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Questions about the Johnson & Johnson COVID-19 vaccines? UF Health experts have answers

After a 10-day pause to further study unusual blood clotting issues, federal authorities have allowed the use of Johnson & Johnson’s COVID-19 vaccine to resume. As of late April, experts reviewing safety reports found that out of more than 8 million doses administered, 15 women who got the J&J vaccine developed blood clots and had low levels of platelets, according to the Centers for Disease Control and Prevention.

Nicole M. Iovine, M.D., Ph.D., chief epidemiology officer for University of Florida Health Shands hospital system, and Marc S. Zumberg, M.D., a professor in the UF College of Medicine’s division of hematology and oncology, provide answers to questions about the J&J vaccine.

Question: Can you put the side effects from the Johnson & Johnson vaccine into context?

Answer: (Iovine) Minor side effects after the J&J vaccine, such as injection site soreness, fatigue and feeling unwell, are similar to those seen with the mRNA vaccines produced by Pfizer and Moderna. The blood clots that have been seen in 15 recipients of the J&J vaccine have not been observed after the mRNA vaccines. It is important to note that these 15 cases occurred after more than 8 million J&J vaccines were given. That’s less than 0.0002%! However, given that these were serious side effects, the FDA recommended a pause on administering the J&J vaccine pending further review of these cases.

The cases of clotting after the J&J vaccine were seen only in women, and primarily those under 50 years of age. Therefore, the Centers for Disease Control and Prevention and the Food and Drug Administration have recommended that “women younger than 50 years old especially should be aware of the rare risk of blood clots with low platelets after vaccination, and that other COVID-19 vaccines are available where this risk has not been seen.” The CDC and FDA reasoned that the J&J vaccine’s known and potential benefits outweigh its known and potential risks for those recommended to receive it, and so recommended resumption of vaccine administration as of April 30.

Q: For those who have gotten or may get the Johnson & Johnson vaccine, what adverse reactions should could be indications of serious problems?

A: (Zumberg) Patients should seek immediate medical attention if they have a severe headache, vision changes, abdominal pain, nausea and vomiting, backache, shortness of breath, leg swelling or pain, petechiae (tiny red, purple or brown spots on the skin) or easy bruising. It is common for patients getting a COVID-19 vaccination to have mild to moderate symptoms, including fevers, chills, headache or muscle pain. These symptoms typically are limited to 24 to 36 hours post-vaccination. Symptoms related to vaccine-induced thrombotic thrombocytopenia — a combination of blood clotting and low blood platelet counts — typically occur four to 20 days following vaccination. This reaction is extremely rare.

Q: What kind of blood clots are at issue?

A: (Zumberg) Clots in atypical locations have been seen with the Johnson & Johnson vaccine. The brain is the most common site, followed by the veins that supply blood to the liver, spleen, pancreas and gastrointestinal tract. These are not usual sites for clotting, which typically occurs in the lungs or lower extremities. None of the affected patients had known risk factors for atypical blood clotting.

Q: What do scientists know so far about whether the vaccine caused the blood clotting condition?

A: (Iovine) It is not known for sure how the clotting complication occurs after the J&J vaccine. Some of the women who developed these clots had an unusual antibody in their blood that was directed against platelets, which are blood cells that assist with stopping bleeding when an injury like a cut or scrape occurs. It seems that this antibody activates the platelets, making them prone to form clots even when there is no injury. A big question is whether this antibody against platelets is somehow triggered by the J&J vaccine. Hopefully, as scientists continue to study this, we will be able to know for sure if this is what is happening.

A: (Zumberg) Knowledge is growing daily, but these clots are also atypical due to their location and association with low blood platelet counts. This confluence of conditions has been termed vaccine-induced thrombotic thrombocytopenia, or VITT. Its origin and development are similar to another condition that can occur in some people after exposure to heparin, a blood thinner. The American Society of Hematology has a frequently updated website with detailed information and answers to common questions.

Q: Is there anything J&J vaccine recipients can do to reduce the potential for blood clots?

A: (Zumberg) Be diligent about seeking medical care if the symptoms mentioned above appear after the vaccine’s initial side effects. It is not recommended to start taking aspirin or anti-clotting medication if you are not doing so already. Also, it is not recommended to stop these medications if you are already taking them.

Q: What is your advice for those who may have concerns about receiving the J&J vaccine?

A: (Iovine) The chance of contracting COVID-19 greatly exceeds the chance of developing a serious clot after the J&J vaccine. This is particularly true for people who are not in the group that experienced the clotting complication (females under the age 50). If you are still unsure, it is always a good idea to discuss your concerns with your health care provider.

Media contact: Ken Garcia at or 352-265-9408

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Doug Bennett
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