How Long After J&J Vaccine Have Blood Clots Occurred? CDC Recommends Pfizer, Moderna Shots

The U.S. Centers for Disease Control and Prevention (CDC) has recommended that people take an mRNA COVID vaccine, such as Pfizer or Moderna, instead of the Johnson & Johnson (J&J) one where possible.

It followed a meeting of the Advisory Committee on Immunization Practices (ACIP) on Thursday, which evaluated the risks and benefits of J&J's Janssen vaccine as well as updates to the rare side effect known as Thrombosis with Thrombocytopenia Syndrome (TTS).

TTS is a rare but serious newly-identified condition that has been reported in some people who have received certain types of the COVID vaccine—namely adenoviral vector vaccines such as the J&J and AstraZeneca shots.

The condition is characterized by the formation of blood clots (thrombosis) as well as a low platelet count (thrombocytopenia), according to the Melbourne Vaccine Education Centre.

As of December 8, there have been 57 confirmed reports of people developing TTS after receiving the J&J COVID vaccine out of 16.9 million doses administered in the U.S., according to the CDC and Food and Drug Administration (FDA).

This compares to three confirmed cases of TTS following the Moderna vaccine out of more than 458 million mRNA-type shots administered.

In the ACIP presentation, which looked at 54 TTS cases in people vaccinated this year between March 2 and August 31, it was noted that the majority of people who developed the condition were female, white non-Hispanic people with a median age of 44.5 years old.

The median time from vaccination to TTS symptoms developing was nine days, and the median time from symptoms starting to hospital admission was five days.

In those who developed TTS, all were hospitalized, 36 were admitted to an intensive care unit, and eight died. Nine deaths have been attributed to TTS following the J&J vaccine, the CDC notes.

Previously in a November update, the CDC had said the symptoms of TTS had started between three and 15 days after vaccination.

The CDC noted on November 15 that people should look out for the following TTS symptoms for "several weeks" following vaccination and that medical help should be sought right away if one or more develop: Severe or persistent headaches or blurred vision; Shortness of breath; Chest pain; Leg swelling; Persistent abdominal pain; Easy bruising or tiny blood spots under the skin beyond the injection site.

"Women younger than 50 years old especially should be made aware of a rare risk" of TTS after vaccination, the health agency added.

While the CDC now endorses ACIP's decision expressing a clinical preference of an mRNA vaccine over the J&J one, the J&J vaccine is still available.

However, it noted on November 15 that people who developed TTS after their initial J&J vaccine should not receive a J&J booster dose.

In response to the CDC's recommendation on Thursday, J&J said in a press release that it "remains confident in the overall positive benefit-risk profile of its COVID-19 vaccine."

"Studies have shown that the Johnson & Johnson COVID-19 vaccine generates strong antibody and cellular immune responses and long-lasting immune memory and breadth of protection across variants."

Dr. Mathai Mammen, global head of Janssen Research & Development, LLC at Johnson & Johnson, said the company "strongly" supports education of rare events like TTS and said: "We appreciate today's discussion and look forward to working with the CDC on next steps."

Janssen COVID vaccine
A photo shows a nurse holding a vial of the Janssen COVID vaccine in Medellin, Colombia, on November 10, 2021. The CDC this week endorsed recommendations that people should opt for an mRNA-type vaccine instead, where possible. Fredy Builes/Getty