Shares of Moderna (NASDAQ:MRNA) climbed 20% in a single trading session (May 18), after the biotech company announced encouraging interim results from an ongoing phase 1 trial of its vaccine for COVID-19, the illness caused by the novel coronavirus.

Now the company is beginning a phase 2 trial to further investigate safety, immune response, and any potential adverse effects.

A researcher injects a trial participant with vaccine.

Image source: Getty Images.

Two key elements I'll be looking for in upcoming reports are the performance of the vaccine in older participants, and information on the production and duration of neutralizing antibodies in participants. And I'd like to see all of this in a larger study group.

Let's take a closer look.

Moderna's phase 1 trial was designed to study its messenger-RNA vaccine in 45 healthy adults. Traditional vaccines introduce a weakened form of virus into the body to incite immune response. But Moderna's method is different. This vaccine uses messenger RNA to give the body's cells instructions to make proteins that would prevent illness.

Neutralizing antibodies

Across all participants age 18 to 55 in two dosage groups, levels of binding antibodies were at or above levels seen in people who have recovered from the virus. Binding antibodies attach themselves to the pathogen, but they don't stop infection. Information on neutralizing antibodies -- those that block infection -- was only available for eight people. In those participants, neutralizing antibodies were also at or above levels seen in recovered coronavirus patients.

The neutralizing antibody data is positive, but it's important to see that result in hundreds and eventually thousands of participants. Moderna's phase 2 trial plans to enroll 600 volunteers. I would like to see if there's a trend in neutralizing antibody production across the rest of the phase 1 participants, and participants in this second larger study.

There also is the question of how long neutralizing antibodies will remain in the body. Recovered COVID-19 patients may be a guide. At the moment, the immunity status of those who once had the illness still is unknown. "There is not enough evidence about the effectiveness of antibody-mediated immunity" to consider former patients risk-free, The World Health Organization wrote in an April brief.

A study published in the journal Immunity last month showed that while most newly discharged COVID-19 patients produced virus-specific antibodies and T-cells, their responses weren't uniform. For instance, some produced high concentrations of neutralizing antibodies, and others didn't. As for Moderna's vaccine, we'll have to wait a while to determine the duration of neutralizing antibodies in the bodies of volunteers. But it is something to monitor through the later stages of trials.

Data concerning older patients

So far, Moderna has only reported data for patients age 18 to 55. As trials move forward, I'll also be closely watching data on older patients. Moderna is enrolling two groups for the phase 2 study: ages 18 to 55, and age 55 and older. The participants will receive placebo or one of two vaccine dosages during two vaccinations 28 days apart. They will be monitored for 12 months following the second dose.

Safety and immune response in older patients are particularly important, since the elderly have been among those most at risk during this pandemic. COVID-19 is fatal for about 13.4% of patients age 80 or older, according to a study published in The Lancet Infectious Diseases. That's compared to 1.25% for patients age 50 to 59. And people 80 and older are also more likely to be hospitalized, the study showed -- 18.4% of that group, compared with 8.16% of people in their 50s.

Moderna plans to initiate a phase 3 study in July. The company hasn't said how many participants will be involved, but we can hope the number will be in the thousands. Strong data across large study groups may be the only way to assuage fears that some people have about the rapid development of a coronavirus vaccine. A recent poll by the Associated Press-NORC Center for Public Affairs Research showed only half of Americans would get the coronavirus vaccine if one is approved -- and possible side effects were the top fear.

So should we be optimistic about this biotech company's vaccine candidate at this point? Sure -- but I'd rather say "cautiously optimistic." Once data on neutralizing antibodies and vaccine performance in older adults is available -- and in a large study group -- it will be easier to bet on whether Moderna will win this coronavirus vaccine race.

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.