Investors in Pfizer (NYSE:PFE) and Moderna (NASDAQ:MRNA) may have breathed a sigh of relief when the U.S. Food and Drug Administration granted the companies' coronavirus vaccines Emergency Use Authorization in December. The risk of failure during clinical trials was in the rearview mirror. The companies began shipping vaccine doses to be administered. Revenue from their contracts with governments was right around the corner.

And then another risk emerged: new coronavirus variants. Over the past few weeks, new strains have emerged: a strain found in the U.K., one that started in South Africa, and more recently new strains in Japan and in Columbus, Ohio. Now, the big question is whether the Pfizer and Moderna vaccines can protect against these new versions of the coronavirus. Let's take a closer look at what we know so far.

The image of a purple medical uniform is shown with a sign saying "mutation of COVID-19" in the pocket.

Image source: Getty Images.

Focused on the spike

Let's start with a little background on the virus itself, the vaccines, and the variants. The coronavirus contains a "spike protein." These are literally spikes on the virus' surface that allow it to attach to cells and infect them. Vaccine makers are focused on this spike. In the cases of Pfizer and Moderna, their vaccines contain instructions so that the body can make a copy of this spike protein. Then the body produces antibodies to fight it. That way, if the actual coronavirus appears, this army of antibodies will recognize it and attack.

The newly discovered coronavirus variants each contain genetic mutations, or changes. And in each variant, the changes affect the spike protein. So now the question is: With these changes to the spike protein, will those antibodies still recognize it?

So far, the situation looks positive for both Moderna and Pfizer. Moderna said its vaccine includes the entire sequence needed to make the spike protein. That's 1,273 amino acids. The new variants include about eight changes in the spike protein amino acids. Overall, that means the "new spike protein" is about 1% different from the original coronavirus spike protein -- the one encoded in Moderna's vaccine. The company is testing its vaccine against the new variants but expects the vaccine to prove effective.

Moderna and Pfizer both comment

Moderna says that if it eventually must update its vaccine, the process wouldn't require big clinical trials, according to an article in MIT Technology Review. So, an updated vaccine could move from drawing board to market in a few months as long as regulators give it the nod.

As for Pfizer, the company recently conducted an in-vitro test against a mutation within the new strains that's linked to rapid transmission. The company said antibodies from people who were vaccinated neutralized the U.K. and South African strains. Still, the test didn't include the entire set of mutations from the new variants. Pfizer said more data are needed to confirm the vaccine's effectiveness against new strains. The company also said that if newer mutations eventually lead to a decline in vaccine efficacity, updating the current vaccine wouldn't be a problem.

What does all of this mean for investors?

Right now, new strains probably represent one of the biggest risks for coronavirus vaccine companies -- and their share performance. So far, it seems the Pfizer and Moderna vaccines can handle the new strains. But that doesn't mean this will be true for additional strains.

The good news is the possibility of a vaccine-resistant variant doesn't mean all of the companies' hard work will be tossed out. As Moderna and Pfizer both said, updates are possible. And this process isn't new. Researchers regularly update the flu vaccine to add new strains.

Still, that's not an ideal situation during a pandemic and after companies have already ramped up production of their original vaccines. It's bad news from a cost and logistics perspective.

Of course, we're not there yet. And that scenario may never happen. But if you hold shares of vaccine makers or plan to buy some, this is a point to watch with careful attention.