The clinical trial of Johnson & Johnson's (NYSE:JNJ) COVID-19 vaccine candidate has been paused due to an unexplained illness in a study participant, according to an article published by STAT on Monday night. The independent data monitoring committee will be convened to review the data and make a recommendation about the situation.

With an expected enrollment of 60,000 participants in the study, it shouldn't be a huge surprise that a participant might develop an issue unrelated to the treatment. At this point, there's no evidence that the unexplained illness is directly tied to the administration of the vaccine. The unexplained illness might have developed regardless of the coronavirus vaccine treatment.

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Investors and industry-watchers are on heightened alert after AstraZeneca (NYSE:AZN) had to pause in its clinical trial after a patient in the study developed an illness. Most of the clinical trials testing AstraZeneca's have been restarted, although the study in the U.S. hasn't been given the green light yet.

AstraZeneca's AZD1222 and Johnson & Johnson's JNJ-78436735 are both adenovirus-based vaccines, where the genetic material to express the coronavirus proteins are delivered via an adenovirus. But Johnson & Johnson's vaccine, JNJ-78436735, only requires a single dose, while AstraZeneca's AZD1222 requires two doses to be effective.

It's hard to do a head-to-head comparison between drugs in two different clinical trials, but the simple answer is that the second dose of AZD1222 might be causing an immune reaction to the adenovirus rather than the coronavirus.