It appears that COVID-19 is resurging in the United States, and that has led some states to reimplement social distancing measures to slow the spread of the coronavirus. In this uncertain economic environment, investors should take steps to help protect their portfolios by investing in coronavirus-resistant companies that can benefit from -- or at least be less hurt by -- stay-at-home requirements. Here are two companies that fit the bill.

The first pick is Activision Blizzard (NASDAQ:ATVI), a video game company with strong intellectual property and a robust pipeline. The second stock is Zynga (NASDAQ:ZNGA), a mobile game developer poised to benefit from growing demand for mobile entertainment. Both companies are poised to outperform the market because of their potential for top-line growth and coronavirus-proof business models.

$100 bill with Ben Franklin wearing a mask.

Image source: Getty Images.

1. Activision Blizzard: Stock is up 36% year to date

Activision Blizzard is a video game developer known for blockbuster titles like Call of Duty and World of Warcraft. The company monetizes its industry-leading intellectual property to deliver value to shareholders, and the stock has soared 36% year-to-date -- trouncing the S&P 500's return of just 2% over that same period.

While revenue dipped 2% to $1.79 billion in the first quarter, the company grew net income by 13% by lowering costs and expenses. And the game developer is poised to return to top-line growth because of several compelling new releases. 

Activision released Call of Duty: Mobile and Call of Duty: Modern Warfare in the fourth quarter of fiscal 2019. Both titles are performing well -- with Modern Warfare selling more units than any prior Call of Duty title at this point after release. In fiscal 2020 the company released Call of Duty: Warzone, a battle royale game that uses a lucrative "freemium" business model. Players can download the game for free, but they will have to pay for cosmetic upgrades and expanded functionality.

Freemium is an effective monetization strategy that has worked for hits like Tencent's League of Legends and Valve's CS:GO. Both games are cult favorites with flourishing esports scenes, and Warzone could be poised for similar success.

2. Zynga: Shares are up about 60% year to date

Zynga is a mobile game developer that has enjoyed practically uninterrupted stock price growth during the coronavirus pandemic. Shares have rallied 60% year to date because of the company's coronavirus-resistant business model and promising acquisition-driven growth strategy.

Zynga reported earnings on May 6, and the results show strength despite this uncertain economic environment. The company achieved the highest first-quarter top-line performance in its history with total revenue soaring 52% to $403.77 million. The strong performance was partially driven by Zynga's earlier acquisition of Small Giant Games, which is performing ahead of expectations according to management.

Zynga bought a majority stake in Small Giant for $700 million in 2018, giving it access to the fast-growing Empires & Puzzles mobile game which helped drive the company's strong first-quarter showing.

Zynga is doubling down on its roll-up strategy with a $1.85 billion acquisition of Istanbul-based Peak Entertainment in July. This deal gives the developer access to the Toon Blast and Toy Blast mobile game franchises, which have ranked among the top 10 and top 20 grossing iPhone games over the past two years. Zynga will also benefit from the talented team behind Peak which could help the company develop new assets in the future.

The takeaway

The coronavirus pandemic is still running its course, and investors shouldn't get complacent about protecting their portfolios from this challenge. Activision Blizzard and Zynga deliver the potential for massive top-line growth coupled with impressive coronavirus-proof business models, and that's why they will likely be good stocks to buy in this uncertain economic environment.