What to Know About a Rare Side Effect Tied to J&J COVID-19 Vaccine

An advisory panel to the US Centers for Disease Control and Prevention (CDC) is recommending the Pfizer and Moderna COVID-19 vaccines over the Johnson/Janssen (J&J) vaccine after considering new data linking the J&J shot to a rare-but-serious blood clotting risk.

Voting 15-0, the CDC's Advisory Committee on Immunization Practices (ACIP) said the mRNA vaccines "are preferred" over the J&J option for everyone ages 18 and older.

During an emergency meeting on December 16, ACIP members reviewed data revealing an elevated risk of thrombosis with thrombocytopenia syndrome (TTS), a condition that's been associated with the J&J vaccine but not the other two COVID-19 vaccines authorized for use in the US.

To date, 54 cases of TTS have been confirmed. Nine people—7 females and 2 males—have died.

While expressing preference for the mRNA vaccines, the panel did not go as far as recommending that the J&J vaccine not be used or that its use be limited to certain older adults at lower risk of TTS.

Earlier in the week, the US Food and Drug Administration (FDA) called out TTS concerns arising from the J&J vaccine. In a fact sheet for both doctors and patients, the FDA noted that people who have had a clotting problem after having a first dose of the vaccine should not receive another.

It's more than understandable to have questions about TTS and how it relates to the J&J vaccine. Here's a breakdown.

What is TTS?

TTS occurs when someone has blood clots (known as thrombosis) along with low blood platelet counts (known as thrombocytopenia), Isaac See, MD, a member of the CDC's COVID-19 response team, told ACIP members.

TTS is a new syndrome that can occur after someone receives an adenovirus COVID-19 vaccine, like the J&J and AstraZeneca COVID vaccines. (AstraZeneca's vaccine is not approved for use in the US.)

How is TTS different from CVST?

You may remember back in the spring how the J&J vaccine was tied to a condition called cerebral venous sinus thrombosis, or CVST. CVST is a type of clot that forms in the venous sinuses—spaces in the skull that allow blood to drain from the brain. A CVST keeps blood from draining out of your brain, which can lead to a stroke.

"CVST is a specific type of clot in a distinct anatomical location—the cerebral venous sinuses," infectious disease expert Amesh A. Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, tells Health. "TTS is a general syndrome, which may or may not include CVST, that involves abnormally low blood platelets and clot."

CVST can happen "with or without TTS also being present, and TTS can occur with or without a CVST also present," Dr. Adalja explains.

How common is TTS?

First, let's put TTS cases into perspective. A little more than 16 million Americans have received the J&J vaccine as part of their primary vaccination, according to CDC data. (That's well below the 73 million people who have received the Moderna vaccine and 113 million who have gotten the Pfizer-BioNTech vaccine.)

Overall, the ACIP counted 3.83 cases of TTS per million J&J doses. The highest rate of TTS—a little more than 10 per million doses—occurred in women between the ages of 30 and 49.

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Who has developed TTS?

CDC data show that the median age of people with TTS has been 44.5, with the age range being 18 to 70. The majority of cases (37) have been in women, while 17 men have been impacted. The data show that 26 of those cases, or 48%, were in women under the age of 50.

Of the TTS cases that have been detected, 29 also had CVST. Seven of the people diagnosed with TTS had tested positive for COVID-19 in the past.