What happened

Johnson & Johnson (JNJ -0.14%) closed 3.6% lower on Friday. A downward move is exactly the opposite of what you'd expect to happen when a coronavirus vaccine developer announces positive clinical trial results, as the company did that morning.

So what

Data indicated that Johnson & Johnson's candidate met all of the primary and key secondary endpoints in its phase 3 clinical trial. In the sprawling global study, the vaccine's overall efficacy was 66% in blocking moderate to severe COVID-19. It was 85% effective in preventing severe forms of the disease.

A child getting a vaccine jab from a medical professional.

Image source: Getty Images.

Better yet, the vaccine's "efficacy against severe disease increased over time, with no cases in vaccinated participants reported after day 49," as the company wrote. It also showed complete protection against hospitalization and death from the disease 28 days after vaccination. No significant safety concerns were reported, and the vaccine was well-tolerated.

One advantage of the pharmaceutical giant's vaccine is that it is administered in a single dose, as opposed to the two-shot vaccines currently authorized in the U.S.

Johnson & Johnson plans to file for an emergency use authorization (EUA) from the Food and Drug Administration in early February.

Now what

An inoculation efficacy rate of 66% is not as good as that of the competition. The Moderna and Pfizer/BioNTech vaccines both had significantly higher rates (94.1% and 95%, respectively). Still, 66% is considered to be a strong result. Plus, Johnson & Johnson's testing involved more contagious variants of the coronavirus. Moderna and Pfizer/BioNTech's results came before the spread of those variants.

All in all, this was excellent news for Johnson & Johnson, so the sell-off seems undeserved.