All the major U.S. indexes are in bear territory now, and many solid stocks have contributed to the market's downward tumble. But even during this rough year, some have been beating the market, and many of those outperformers can be found in the healthcare sector. Investors often favor that industry during tough times because its earnings are less at risk -- patients can't do without the products these companies sell.

Shares of Axsome Therapeutics (AXSM 0.64%), Regeneron Pharmaceuticals (REGN 1.31%), and Vertex Pharmaceuticals (VRTX 0.02%) have actually been climbing this year, and it doesn't look as if those trends will end any time soon.

1. Axsome Therapeutics

Axsome Therapeutics attracted investors' attention as it moved closer to product commercialization. First, it acquired Sunosi, a drug to treat sleep disorders, from Jazz Pharmaceuticals, and began selling it this spring. The drug brought in more than $8 million in about six weeks. If we use its recent sales at Jazz as a guide, there's reason to be optimistic: Sunosi sales rose 104% last year.

The biggest news for Axsome, though, is that the Food and Drug Administration approved its drug Auvelity as a treatment for major depressive disorder. You may be thinking, "Wait a minute, that's a pretty crowded market." And you'd be right. But Auvelity has a key feature that could lead to blockbuster sales. It's fast acting. While it takes weeks for patients taking most antidepressants to show improvement in their symptoms, Auvelity can have an impact in just a week.

According to a forecast by GlobalData, Auvelity's sales could reach $1.3 billion annually by 2029.

Another important element is the fact that all five candidates in Axsome's pipeline are already in phase 2 trials or farther along the clinical development path. So it may not be too long before it has other products ready for commercialization.

Axsome shares have given up some of the gains they made in recent months. They're now up by around 9% year to date after having been up by more than 80%. But over the long term, there could be a lot more share price growth to come. And driven by growing earnings, these gains could be lasting.

2. Regeneron

During the earlier days of the pandemic, Regeneron Pharmaceuticals brought in billions of dollars from its coronavirus treatment. But it no longer sells that product since it isn't efficacious against the omicron variant.

However, Regeneron still sells seven products, including the blockbuster eye drug Eylea. In fact, the company has a solid track record of billion-dollar revenues and profits. And in the second quarter, revenue excluding its coronavirus treatment increased by 20% year over year. The company also reported record net product sales for Eylea, the eczema drug Dupixent (which it shares with Sanofi), and cancer drug Libtayo.

The eras of sales growth for some of these key drugs may not be over. Eylea had U.S. sales of $1.62 billion in the second quarter. And Regeneron's collaboration revenue from Sanofi climbed 55% to $678 million. But their expansion into new indications could keep the growth of Regeneron's treatments going. For example, the FDA recently approved Dupixent as the first treatment for the skin disease prurigo nodularis.

Regeneron shares are up 15% this year, and the stock today trades at about 14 times trailing-12-month earnings. That's around its lowest level of the past five years. At the same time, its top line is rising -- even if it isn't as high as it was when Regeneron's coronavirus treatment was still generating sales.

At this level, Regeneron could attract investors looking for a bargain today -- and lasting returns in the future.

3. Vertex Pharmaceuticals

Vertex Pharmaceuticals is another player with a great track record. The company is the leader in cystic fibrosis  treatment. Its best-selling product is the triple-drug therapy Trikafta, which generated $5.6 billion in revenue last year. And that led to a profit of $2.3 billion for Vertex. The company also is moving through phase 3 trials with a candidate that could work even better than Trikafta. So, it's likely its dominance in the cystic fibrosis market will continue.

But Vertex isn't stopping at cystic fibrosis. It may be close to commercializing another potential blockbuster. Exa-cel is a one-time curative treatment for the inherited blood disorders beta-thalassemia and sickle cell disease. Vertex plans to complete regulatory submissions for exa-cel in the U.K. and Europe this year -- and a submission in the U.S. in 2023's first quarter.

Treatment options are limited for blood disorders right now, so exa-cel could be a game-changer. Doctors and patients may flock to this treatment, and that could lead to major revenue for Vertex.

In the near term, exa-cel news could drive Vertex's share performance. And in the longer term, growing revenue could keep that performance going. Vertex also has some intriguing candidates in its pipeline, targeting both rare diseases and common problems like pain. If it brings even a couple of these candidates to market, its already-solid earnings could skyrocket, and its share price might follow.

What does this mean for investors?

All three of these healthcare stocks would make great buys today because each company offers the potential for product revenue growth down the road. Investors have recognized this -- and, as revenue climbs, share price performance may follow. Of course, there may be pauses in the share price action here and there. But it looks as if these red-hot stocks have what it takes to keep growing over time.