A big concern for investors looking to buy Moderna (MRNA -0.58%) stock has been what the future holds for the business in the long run. While it has generated billions in revenue this year from its COVID-19 vaccine, sales from Spikevax will inevitably decline as concerns about the pandemic subside. 

The company is attempting to diversify, and it has multiple trials ongoing, which include testing vaccines for the flu and the respiratory syncytial virus (RSV), which could help the business offset some of the decline in revenue. And earlier this month, Moderna gave investors some exciting news that has made them more bullish on the stock.

Personalized cancer vaccine candidate shows promise

On Dec. 13, Moderna issued a press release stating that its personalized cancer vaccine candidate, mRNA-4157/V940, when used in combination with Merck's top-selling cancer drug, Keytruda, was effective in treating people with advanced melanoma. According to the data, it reduced the risk of death and recurrence by 44%. 

Moderna CEO Stéphane Bancel was excited with the results, noting: "Today's results are highly encouraging for the field of cancer treatment. mRNA has been transformative for COVID-19, and now, for the first time ever, we have demonstrated the potential for mRNA to have an impact on outcomes in a randomized clinical trial in melanoma."

Given that the market for cancer drugs is worth around $150 billion and could grow to nearly $290 billion by 2030, it's easy to see why this would get investors excited about the results, as having an effective oncology treatment could be a huge moneymaker for both Moderna and Merck.

The positive results may not be entirely a surprise since earlier this year Merck exercised an option to jointly develop and commercialize this vaccine with Moderna. At a cost of $250 million, the company must have had reason for optimism.

Time to temper investor expectations?

The positive news surrounding the personalized cancer vaccine sent shares of Moderna up to over $200 for the first time since January. However, the stock is still down close to 40% from its 52-week high of $321.30. And trading at a multiple of 30 times its future earnings (based on analyst expectations), the healthcare stock doesn't look like a cheap buy -- the industry averages a multiple of 17.

One reason investors may want to scale back the excitement is that these recent results weren't for a late-stage trial, and Merck and Moderna will only initiate a phase 3 trial for the personalized cancer vaccine next year. So it could be at least several years before the vaccine, assuming it obtains approval from the Food and Drug Administration, might bring in any revenue for Moderna. And there's still considerable risk for risk investors as oncology treatments typically have the worst odds for success, with roughly 3% of them making it all the way through clinical trials and to approval.

That doesn't mean Moderna's personalized cancer vaccine won't be successful, but it's a reminder to investors to tread carefully and not to set expectations too high.

Moderna's stock still isn't a buy

Moderna isn't a stock I would invest in today because the risks are far from gone. The company has already seen its profits decline by nearly 70% to just over $1 billion for its most recent quarter (ended Sept. 30).

A softer market for Spikevax next year could mean even worse results are ahead for the business. Moderna expects to bring in up to $19 billion in revenue this year, and it may be a long time before the business gets back to these levels again -- assuming it ever does.

The company's reluctance to pursue acquisitions and diversify means investors are still taking on significant risk with Moderna, and given its volatility, this is an investment I'd steer clear of, as there are better, safer growth stocks out there to invest in today.