New dangerous coronavirus variants aren't on the way to the U.S. -- they're already here.

Many Americans have begun to feel increasingly confident that the worst of the COVID-19 outbreak is behind us. In India, though, the story is much worse. A mutation of the novel coronavirus called the Delta variant swept across the sub-continent. Unfortunately, this strain appears to be more contagious and more severe than earlier variants.

It didn't take very long for the Delta variant to spread to other countries. As of last week, it accounted for more than 10% of new cases of COVID-19 in the U.S.

Could the rise of this concerning variant present problems for leading COVID-19 vaccine makers Pfizer (NYSE:PFE) and Moderna (NASDAQ:MRNA)? Actually, it could instead present a major opportunity. Here's how the deadly Delta variant could boost Pfizer's and Moderna's COVID-19 vaccine sales.

Person looking at a computer image of coronavirus and a world map with a city street in the background.

Image source: Getty Images.

Some good news

The greatest hope for curbing the spread of the Delta variant is for vaccines to be effective against the strain. Fortunately, there's reason to be optimistic.

In results from a laboratory study announced in May, vaccines developed by Moderna, and by Pfizer and its partner BioNTech (NASDAQ:BNTX) produced neutralizing antibodies against the coronavirus variant wreaking havoc in India. While the antibody levels were seven times lower than those produced by the vaccines against the previous predominant strain, the vaccines could still provide protection against the Delta variant.

Another study conducted by Public Health England found that two doses of the Pfizer-BioNTech vaccine demonstrated 88% efficacy against infection by the Delta variant. In this same study, two doses of AstraZeneca's (NASDAQ:AZN) vaccine was found to be 60% effective against the strain.

The study didn't include Moderna's vaccine. However, both the Pfizer-BioNTech and Moderna vaccines are based on similar messenger RNA (mRNA) approaches. The vaccines have demonstrated similar efficacy with other coronavirus strains, so it seems likely that Moderna's vaccine is close to as effective as the Pfizer-BioNTech vaccine is against the Delta variant.

Meeting the demand

If worries about COVID-19 were quickly fading across the world, the demand for vaccines could also fall off significantly. However, the increasing spread of the Delta variant seems to make that scenario less likely every day.

Dr. Anthony Fauci, chief medical advisor to President Biden, stated earlier this week that the Delta variant presents the "greatest threat" to the U.S. in fighting COVID-19. That threat should present a major opportunity for both Pfizer and Moderna.

Both companies' vaccines appear to be much more effective against the deadly variant than rival vaccines. Perhaps more importantly, Pfizer and Moderna have proven that they can meet the demand, while other drugmakers, including AstraZeneca, have stumbled. 

These factors almost certainly explain why the leading COVID-19 vaccine makers continue to land additional supply deals. Earlier this month, the U.S. government agreed to buy 500 million more doses of the Pfizer-BioNTech vaccine to donate to poor countries.

Moderna recently announced a deal to supply the European Commission with another 150 million doses of its vaccine. This followed on the heels of a similar agreement with the U.S. to buy an additional 200 million doses.

Getting a boost

The rise of the Delta variant could increase the chances that booster shots will be required. Indeed, some experts are calling for governments to make plans as soon as possible for administering booster vaccines in the fall.

At this point, Pfizer (along with BioNTech) and Moderna are clearly in the best position to benefit from additional sales of their respective COVID-19 vaccines. The big question for investors, though, is how much higher their shares will move.

Pfizer stock is up only around 6% year to date, despite the tremendous success of its vaccine. Moderna has been a much bigger winner, with the biotech stock soaring close to 100%.

It's likely that the growth related to the spread of the Delta variant is already priced into the stocks to some extent. However, I suspect that shares of both Pfizer and Moderna could get a boost as new supply deals roll in over the next few months.

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.