Think back to one year ago. No one knew then whether any of the COVID-19 vaccines in development would actually work. Fortunately, several of them did.

Pfizer (PFE) (along with its partner, BioNTech (BNTX -1.44%)), and Moderna (MRNA -0.74%) never experienced the worst-case scenario of their vaccines failing miserably. These companies have gone on to generate billions of dollars in sales for their respective vaccines, and hope to make a lot more money.

But there are some new study results that just might put a damper on how much recurring revenue these vaccines will make. Could Pfizer and Moderna now face their second-worst scenario?

Healthcare professional giving a person a shot.

Image source: Getty Images.

The second-worst scenario

The worst-case scenario for Pfizer and Moderna was that their COVID-19 vaccines wouldn't work. The second-worst scenario might be that their vaccines work too well.

Washington University School of Medicine immunologist Ali Ellebedy led a team of researchers attempting to determine how long the protection provided by messenger RNA (mRNA) vaccines would last. Their findings were recently published in Nature magazine.

The key takeaway from this research was that the Pfizer-BioNTech vaccine could provide immunity for years. And while Moderna's vaccine wasn't included in the study, Ellebedy thinks it would likely provide similar longevity because of its similar mRNA approach.

There's a major financial implication with this recent finding for Pfizer and Moderna. If their vaccines provide protection against COVID-19 for years, the companies won't sell nearly as many vaccine doses in future as they will in 2021 and 2022.

Not so fast

Don't write off the prospects for these vaccine stocks too quickly, though. There are several things to consider about the results published in Nature.

First, the study conducted by Ellebedy and his team was really small: Only 41 participants were included. And the lymph-node samples that were key to the team's findings about potential longevity for mRNA vaccines were collected from only 14 people.

Also, the researchers didn't actually prove that any COVID-19 vaccine provided immunity for years. Instead, they extrapolated based on an observation that immune-system cells were being trained to fight SARS-CoV-2, the novel coronavirus that causes COVID-19, for at least 15 weeks after the first dose of the Pfizer-BioNTech vaccine. Ellebedy and team theorized that memory cells would likely last for years because of this response lasting for months.

More importantly, these findings could be a moot point because of the emergence of coronavirus variants. Even if the mRNA vaccines provide immunity for years against the virus strain that has been prevalent in the U.S., it doesn't mean that they will do so against new strains.

Bad news could be good news

There has been some reason to be optimistic about the efficacy of the Pfizer-BioNTech and Moderna vaccines against new variants, including the highly contagious Delta variant. Studies have found that the mRNA vaccines produce neutralizing antibodies against the emerging variants. One study conducted by Public Health England found an 88% efficacy for two doses of the Pfizer-BioNTech vaccine against the Delta variant.

Is this good news for vaccine recipients potentially bad news for Pfizer's and Moderna's hopes of generating strong recurring revenue from their vaccines? Not necessarily. There's some additional bad news for the vaccinated that could be good news for the companies.

The Israeli Ministry of Health recently announced that based on real-world data, the Pfizer-BioNTech vaccine appears to be only 64% effective against the Delta variant. That's much lower than the 94% efficacy for the vaccine reported in May.

On one hand, Pfizer doesn't seem to have any worries that its vaccine is ineffective against the deadly new variant. However, the lower efficacy increases the likelihood that governments will want booster doses and new versions of the vaccines that target the variants.

Pfizer and Moderna want their vaccines to be highly effective, but they also hope that booster doses are needed at least annually. That's the best-case scenario for both companies. And it's a scenario that seems more likely to happen, thanks to the recent data from Israel.