As Pfizer (PFE -0.85%) and Moderna (MRNA -0.07%) vaccines entered the U.S. market in December, you might have breathed a sigh of relief. A potential end to the COVID-19 pandemic had appeared on the horizon. And then came along two new strains of the novel coronavirus that threw this newfound optimism into jeopardy. One big question emerged: Could today's vaccines beat these and other variants?

We don't yet have a definitive answer. But two companies have announced plans that could position them for victory. I'm talking about Moderna and Novavax (NVAX -3.15%). Let's take a closer look at what each is doing to assure the efficacy of its vaccine against threatening new variants.

Female healthcare worker in blue scrubs, mask, and gloves administers a vaccine to an older male patient.

Image source: Getty Images

First, some quick background on today's most common variants: the U.K. and South African strains. They both involve mutations that cause changes to the coronavirus's spike protein. The spike protein is what a coronavirus uses to attach to cells and infect them; it's targeted by all the vaccines currently on the U.S. market, or close to authorization. The South African strain is of particular concern, because one of the mutations helps it evade the antibodies your body produces post-vaccination.

Moderna's tests

Moderna started by testing its vaccine against the U.K. and South African strains in vitro. Researchers used blood serum from individuals who had received the Moderna vaccine. In studies involving the South African strain, they noted a decline in antibody levels -- which may indicate a shorter duration of immunity against that strain. But antibodies to fight both strains still remained at high enough levels to provide protection.

Overall, Moderna's conclusions are encouraging. But what's even better is its next step. Moderna is testing a booster dose of its vaccine. This shot could be administered some time after the first two doses to reinforce the body's ability to fight new strains. That isn't all. Moderna is also launching a booster candidate into preclinical and phase 1 studies, which is specifically designed to target the South African strain's spike protein. Moderna will evaluate this idea of adding a strain-specific booster to the original vaccination regimen. If it works, the booster could target other strains in the future.

Novavax's real-world setting

Novavax's clinical trials unfolded in the U.K. and South Africa -- as the new strains were becoming more and more prevalent. That gave the company a chance to test its vaccine candidate against those strains in a real-world setting.

The results? A phase 3 trial involving 15,000 volunteers in the U.K. showed 85.6% efficacy against the U.K. strain. Novavax detected the U.K. strain in more than 50% of coronavirus cases that occurred in that trial. And a phase 2b trial including more than 4,000 volunteers in South Africa demonstrated 60% efficacy. In that study, the South African strain accounted for more than 90% of the coronavirus cases that arose.

Sure, those figures are lower than the 95.6% efficacy Novavax's candidate produced against the original coronavirus. But they are higher than 50% -- the original bar the U.S. Food and Drug Administration (FDA) set for considering a candidate for Emergency Use Authorization (EUA). So Novavax's performance against the variants is actually rather strong.

Like Moderna, Novavax isn't stopping here; it's studying the possibility of a booster for new strains, or a combined vaccine. Novavax aims to select candidates soon and start clinical trials in the second quarter of this year.

The main risk

The main risk for both Moderna and Novavax is the design of a new booster or combined vaccine. Such a product must be one that each company could adapt to new variants shortly after they arise. Otherwise, they'll find themselves constantly chasing new variants -- and launching products when it's almost too late.

For now, though, Moderna and Novavax are on the right track. In the near term, it looks like Moderna's vaccine and Novavax's candidate may be able to protect many people from today's most worrisome variants. More importantly, work on potential boosters shows these biotech companies are proactive. And that's why Moderna and Novavax are most likely to prevail in this new race to conquer emerging coronavirus strains.