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Patients with a prior history of an episode of an immune-mediated syndrome characterized by thrombocytopenia and thrombosis, such as heparin-induced thrombocytopenia, should avoid the Johnson & Johnson COVID-19 vaccine in the immediate aftermath of their illness, CDC staff said in a call with clinicians Tuesday.

Because the etiology of thrombosis and thrombocytopenia syndrome (TTS) appears similar to heparin-induced thrombocytopenia, patients with a similar immune-mediated syndrome should be offered another FDA-authorized COVID-19 vaccine for at least 90-180 days after their illness resolves.

While women under age 50 can receive any authorized COVID-19 vaccine, they have an increased risk of TTS associated with the Johnson & Johnson vaccine. CDC staff noted they should "be aware of the rare risk of TTS" and may opt for one of the other authorized vaccines, such as the mRNA vaccines from Pfizer and Moderna.

However, there were no such caveats for patients with risk factors for venous thromboembolism (VTE), or a prior history of thromboses not associated with thrombocytopenia. CDC staff noted the biologic mechanisms for VTE and arterial thrombi were different from the "underlying immune mechanism" for heparin-induced thrombocytopenia.

Therefore, these patients were unlikely to be at increased risk for TTS, they said.

Pregnant women can also receive the Johnson & Johnson COVID-19 vaccine. While thrombosis risk is higher during pregnancy and postpartum, as well as with hormonal contraception, "experts believe these factors do not make people more susceptible to TTS," CDC staff noted.

Sara Oliver, MD, of the CDC, urged people taking aspirin or anticoagulants as part of their routine medications to not stop taking them prior to the Johnson & Johnson COVID-19 vaccine, nor should people start taking these medications prior to receiving Johnson & Johnson or any other COVID-19 vaccine.

Clinicians asked whether patients with a history of stroke or risk factors for stroke should avoid the vaccine. Oliver said because the phenomenon appears to be immune-mediated, "people who have a history of other embolic events, including stroke, do not appear to be at increased risk for developing" TTS, and there were no recommendations for people with a history of stroke to avoid this vaccine.

Similarly, those with other risk factors, such as smoking or obesity, did not appear to be at increased risk of developing these rare blood clots.

Oliver added that CDC has partnered with the American Society of Hematology about for patients.

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