture vaccines. Even if a supply glut was on the horizon, it would be a good problem to have -- since it would drive down the costs of the vaccines.