Moderna (MRNA -1.22%) delivered its biggest one-day gain in a long time on Tuesday. Its shares skyrocketed more than 20% after the company and its partner, Merck (MRK 0.36%), announced great results from a phase 2 study of experimental personalized cancer vaccine mRNA-4157/V940 combined with cancer immunotherapy Keytruda. 

Even with yesterday's surge, the biotech stock remains down significantly below its highs. But is Moderna stock a buy after its impressive cancer vaccine results?

About those results

Make no mistake about it: Moderna's and Merck's results were definitely impressive. The combination of mRNA-4157/V940 and Keytruda reduced the risk of recurrence of melanoma or death by 44% versus Keytruda alone. That's huge.

The two companies now plan to advance the cancer combo into late-stage testing in 2023, pending a green light from regulators. It's quite possible that Moderna and Merck could have a winning melanoma treatment within a few years.

Phase 2 success doesn't guarantee that everything will go well in the days ahead, though. Indeed, less than 36% of oncology therapies that enter phase 3 testing win regulatory approval, based on data published in the scientific journal Biostatistics in 2018.

Even if the combination of mRNA-4157/V940 and Keytruda eventually secures regulatory approvals and has a successful commercial launch, Moderna won't pocket all of the proceeds. Two months ago, Merck exercised its option to jointly develop and market the combo therapy. The partners will split any profits made 50-50. 

But there's also reason to be more optimistic about Moderna's prospects now. The positive results announced this week are encouraging about the opportunities for personalized cancer vaccines to be used in treating other types of cancer. Moderna CEO Stéphane Bancel underscored this opportunity in his comments, stating, "We will begin additional studies in melanoma and other forms of cancer with the goal of bringing truly individualized cancer treatments to patients." 

Still centered on COVID

However, Moderna's near-term fortunes continue to hinge on its COVID-19 vaccines. It's fair to say that the outlook on this front is murky at best.

Moderna has demonstrated that its bivalent booster mRNA-1273.222 works well against coronavirus omicron variants. It has also been successful in winning regulatory authorizations and approvals for this booster. As a case in point, Moderna picked up U.S. Emergency Use Authorization for mRNA-1273.222 in children ages six months through five years last week.

But far fewer Americans have received the bivalent boosters than received the initial primary series of shots. That's the case even in the higher-risk group of senior adults. 

Applying Buffett's test

Warren Buffett stated several years ago that his first step in deciding whether or not to buy a stock was to estimate the business' earnings for the next five years or more. If he can't estimate future earnings, he doesn't buy the stock. 

In my view, the hardest question for Moderna is estimating the company's earnings going forward. There are significant uncertainties surrounding the COVID-19 vaccine market. Analysts can take a stab at projecting Moderna's profits, but it's just guesswork at this point.

The latest results for Moderna's personalized cancer vaccine are promising. Again, though, it's very difficult to accurately predict what impact the vaccine might have on Moderna's financial position in the future. The same is true for the company's other pipeline candidates.

I truly believe that Moderna could achieve overwhelming success with its mRNA programs over the long run. However, Buffett's process of picking stocks is a good one. Despite the impressive results announced this week, I don't think that Moderna passes the Buffett test -- for now.