Successful investing in stocks typically takes time. And over the long term, stocks can make for amazing wealth creators. But occasionally investors get lucky and latch onto a stock that soars quickly, doubling -- or even better -- in a year or less. And while plenty of investors will say "take profits" when a stock price doubles, sometimes the best thing to do with your biggest winners is to let them keep winning. 

What about MercadoLibre Inc. (NASDAQ:MELI)Shopify Inc. (US) (NYSE:SHOP), and Dynavax Technologies Corporation (NASDAQ:DVAX)? These three companies not only delivered doubles -- and then some -- in 2017, but two of the three have continued to deliver big gains so far in 2018. 

Three Motley Fool investors answer the big question: "Are they still buys?" Here's why they say yes -- even the one that surged an incredible 373% in 2017. 
Man with a surprised look on his face holding a toy rocket that's starting to take off.

Even after rocketing higher in 2017, we think these stocks are still buys. Image source: Getty Images.

How much juice does MercadoLibre have left?

Brian Stoffel (MercadoLibre): Shares of Latin America's biggest e-commerce player, MercadoLibre, more than doubled in 2017. That leaves shares trading at an expensive valuation: over 100 times trailing earnings and over 70 times trailing free cash flow. That said, the company's growth rates have been through the roof -- helping to justify the price tag.

But does that make the company a buy?

I've firmly put my own skin in the game, with shares accounting for over 6% of my real-life portfolio. The way I see it, there are three ways the battle for supremacy of e-commerce in Latin America -- likely with either or Walmart -- will go down, and two of them are favorable for MercadoLibre shareholders.

In the first scenario, MercadoLibre's head start over its American counterparts will be too much to overcome. It already has a positive brand locally, and it is starting to match any potential scale that Amazon or Wal-Mart could accomplish with its fulfillment network. Just as importantly, MercadoPago has high switching costs. MercadoLibre worked hard to help create relationships that allow the unbanked in Latin America to use the payment tool, making it hard to displace.

The second scenario sees one of the two larger rivals buying MercadoLibre, though that looks like a more difficult proposition by the day as its shares continue their ascent.

The third, much-uglier scenario envisions a price war among the players where the buildout of fulfillment centers continues unabated and all three try to undercut the competition on price, leading to unprofitable business models until one winner is left. With the smallest cash balance on the books, MercadoLibre wouldn't fare as well here, but it's also a scenario that would hurt both Amazon and Walmart as well -- that's why I find it unlikely.

Only time will tell how it all shakes out, but for now, I firmly believe that MercadoLibre shares are still worth holding.

This e-commerce disruptor may just be getting started

Jason Hall (Shopify): Shopify's stock price didn't just double in 2017.  It's up an incredible 219% to date, since the start of 2017. Even with that remarkable performance, I think that even near its all-time high, Shopify is worth buying and holding for the long term. As the clear leader in e-commerce platforms, Shopify is set to profit and grow for years as e-commerce becomes a bigger and bigger piece of the way we buy things. 

But Shopify's success isn't its role in the supposed "death" of bricks-and-mortar retail. The lines between traditional retail and e-commerce are blurring, and Shopify provides a suite of powerful tools for merchants to successfully integrate e-commerce with their physical presence. This is important, because a large portion of traditional retail is healthy and growing, and can utilize e-commerce to augment -- and help grow -- their physical operations. Lastly, some of the biggest merchants in the world -- Amazon and eBay, not to mention social media giant Facebook -- have adopted Shopify as their preferred tool to use to sell through their websites. 

Sales are absolutely rocketing higher, climbing 73% last year and an incredible 541% since going public in early 2015. But total revenue was only $673 million over the past 12 months. There's still a massive amount of growth in Shopify's future. 

SHOP Revenue (TTM) Chart

SHOP revenue (TTM). Data by YCharts.

Shopify's stock will almost certainly be volatile going forward, and it could take a beating in market downturn. But if that happened, I'd be taking that opportunity to buy even more shares, since the company's results and prospects would be largely unaffected. 

A cheap growth stock

George Budwell (Dynavax Technologies Corporation): Shares of Dynavax Technologies nearly quadrupled in value last year, thanks to the Food and Drug Administration approval of the mid-cap biotech's hepatitis B vaccine, Heplisav-B. Even so, the stock is still grossly undervalued relative to the company's long-term growth prospects. 

With an FDA approval finally in hand, Heplisav-B is set to rapidly become the market share leader. The vaccine, after all, has been shown to be more potent than GlaxoSmithKline's Engerix-B in clinical trials, and it also requires fewer doses (two doses administered over a month, compared to three doses over six months for other FDA-approved hep B vaccines). The vaccine also recently received a key endorsement from the Centers for Disease Control and Prevention's advisory committee on immunization practices. 

As a result, Dynavax estimates that Heplisav-B can generate at least $500 million in sales at peak. Some Wall Street analysts, on the other hand, have put this figure closer to $750 million. Dynavax's shares are therefore trading at about twice Heplisav-B's more conservative peak at present, which is rock bottom for a biotech with an important new product coming to market.

Why is Dynavax so cheap? The big issue is that investors apparently preferred Dynavax to strike a lucrative licensing deal, rather than plow capital into a building a sales force from scratch. The company, after all, has no experience handling a commercial launch, implying that Heplisav-B might take longer than expected to gain traction in the marketplace.  

Beyond Heplisav-B, Dynavax is also building out a portfolio of promising immuno-oncology products for indications such as lung cancer, head and neck cancer, and advanced melanoma. The market clearly isn't putting much faith in Dynavax's immuno-oncology pipeline based on its current valuation. But these experimental cancer drugs may eventually be a major growth engine for this deeply undervalued biotech stock.  

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.