One of the best ways to build wealth is to purchase stocks -- particularly those of companies with strong economic moats -- and hold these stocks for a long time. Witnessing the stock market get clobbered, as it has as a result of the COVID-19 pandemic, can make it hard to trust this process.

Still, you can be reasonably confident that this time-tested strategy will eventually pay off, and despite the recent market correction, now is as good a time as any to look for excellent stocks that can earn above-average returns in the long run. With that in mind, here are two such stocks to buy with an extra $2,000: Zoetis (ZTS -4.32%) and Guardant Health (GH -3.60%).

The leader in animal health

Zoetis has trounced average market returns since it spun off from Pfizer (PFE 2.40%) in 2013.

Zoetis stock

ZTS data by YCharts

The company was able to achieve such returns thanks to its leadership position in the market for animal health products. The company develops and markets several products, including medicines, vaccines, and diagnostic instruments, for eight core species of animals, including cattle, poultry, and fish.

Several facts provide strong evidence of Zoetis' dominance in its industry. Here are two of them. First, the company's operational revenue growth has consistently outperformed that of its industry since 2013. For instance, Zoetis' operational revenue increased by 8% in 2018 and 2017, compared to the market averages of 6% and 4% for 2018 and 2017, respectively. Second, 90% of Zoetis' portfolio comprises products for which the company holds a leadership position in the market.

Veterinarian holding a dog and a cat.

Image Source: Getty Images.

Zoetis is looking to continue on its upward trajectory, and there are good reasons to think it can do so. With a growing population and a growing middle class ready to spend money on pets, there will be an increasing need for animal health products, and Zoetis is well positioned to profit from these trends. Zoetis -- whose revenue for the fiscal year 2019 was $6.3 billion -- sees a $40 billion market opportunity ahead for products within its market. If Zoetis maintains its strong standing within its industry, the company could continue to provide market-beating returns for many years to come.

The future of cancer diagnostics?

Guardant Health is a leader in the market for liquid biopsies, tests that allow physicians to look for cancer cells from tumors in samples of blood from patients. This cancer diagnostic method offers several advantages over the old-fashioned method, tissue biopsy, which requires direct, physical access to sample tissue from a patient's body. Tissue biopsies are often more expensive and more time consuming, not to mention more invasive, and are less adept at detecting cancer early than liquid biopsies.

Guardant Health's core products include its Guardant360, which is a molecular diagnostic test that allows physicians to find the best treatment option for cancer patients. Also, there's the GuardantOMNI, which is used by pharmaceutical companies as a tool to "help identify patients whose cancer has the right molecular profile for their clinical program."

Both of these products have been racking up strong and growing sales. For instance, during the fourth quarter, Guardant Health recorded total revenue of $62.9 million, representing a 91% increase compared to the year-ago period. The company's precision oncology testing revenue -- which is generated from the sale of its Guardant360 and GuardantOMNI products -- was $57.4 million, increasing by 104.3% year over year.

Guardant Health has provided splendid returns in recent years.

GH Stock

^SPX data by YCharts.

The company might see even better days ahead. Guardant Health's next wave of liquid biopsies -- Lunar-1 and Lunar-2 -- could be significant growth drivers in the future. Lunar-1 can be used to match early-stage cancer patients with neoadjuvant (therapies administered before the primary treatment) and adjuvant (therapies administered after the main treatment to target remaining cancer cells) treatments. On the other hand, Lunar-2 helps with the early detection of cancer, which significantly increases the chance of survival.

While neither of these products has been approved yet, Guardant Health believes that their combined market opportunity reaches about $45 billion. Considering that combined with Guardant Health's already successful Guardant360 and GuardantOMNI, the company looks poised to continue growing its top line at a good clip. Investors would do well to consider buying shares of Guardant Health right now.

Investor takeaway

While both of these healthcare stocks present strong prospects, nothing is set in stone. The ongoing COVID-19 outbreak could severely impact both Zoetis and Guardant Health. And given the current uncertain economic conditions, you should only invest in stocks if you have enough savings in the bank, as well as some disposable income with which you are comfortable parting ways. If those criteria apply to you, Zoetis and Guardant Health are both excellent companies to invest in.