The third coronavirus vaccine authorized for emergency use in the United States is going to sit in the corner by itself for a little while. On Tuesday morning, the Centers for Disease Control and Prevention and the Food and Drug Administration issued a joint statement regarding Johnson & Johnson's (JNJ 0.03%) COVID-19 vaccine.

The agencies recommended that health care providers pause their use of Johnson & Johnson's vaccine until an advisory committee can review six cases in which inoculations were followed by dangerous blood clots. All six occurred in women between the ages of 18 and 48 years old. Most importantly, they were associated with a low concentration of blood platelets.

Physician with hand up.

Image source; Getty Images.

If there is an associated risk, it's an extremely small one. As of Monday, more than 6.8 million people had received Johnson & Johnson's COVID-19 vaccine.

At a rate of less than one patient in a million, this pause might seem like an excessive overabundance of caution. The agencies won't please anyone with this decision, but they really don't have another option.

Both AstraZeneca's (AZN -1.61%) COVID-19 vaccine and Johnson & Johnson's employ a non-replicating virus that delivers genetic instructions. The European Medicines Agency recently said unusual blood clots that present in combination with low platelet levels should be listed as a very rare side effect of AstraZeneca's COVID-19 vaccine after assessing dozens of cases similar to the six reported in association with J&J's vaccine.

Blood clots that break free and travel to the blood vessels that serve the heart and other organ systems can be fatal. Strong blood thinners are standard treatments for dangerous clots, but giving those to patients with low platelet concentrations can be just as dangerous. 

The Advisory Committee on Immunization Practices will meet on Wednesday, so based on its determination, injections with Johnson & Johnson's vaccine could be allowed to resume before the end of the week.