Shares of Moderna (NASDAQ:MRNA) and Inovio Pharmaceuticals (NASDAQ:INO) have skyrocketed 202% and 534% respectively year to date, as investors bet on the biotech companies' coronavirus vaccine programs. Now, as both have released interim trial data from the first human trials and head into late-stage testing, investors may wonder which of the two makes the best buy right now.

Let's take a closer look at both companies' trials, as well as other factors that could help or hinder their efforts to bring a safe and effective COVID-19 vaccine to the public.

Masked and gloved healthcare worker holding a vial labeled COVID-19 Vaccine

Image source: Getty Images.

Moderna was the first to start testing a COVID-19 vaccine on humans when it began clinical trials in March. The company has since released positive interim data, showing participants' levels of binding and neutralizing antibodies at or above levels seen in recovered COVID-19 patients. Binding antibodies alert the body's immune system to the presence of a pathogen, but neutralizing antibodies actually block infection.

The phase 1 trial involved younger patients, age 18 to 55. Moderna recently began a phase 2 study, which will involve that age group as well as older patients. And finally, Moderna said that it's set to start a phase 3 study this month in as many as 30,000 people.

What should we be looking for now?

In Moderna's interim report, neutralizing antibody data were only available for eight patients. Regulators will have to see those positive results in many more patients to determine whether the vaccine helps the body fight infection. It also will be crucial to see how the vaccine performs in older patients since elderly people are among the most at risk during this health crisis.

Moderna remains a leader in this race -- but not the leader. The U.S. government included Moderna in its list of five companies it thinks is most likely to find a vaccine first and offered Moderna as much as $483 million in support of its program. Still, big pharma AstraZeneca (NYSE:AZN), which teamed up with University of Oxford researchers on a vaccine, garnered even more funding from the U.S. government -- $1.2 billion -- and has already started a phase 2/3 trial.

What happens if Moderna wins the race? Well, there are still manufacturing challenges. The company is scaling up manufacturing with its partner Lonza Group (OTC:LZAG.Y) and expects to produce as many as 1 billion doses annually as of next year. Still, according to Fierce Pharma, Moderna's CEO Stephane Bancel, speaking at a BIO virtual conference, expressed concern about the supply of raw materials. Other members of the panel echoed those concerns and spoke of other issues such as staffing and speed.

Let's turn to Inovio...

Inovio recently reported interim data from its phase 1 trial on patients age 18 to 50. The company said 94% of participants demonstrated immune response, based on binding and neutralizing antibodies and T-cell response. Inovio didn't provide details on antibody levels.

Inovio plans to begin a phase 2/3 efficacy study this summer. Inovio, like Moderna, still must show neutralizing antibody data across a broad set of trial participants and it has to prove its vaccine is safe and efficacious in older patients.

While the Department of Defense offered Inovio $71 million, the White House didn't include Inovio on its list of five most promising vaccine developers.

From a timeline perspective, Moderna -- and others like AstraZeneca -- are clearly ahead of Inovio. But that doesn't mean Inovio is out of the race. Anything can happen in clinical trials, and if companies that are a few steps ahead end up stumbling, Inovio could move into the lead. Or Inovio's vaccine may get to market later, but it could be a better vaccine and eventually gain market share. That's why it's as important to examine trial results, and the company's ability to manufacture the product, as it is to focus on the question of "who will be first to market?"

Manufacturing is a potential hurdle for Inovio. The company recently filed a court complaint against VGXI, supplier of DNA plasmids used in its vaccine. Inovio said that VGXI can't support the large-scale manufacturing needed, and must transfer its technology to other companies in order to assure production. The court turned down Inovio's request, saying it wouldn't order VGXI to turn over its technology. Inovio has filed a notice of appeal.

What does all of this mean for investors?

It's impossible to predict the performance of Moderna's and Inovio's vaccines in late-stage clinical trials. Based on early data, we can be cautiously optimistic. We have slightly more detail from Moderna's trial so far, and Moderna has the advantage of heftier government support.

While manufacturing for any player may have its rough patches, Inovio's situation seems more worrisome. If the company is mired in a legal dispute with VGXI now, I wonder how it can resolve the issue in time to scale up manufacturing for early next year. (The U.S. government initiative to support vaccine development set January as a goal for vaccine deployment.) Though the market's focus has been on the development of a vaccine, a key issue is actually getting that vaccine to people who need it.

I would hesitate to buy shares of either Moderna or Inovio right now: Their stock prices have become dependent on coronavirus vaccine news rather than on their pipelines as a whole. But if I had to choose the best option of the two, I would favor Moderna for the strengths mentioned above. If trial results continue to be encouraging, shares of this biotech company may climb higher.