Johnson & Johnson announced the start of phase three of its coronavirus vaccine trial this week, citing "positive interim results" from earlier stages of its study. Those were published Friday, and they were indeed promising.

The pharmaceutical giant reported that 99 percent of the participants between the ages of 18 and 55 in early-to-mid stage clinical trials developed neutralizing antibodies against the novel virus. The analysis also found that most of the side effects associated with the vaccine were mild and resolved within a matter of days.

It wasn't clear, however, whether participants over 65 were well-protected since immune response results were available for only 15 people in that demographic. Additionally, Reuters reports, the rate of adverse reactions — like fatigue and muscle aches — to the vaccine in that age group was just 36 percent, far lower than those seen in 64 percent of the younger participants. That might sound like good news, but it actually suggests the immune response in older people may be weaker.

One of the key aspects of Johnson & Johnson's trial is that just a single dose produced a strong immune response in participants. Other companies developing vaccines like Moderna and Pfizer are using a two-dose approach. If Johnson & Johnson's phase three trial, in which 60,000 volunteers will enroll across three continents, eventually proves the single dose is safe and effective, it could simplify distribution of the vaccine. Read more at Reuters and CNN. Tim O'Donnell