A safe and effective vaccine would be a shot in the arm for a world that's grown weary of the COVID-19 pandemic. The good news is that there's a lot of work going on to produce just such a vaccine. At least 19 novel-coronavirus vaccine candidates are now in clinical testing, according to the World Health Organization (WHO). Another 130 candidates are currently in preclinical trials.

But you shouldn't get your hopes too high for COVID-19 vaccines. Here are three reasons why.

Healthcare professional holding syringe up to small globe

Image source: Getty Images.

1. The probability of success isn't as great as you might think

Many Americans assume that regulatory approval of a vaccine is right around the corner. President Donald Trump even publicly suggested that a "vaccine solution" for COVID-19 will be available "long before the end of the year." But these assumptions could be off-base.

WHO's list of COVID-19 vaccines includes only one U.S.-made candidate in phase 2 testing. Moderna (NASDAQ:MRNA) recently announced that its late-stage study of COVID-19 vaccine candidate mRNA-1273 would be delayed. The biotech still hopes to begin the trial in July, however. Meanwhile, AstraZeneca (NYSE:AZN) and its partner, the University of Oxford, are already recruiting for participants in a phase 3 study for their COVID-19 vaccine candidate.

The probability of these or other vaccines being successful isn't as great as you might think. Only around 24% of vaccines in phase 2 clinical testing go on to win approval from the Food and Drug Administration, according to a historical analysis conducted by biopharmaceutical trade group BIO. That percentage jumps to 74% for vaccines in phase 3 testing. But that's still a 1-in-4 chance of failure.

2. COVID-19 vaccines might not be as effective as you expect

Even if one or more COVID-19 vaccines win FDA approval, they might not be as effective as you'd expect. Why? The bar isn't all that high when it comes to efficacy.

Last week, the FDA issued guidelines for its review and approval process for COVID-19 vaccine candidates. To be considered effective, a vaccine only has to "prevent disease or decrease its severity in at least 50% of people who are vaccinated."

This threshold isn't unusual for the first vaccines against a virus for which no vaccines currently exist. However, it also means that there's a real possibility that among those who receive a COVID-19 vaccine, nearly half won't be effectively immunized against the novel coronavirus.

3. Many Americans will refuse to get a COVID-19 vaccine

Michael Jordan once said, "You miss 100% of the shots you don't take." He was, of course, referring to basketball. However, the idea is also relevant to COVID-19 vaccines.

A survey conducted by the Associated Press-NORC Center for Public Affairs Research in May found that only 49% of Americans said that they planned to get vaccinated if a vaccine against the novel coronavirus becomes available. That number isn't too surprising, considering that it's roughly in line with the percentage of American adults who get the flu vaccine.

It's possible that more Americans would want to be vaccinated against the novel coronavirus, though. Another 31% of the survey respondents stated that they weren't sure about getting a COVID-19 vaccine. However, if the percentage of Americans who refuse to be immunized isn't high enough, even an effective vaccine won't be enough to prevent COVID-19 from spreading.

Still a big opportunity

The probabilities for approval, efficacy, and potential immunization rates don't paint an encouraging picture. However, there's still a chance that one or more COVID-19 vaccines that are highly effective will win regulatory approval and gain widespread public acceptance.

And there's still a big opportunity for investors hoping that coronavirus-focused biotech stocks pay off in a huge way. For example, even though its shares have tripled so far this year, Moderna would almost certainly soar even higher if mRNA-1273 is successful in late-stage testing.

Any vaccine that's safe and effective enough to secure approval will help in the fight against COVID-19. COVID-19 vaccines might not be the magic bullet that many hope for. But combined with new treatments and better testing, they could be part of an overall arsenal that enables the world to move past the pandemic and return to normal.