By definition, a good growth stock will deliver outsized revenue gains compared with its peers. However, a top-tier growth stock can sustain that faster pace of sales increases over the long term. 

Among the firms that fit into that second classification at the moment are Fastly (FSLY -3.00%) and BioNTech (BNTX 0.43%). In the past 12 months, their share prices have risen by 20.34% and 273.1%, respectively.

Let's look at what tools each of them has to keep that momentum going. 

Piggy bank on top of coin stacks - growth concept.

Image source: Getty Images

Edge computing specialist Fastly dances ahead

Fastly is one of the best bargains available in the cloud-computing space, trading at only 12.7 times forward revenue. Last year, the company lost approximately 12% of its annual sales after ByteDance -- the owner of the popular app TikTok -- pulled the plug on its contract. This was a result of the Trump administration's threats to ban the video-sharing app in the U.S. Despite a change of administration in Washington, however, ByteDance -- once Fastly's largest customer -- has not renewed their relationship.

Luckily, Fastly is on track to find new sources of revenue to compensate. During the first quarter, Fastly's sales increased by 35% year over year to $83 million. Further, it achieved a net retention rate of 133%, indicating that it can grow revenue even without adding new customers.  

Its number of enterprise clients increased to 336 vs. 324 a year prior. What's more, per-capita customer spending in this segment grew year over year from $782,000 to $800,000.

Fastly plays a leading role in our increasingly digital economy. It delivers over 130 terabytes of content (such as video streaming) every second across the high-tech, education, e-commerce, and gaming sectors to users in 26 countries.

For 2021, the company expects to generate $385 million in sales and an operating loss of $45 million -- substantially better than the $291 million in revenue and $57 million in operating losses it booked in 2020. Tech investors should definitely check out this stock. 

BioNTech will be battling COVID-19 for years

This year, BioNTech and its partner Pfizer (PFE 0.76%) expect to sell $26 billion worth of their coronavirus vaccine, Comirnaty. After accounting for a 50-50 revenue split and a pre-tax margin in the 26% to 29% range, that would give BioNTech estimated earnings of around $3 billion. Not bad for a firm with a market cap of $44.37 billion.

The COVID-19 vaccine is authorized for use in more than 56 countries, and BioNTech expects to deliver 1.6 billion doses this year jointly with Pfizer. Comirnaty has demonstrated an estimated 97% efficacy at preventing severe cases of COVID-19. 

The partners are also in negotiations with governments worldwide to supply 3 billion doses of the vaccine by 2022. But for those who think that coronavirus vaccines will provide their developers with nothing more than short-term boosts to cash flow, think again.

Medical experts anticipate that booster shots will likely be necessary for people to maintain their resistance to COVID-19. And while mutations have created numerous new variants of concern, those annual vaccinations can and will be tailored to be more effective against them.

That's the most sustainable part of BioNTech's growth story. In addition, the company has been reinvesting its mRNA vaccine earnings into research and development. Currently, it is on track to deliver five updates on clinical trials of its solid tumor therapeutic candidates.

On top of that, BioNTech seeks to carry over its mRNA technology to develop HIV and tuberculosis vaccines. Overall, this emerging biotech is a promising candidate for investors' portfolios.