Volatility has returned to the stock market, but finally, the Nasdaq Composite (NASDAQINDEX:^IXIC) is starting to make waves once again. The tech-heavy index is making a run toward all-time highs, trading within 1% of its high-water mark on Monday afternoon. As of just before 2 p.m. EDT today, the Nasdaq  was higher by three-quarters of a percent.

It wasn't that long ago that investors figured that stocks of COVID-19 vaccine manufacturers had already seen their best days. Companies like Moderna (NASDAQ:MRNA) and BioNTech (NASDAQ:BNTX) had seen their share prices start to give up ground as many believed that a vaccinated world would eventually cause revenue and profits to dry up for the vaccine makers. Now, though, it's becoming increasingly clear that the two companies could well have a much brighter future than many had thought.

More moves for Moderna and BioNTech

Shares of the vaccine manufacturers were among the leaders on the Nasdaq today. Moderna's gains amounted to more than 5%, while BioNTech boasted gains of 6% or more on the day.

Health professional administering vaccine shot to patient.

Image source: Getty Images.

The general sentiment toward BioNTech and Moderna has been positive because of just how effective their vaccines have been. Last month, the U.S. Centers for Disease Control and Prevention released the latest figures on efficacy for the messenger-RNA-based vaccines from the two companies. Data from real-life use showed a reduction in infection risks of 91%. Those who got infected had a 60% lower risk of showing symptoms, and they spent on average six days fewer being sick and two days fewer stuck in bed recovering.

In addition, the companies have benefited from sustained demand for COVID vaccines from countries around the world. On Monday, BioNTech said that it had received provision approval of its vaccine from regulators in New Zealand. Over the weekend, the government of the Philippines announced a 40-million-dose agreement with BioNTech and Pfizer (NYSE:PFE) for more vaccine doses as well.

More broadly, some health officials have started talking about the potential need for vaccine booster shots. It's uncertain at this point whether and how quickly antibody levels from initial vaccinations decline, and so it's entirely possible that even those who've already received vaccinations could need additional doses in the future. From a business standpoint, that would create even further demand for Moderna and BioNTech that could dramatically lengthen the expected flow of revenue stemming from COVID vaccines.

Will existing vaccines be enough?

The biggest threat on the COVID front comes from the potential for the virus to mutate into more-dangerous variants. Already, the Delta variant has proved to be more easily transmitted among infected patients and with more-severe health impacts. Future variants could prove even more problematic, and there's no guarantee that existing vaccines will provide protection against them all.

For the most part, both Moderna's and BioNTech's stock prices seem to reflect little expectation of success beyond the current COVID vaccine products. Yet if anything, COVID has proved that the broader-based investing thesis behind mRNA-based treatment development is sound. Both companies have plans for vaccines and other treatments for a wider variety of different medical conditions, and success anywhere on that front could provide the positive surprise investors need to gain confidence in the long-term futures of these stocks.

If you made the mistake of thinking that COVID vaccine stocks would be done once much of the U.S. population had been vaccinated, you aren't alone. But you might be surprised at how much staying power BioNTech and Moderna could have -- especially if a few things end up working out in their favor.

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.