When reviewing the companies with the fastest growth in 2020, it immediately becomes clear that the COVID-19 pandemic is behind most of the changes that have propelled sales for diagnostics and remote-work companies. Fulgent Genetics (FLGT -0.33%) and Zoom Video Communications (ZM 0.43%) have captured those trends and are delivering some of the highest growth around.

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1. Fulgent Genetics

In the competitive market of diagnostic testing, Fulgent is differentiating itself by offering flexibility and affordability to its customers. The company has one of the largest genomic test menus in the industry (almost 19,000 tests) and can provide pre-established tests or customize them for specific needs. Management reported year-over-year revenue growth of 880% in the third quarter, thanks to a 5,000% increase in testing volume. The company expects full-year revenue of $235 million, compared to $32.5 million in 2019 -- good for 623% growth.

In case you think the increase is only due to the coronavirus, that 2019 revenue was 52% higher than in 2018, and management said non-coronavirus revenue growth in the latest quarter was 57%.

The increase is being driven by new agreements with large customers such as commercial organizations, municipalities, medical institutions, and biotech companies. Wider acceptance of Fulgent's tests by insurers is also helping. The company filed 590,000 claims with insurance companies in the third quarter, a 9,000% increase over the second quarter. Customers had previously been paying out of pocket.

The business is also demonstrating scalability. While year-over-year testing volume grew 5,000% in the third quarter, operating expenses only climbed 72.5% to $11.9 million. This is a great indication that as the business grows, it will become more profitable. In fact, cost per test decreased from $35 in the second quarter to only $25 in the third quarter. Although growth won't remain at these levels after a vaccine is more broadly rolled out, the company expects the demand for antibody tests to persist.

Despite the growth, the company's price-to-sales ratio (P/S) sits at 8.4, slightly above where it did at the end of 2019. If the demand for COVID-19 testing persists, as the company expects, the current stock price could be a bargain.

2. Zoom 

When Zoom reported its fourth-quarter 2020 earnings on March 4 for the quarter ending Jan. 31, management was already outlining the increased use it was seeing. When the coronavirus crisis began, company leaders offered plans to increase capacity, help educational institutions, and remove time limits on free accounts in China to facilitate the transition to a remote world. Despite analysts' focus on how the outbreak might affect the sales force, company executives continued to stress the increased use they were seeing and the need for empathy and humanity above revenue.

Since then, the shift to a remote work environment and rapid adoption of Zoom's web-based meeting application have driven sales and shares up significantly. The stock is up about 425% so far this year, and for the first nine months of the company's fiscal 2021 (February through October), sales have climbed to $1.77 billion, up 307% over the same period in 2019. Revenue for the August-to-October quarter was $777.2 million, up 367% over the same period in 2019. The increase fed through to the bottom line: Zoom went from $0.03 in earnings per share during those nine months of 2019 to $1.38 in 2020.

It would be natural for sales to fall off as COVID-19 vaccines are distributed and people get comfortable returning to the office, traveling for work, and meeting in person with friends and family. But Zoom has a plan to keep the growth going. The company is reportedly developing broader enterprise tools such as email and other productivity applications to compete with the likes of Microsoft and Alphabet's Google. The company has a track record: It has already introduced conference software, cloud-based telephony, and an online events marketplace this year. 

CEO Eric Yuan is not resting on his laurels. Zoom was a rare combination of fast growth and profitability coming into 2020, and the pandemic has only accelerated those trends. If the company plans to grow beyond its current $107 billion market capitalization, it makes sense to replace tools that are necessary but mostly disliked. Zoom took the videoconferencing market by storm when it introduced software that was easy to use and solved specific customer pain points. If it can do the same with email, then revenue -- and the stock price -- could continue climbing long after COVID-19.