A handful of biotech companies have been stock-market winners even as the coronavirus outbreak has pushed the market as a whole into bearish territory. The S&P 500 is down 25% since the start of the year, while Moderna (NASDAQ:MRNA) has gained 47%, Regeneron Pharmaceuticals (NASDAQ:REGN) rose by 34%, Gilead Sciences (NASDAQ:GILD) climbed by 24%, and Inovio Pharmaceuticals (NASDAQ:INO) soared by 96%.

There is no secret to these companies' short-term performance. Each one is working on a treatment or vaccine for coronavirus, the pandemic that so far has resulted in more than 207,000 cases of COVID-19 worldwide. Now investors are wondering: Considering the gains, is it too late to buy shares of these companies?

A vaccine dose is shown against a blurred background.

Image source: Getty Images.

With countries including France and Italy on lockdown, and coronavirus cases increasing in the U.S. and elsewhere, the outbreak isn't over. As long as it continues and biotech companies make progress on their treatment or prevention programs, upside is possible. But that remains a short-term investment strategy.

Revenue gains from coronavirus treatments may be limited, because of the short time frame of pandemics and the ethical question of vaccine pricing. I favor long-term investing, based on each company's entire pipeline. Let's have a closer look at these recent stock-market stars.

Moderna

Moderna is working on a coronavirus vaccine based on its messenger RNA (mRNA) technology, which uses mRNA to instruct the body's cells to make proteins that would prevent disease. Moderna this week said the first participant in its Phase 1 clinical trial for the vaccine has been dosed. The trial, conducted by the National Institutes of Health, will study the safety of the investigational vaccine and the immune responses it generates.

Moderna, on its own, aims to begin a Phase 2 study in a few months. The company has more than 20 candidates in the pipeline for treatment areas including infectious diseases and cancer, but hasn't yet completed Phase 2 studies for any of them. That means we don't yet know whether Moderna's mRNA technology works in humans.

Regeneron

Regeneron this week announced the discovery of hundreds of antibodies that could neutralize coronavirus in its work toward a treatment with the U.S. Department of Health and Human Services. Regeneron said it will select the top two antibodies for its potential treatment and aims to start testing in humans in early summer.

Separately, the company is working with French drugmaker Sanofi (NASDAQ:SNY), and as part of that project just started a Phase 2/3 trial for rheumatoid arthritis drug Kevzara in New York patients suffering from COVID-19. Beyond work on coronavirus treatments, Regeneron has nearly 30 products in the pipeline in areas such as cardiovascular health and oncology, with six approved drugs. The company has increased annual revenue since 2012.

Gilead

Earlier this month, Gilead began two Phase 3 studies for its investigational antiviral, remdesivir, in coronavirus patients. More recently, the director of the U.S. Centers for Disease Control and Prevention said remdesivir was available via "compassionate use" to coronavirus patients in Washington state, the U.S. epicenter of the outbreak.

Gilead has a full pipeline and nearly 30 products on the market in treatment areas including HIV and cardiovascular health. The company posted a 12% gain last year in sales of HIV products. Analysts predict the HIV drug market will reach $26 billion by 2022 from $20 billion in 2015. And the U.S. Food and Drug Administration is now reviewing filgotinib, Gilead's drug for rheumatoid arthritis, another growing treatment area.

Inovio

Inovio announced recently that it would begin human trials of its coronavirus vaccine next month and aims to present data in the fall. The company's timeline predicts the production of 1 million doses for additional trials or emergency use by the end of the year.

Inovio's investigational products are based on its DNA technology. Here's how it works: Inovio's handheld smart device delivers optimized plasmids into cells via an injection into either the muscle or the skin. These plasmids are circles of double-stranded DNA that have been reorganized by computer sequencing so that the body will produce a specific immune response. Once in the body, these plasmids replicate.

Inovio has seven candidates in the pipeline for multiple tumor types and infectious diseases. This year could be significant for its most advanced product, an immunotherapy for cancers caused by human papillomavirus, as the company expects to report top-line efficacy data from a Phase 3 trial in the fourth quarter.

So what's an investor to do?

I tend to favor biotech companies such as Gilead and Regeneron thanks to the revenue stream they enjoy from products already on the market. But clinical-stage companies like Moderna or Inovio may offer investors just as much opportunity -- if the pipeline is strong enough and the share price hasn't been skyrocketing.

Moderna and Inovio have gained too much too quickly based only on optimism about their coronavirus treatments, so I don't recommend buying them for the long term right now. But once the coronavirus storm passes and the share prices settle, they may be worth a second look.