I think there's a decent chance that a COVID-19 vaccine will be available by the end of this year. I'm in pretty good company. Dr. Anthony Fauci, longtime director of the National Institute of Allergy and Infectious Diseases, also recently predicted that there will likely be a safe and effective vaccine ready by year's end. Pfizer (NYSE:PFE) CEO Albert Bourla agrees.

Don't get too excited yet, though. You and I probably won't receive a coronavirus vaccine shot by the end of December. Here are three reasons why.

"Vaccine?" printed on a mask next to a clock, calendar, and hand sanitizer bottle.

Image source: Getty Images.

1. Few (if any) vaccines will be ready for distribution

While there are reasons to be cautiously optimistic that a COVID-19 vaccine will be available by the end of this year, it's important to keep in mind that there's no guarantee that will be the case. Even Bourla acknowledged that there's around a 60% chance that Pfizer and BioNTech (NASDAQ:BNTX) will know by the end of October if coronavirus vaccine candidate BNT162b2 will work. This, of course, implies that there's a 40% chance that the two drugmakers won't know if the vaccine works by the end of next month.

The main problem is timing. Late-stage clinical studies must first progress to the point that a company feels the drug has enough data to support requesting emergency use authorization (EUA) from the U.S. Food and Drug Administration. Next comes the review process. FDA Commissioner Stephen Hahn and Center for Biologics Evaluation and Research Director Peter Marks have stated that an advisory committee will first review any COVID-19 vaccine candidate seeking FDA approval or EUA before the agency makes its final decision. 

There are only three COVID-19 vaccine candidates that even have a shot at winning EUA from the FDA soon enough for any American to be immunized by the end of 2020. Pfizer and BioNTech's BNT162b2 is arguably in the lead. Moderna's (NASDAQ:MRNA) mRNA-1273 is another potential contender. AstraZeneca's (NASDAQ:AZN) AZD1222 is also a possibility. But it's less likely to be ready for EUA and distribution now because of the temporary pause of its phase 3 testing after an individual experienced a severe reaction during the trial.

2. Supply chain problems are likely

Let's assume that one or more of these drugmakers receive EUA for their respective COVID-19 vaccines in November -- which would be an astonishingly fast timeline. Would those vaccines be available to most Americans soon thereafter? Don't count on it.

Any authorized vaccines will likely be in short supply at first. The leaders in the coronavirus vaccine race are still ramping up manufacturing capacity. Pfizer and BioNTech think they'll be able to supply up to 100 million doses of BNT162b2 by the end of this year, but those doses won't be only for the U.S. market.

Supply chain problems are likely. Distribution sites will have to be set up. States will need to acquire sufficient quantities of syringes and needles, in addition to the vaccine itself. Refrigerated transportation for the vaccines must be lined up. There's a lot of work to be done in a very short period of time for COVID-19 vaccines to be widely available by the end of the year.

3. You're probably not in the phase 1 group

What if everything goes smoothly with coronavirus vaccines securing FDA authorization and supply chains snapping into place quickly? You're still statistically unlikely to receive a vaccine by the end of this year.

Because of the limited initial supplies of vaccines, immunization will probably be done in phases. The National Institutes of Health and the Centers for Disease Control and Prevention requested a proposal from the National Academies of Sciences, Engineering, and Medicine on how to distribute COVID-19 vaccines in the U.S. The draft proposal calls for distribution in four phases. 

The first phase to be vaccinated includes healthcare workers and first responders at high risk of infection, individuals with serious underlying health conditions, and older adults in "congregate or overcrowded settings," such as nursing homes. All of these individuals combined make up an estimated 15% of the U.S. population. Unless you fall into these categories, you probably won't be in the phase 1 group.

Wait until next year

So when might you be able to receive a COVID-19 vaccine? Your chances should be better next year. 

For one thing, there will be more drugmakers far along enough in late-stage testing in the first half of 2021 to seek EUA from the FDA. Production capacity will be significantly higher by then as well. In addition, it's likely that many of the early supply chain kinks will have been worked out by that time.

Of course, everything depends on the vaccine candidates succeeding in clinical testing. Even if you aren't a fan of biopharmaceutical companies, it makes sense to root for Pfizer, BioNTech, AstraZeneca, Moderna, and others with COVID-19 vaccine candidates in development. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.