Next year is going to be challenging for COVID-19 vaccine maker Moderna (MRNA 1.58%). With up to $19 billion in revenue this year potentially coming from its vaccine, there could be a sizable drop off in sales next year, even if health officials continue to administer booster shots. The company's pipeline is by no means bare, but it won't be easy to make up for a significant decline in revenue. In the U.S., Moderna projects the COVID vaccine market could be worth between $5.2 billion and $12.9 billion, depending on the price that it obtains per shot.

But there are a couple of trends that could make the stock even riskier next year. Before investors decide to buy the healthcare stock, they should consider the following two items:

Expenses are soaring

For the first three quarters of the year the company has incurred operating expenses totaling $6.3 billion. That's an increase of 88% from the same period last year. While some of the company's expenses will decline if the business isn't as busy, some could remain elevated. Selling, general and administrative costs have doubled to $757 million, and research and development costs of over $2 billion are also up over 55%.

The company's expenses may also continue to increase, as Moderna is investing $500 million to build a manufacturing facility in Kenya that could be producing vaccines as early as next year. Plus, with the business investing into its pipeline to develop future blockbusters that can help generate billions in revenue besides the COVID vaccine, rising costs and lower profits could be something investors need to accept.

Year to date, the company has reported revenue of $14.2 billion, representing an increase of 26% year over year. However, net income of $6.9 billion has declined by 6%.

Cash flow is declining

Rising expenditures won't just hurt the company's profit, but will also lead to less cash flow. Over the past three quarters, Moderna has brought in $3.3 billion in operating cash from its day-to-day operating activities, versus $10.3 billion this time last year. And when looking at just an individual quarter, the trend looks even worse:

MRNA Cash from Operations (Quarterly) Chart

MRNA Cash from Operations (Quarterly) data by YCharts

It's not an overly concerning issue right now, as the company has $8.3 billion in cash and investments on its books as of the end of September. However, if spending increases, this could become a problem in the future, especially with the company spending heavily on share repurchases. This year it has announced multiple share repurchase programs, each for $3 billion. It has completed one already, and another is still in effect.

For a company with question marks about its future growth, buying back shares is not something I'd want to see as an investor. Even if the company doesn't see deals out there right now, the more cash it has available, the stronger of a position the business will be in, especially when an opportunity arises to expand the business via acquisition.

Why I'd avoid the stock

How a company allocates capital and controls spending is crucial for its long-term success. Besides hoping for Moderna's pipeline to pay off, I'm not sure there's enough behind the business today to make it a good buy, even at a single-digit earnings multiple.

A few years down the road, the company could be generating revenue from a vaccine for the respiratory syncytial virus and a shot covering the flu and COVID -- but even combined, they likely won't be anywhere near as lucrative as the COVID-19 vaccine market has been for Moderna over the past two years. Until there's a stronger path for long-term growth, the stock simply isn't worth the risk.