The CDC offered guidance to clinicians for the Johnson and Johnson (J&J) Covid-19 vaccine Tuesday after six women, ages 18 to 48, suffered a rare type of blood clot. One patient died.

Symptoms of the clot, a cerebral venous sinus thrombosis (CVST), were in combination with low levels of blood platelets (thrombocytopenia) and began from 6–13 days after the one-shot vaccine, the CDC said.

Roughly 6.85 million doses of the J&J vaccine had been administered before it was put on pause Tuesday while the CDC and the Food and Drug Administration investigates.

The CDC cautioned providers about the use of heparin in any suspected cases, which is often used to treat clots.

“When these specific type of blood clots are observed following J&J Covid-19 vaccination, treatment is different from the treatment that might typically be administered for blood clots,” the CDC said in a Health Alert Tuesday afternoon. “Based on studies conducted among the patients diagnosed with immune thrombotic thrombocytopenia after the AstraZeneca COVID-19 vaccine in Europe, the pathogenesis of these rare and unusual adverse events after vaccination may be associated with platelet-activating antibodies against platelet factor-4 (PF4), a type of protein.

“Usually, the anticoagulant drug called heparin is used to treat blood clots. In this setting, the use of heparin may be harmful, and alternative treatments need to be given.”

Guidelines for clinicians

The CDC provided this additional guidance:

1. Pause the use of the J&J Covid-19 vaccine until the ACIP is able to further review these CVST cases in the context of thrombocytopenia and assess their potential significance.
2. Maintain a high index of suspension for symptoms that might represent serious thrombotic events or thrombocytopenia in patients who have recently received the J&J Covid-19 vaccine, including severe headache, backache, new neurologic symptoms, severe abdominal pain, shortness of breath, leg swelling, petechiae (tiny red spots on the skin), or new or easy bruising. Obtain platelet counts and screen for evidence of immune thrombotic thrombocytopenia.
3. In patients with a thrombotic event and thrombocytopenia after the J&J Covid-19 vaccine, evaluate initially with a screening PF4 enzyme-linked immunosorbent (ELISA) assay as would be performed for autoimmune HIT. Consultation with a hematologist is strongly recommended.
4. Do not treat patients with thrombotic events and thrombocytopenia following receipt of J&J Covid-19 vaccine with heparin, unless HIT testing is negative.
5. If HIT testing is positive or unable to be performed in patient with thrombotic events and thrombocytopenia following receipt of J&J COVID-19 vaccine, non-heparin anticoagulants and high-dose intravenous immune globulin should be strongly considered.
6. Report adverse events to VAERS, including serious and life-threatening adverse events and deaths in patients following receipt of Covid-19 vaccines as required under the Emergency Use Authorizations for Covid-19 vaccines.

Guidelines for patients

The CDC also provided guidance for people who have received the J&J vaccine, telling them to seek medical attention if they suffer a severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after the shot.