Shares of COVID-19 vaccine-maker Moderna (NASDAQ:MRNA) have been falling quickly in recent months. In the past three months, the stock has declined 40%, while the S&P 500 has risen by 5%. To make matters worse, the company is coming off a disappointing earnings performance that could send its shares even lower.

But the disappointing results should be the least of investors' worries. There could be much greater challenges ahead for Moderna.

Person receiving a vaccine.

Image source: Getty Images.

The company missed on earnings, revenue, and lowered its forecast

Moderna released its third-quarter results in November. For the period ending Sept. 30, its earnings per share of $7.70 came in much lower than the $9.05 that analysts were expecting. Revenue of $5 billion was also significantly less than Wall Street projections of $6.2 billion. The company also expects to deliver fewer vaccines this year, and its vaccine-related sales will be no higher than $18 billion. Previously, it was forecasting about $20 billion.

Generally, falling short on even one of these metrics can be tough, but Moderna fell short on revenue, earnings, and guidance. These failures could make it easy for its shares to drop even lower in the weeks and months ahead.

Why 2022 could be another big disappointment

For next year, Moderna anticipates that revenue will come in between $17 billion and $22 billion. Moderna says that it already has $17 billion in signed advance purchase agreements in place for 2022. However, there are multiple headwinds the company is facing that could potentially derail what looks like should otherwise be a strong year for the business.

The first is that there are more treatment options available. Healthcare companies Merck and Pfizer both have effective pills to treat COVID-19, and countries are eagerly buying them up. By having a pill at their disposal, countries may be less inclined to purchase vaccines. In addition, it gives unvaccinated individuals who may be reluctant to get the vaccine another reason to avoid doing so. Plus, there's also Regeneron's antibody cocktail that the European Commission approved this month to treat non-hospitalized patients (ages 12 and up) with COVID-19. This gives countries another option in the fight against illness.

Second, there's the issue of bad press surrounding Moderna's vaccine and cases of myocarditis, a rare heart condition. Although instances may be rare, they're concerning enough for some countries to take steps to not use the vaccine for young males -- the group that appears to be among the likeliest to have the condition. Data in France showed that among males between the ages of 12 to 29, myocarditis occurred in 13.3 cases per 100,000, compared to 2.7 cases for Pfizer's vaccine. Multiple countries, including Germany, France, Finland, and Sweden, are now recommending people under 30 (especially males) take the Pfizer vaccine instead. As studies on these issues continue and fears heighten, the demand for Moderna's vaccine may prove to be underwhelming.

Finally, there's also the issue of more potential vaccine makers entering the field. Biotech company Novavax is planning to file its COVID-19 vaccine for Emergency Use Authorization in the U.S. before the end of this year. And healthcare companies Ocugen and Bharat Biotech are applying for a Biologics License in the U.S. for their vaccine, Covaxin. Internationally, there will certainly be more competition as the World Health Organization recently added Covaxin to the list of vaccines for which it has issued an Emergency Use Listing. It's the eighth vaccine on its approved list. Novavax could soon be on there as well, as it also has applied for that listing.

Should you avoid Moderna's stock?

Moderna's sell-off may not be over, especially if the company's COVID-19 vaccine sales underperform next year and if the guidance isn't strong. The increase in competition, more options for countries (e.g. COVID pills), and negative press surrounding the Moderna shot are more reasons to suggest that Moderna may fall short of expectations rather than exceed them.

Trading at just over $230, the healthcare stock hasn't been this cheap since July. Multiple brokerages still have price targets set above $300 for Moderna. And while that could be a potentially decent 30% return from where the stock is today, the risk doesn't appear to be worth it. Analyst price targets also frequently change.

The further out your outlook, the more uncertain Moderna's future becomes, especially post-COVID. While the company's cytomegalovirus vaccine is in phase 3, at its peak, sales will be between $2 billion and $5 billion. Outside of that, there are many question marks in the company's pipeline, as nothing else is as far along.

With so much uncertainty and the headwinds facing the company, this isn't a stock I'd take a chance on right now. Although it's been a great investment over the past few years, it may be wishful thinking to expect that Moderna will continue to be a good buy moving forward.

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.