Warren Buffett's holding company Berkshire Hathaway (NYSE:BRK.A) (NYSE:BRK.B) has been able to produce jaw-dropping gains for its shareholders over the years by adhering to three basic principles:

1. Buy high-quality companies with strong brand recognition, healthy balance sheets, and a well-established competitive moat. These types of companies can better handle economic downturns and even periods of poor management.
2. Buy stocks with the expectation of holding them for no less than a decade, if not longer.
3. The best time to load up on stocks is when everyone else is selling. As Buffett himself puts it, "The best chance to deploy capital is when things are going down."

WARREN BUFFETT. IMAGE SOURCE: THE MOTLEY FOOL.

But even Berkshire Hathaway hasn't been able to escape the ravages of the marketwide meltdown over the COVID-19 coronavirus. This indiscriminate wave of selling, though, is undoubtedly creating some once-in-a-lifetime buying opportunities for patient investors. Within Warren Buffett's own portfolio, for instance, the healthcare giants Biogen (NASDAQ:BIIB), Johnson & Johnson (NYSE:JNJ), and Teva Pharmaceutical Industries (NYSE:TEVA) all appear to be trading at bargain basement prices after their steep declines over the past few weeks. Here's why.

Biogen: A super-cheap large-cap biotech

Biogen is a leader in the treatment of multiple sclerosis and spinal muscular atrophy, and in the development of cheaper versions of biologically based therapies, known as biosimilars. Additionally, the biotech is developing several high-value clinical assets for hard-to-treat neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease. Further down the line, Biogen also has designs on becoming a major player in the red-hot field of gene therapy, evinced by its acquisition of Nightstar Therapeutics and recent partnering deal with Sangamo Therapeutics.

At last count, the company had approximately $4.5 billion in cash and a fairly reasonable debt-to-equity ratio of 48.2. Since the biotech doesn't have to worry about covering a dividend and its revenue stream should be more or less immune to the COVID-19 threat, Biogen should thus have little trouble weathering this ongoing public health and economic crisis. Biogen, in effect, is a prototypical Buffett stock in many ways.

Should investors buy this blue chip biotech in the wake of its 6% decline in 2020? Biogen's near-term outlook is undoubtedly closely tied to the prospects of its high-risk Alzheimer's disease candidate aducanumab. That being said, the company does have some truly exciting early stage assets under development following its recent business development activities, as well as an overall healthy balance sheet and a stable near-term outlook. Those are all highly prized commodities for long-term investors, especially in this turbulent market. As such, bargain hunters might want to consider this top biotech stock soon.

Johnson & Johnson: A rock-solid dividend you can count on

J&J's shares have fallen by almost 18% in 2020. This giant of the healthcare landscape is thus trading at just 3.5 times next year's sales, and its dividend has ballooned to a handsome 3.17% yield after this sizable pullback.

Investors appear to be worried that J&J's consumer healthcare segment, which sells baby powder, Band-Aids, and skin and hair care products, among many others, may take a major hit from the COVID-19 economic downturn. Perhaps it will, but it's hard to imagine J&J's consumer healthcare sales dipping to the point that would justify a nearly $80 billion drop in the company's market capitalization since the start of the year.

The big picture is that J&J has one of the strongest balance sheets in the entire corporate world, it is a Dividend Aristocrat with over a half-century of consecutive dividend increases, and it has been a cornerstone of the American healthcare landscape for over 130 years. This isn't J&J's first economic crisis, and it won't be the company's last by a long shot.

J&J will survive COVID-19, perhaps with only minor dings to its core businesses. So do yourself a favor financially and buy J&J's stock on this unprecedented pullback. You will probably never have this chance to buy the shares at these rock-bottom levels ever again.