COVID-19 Vector Vaccines & Hematological Events: J & J, AstraZeneca Vaccines

COVID-19 Vector Vaccines & Hematological Effects: J & J, AstraZeneca Vaccines

The link between adenovirus vector vaccines, blood clotting, and low platelets.

The World Health Organization (WHO) lists about 180 COVID-19 vaccines that have been developed or are being developed around the world. Each vaccine uses a slightly different approach to initiate the immune response to recognize and fight SARS-CoV-2, the virus that causes COVID-19.

Vaccine technologies are grouped into five main types:

  1. DNA-Based or RNA-Based (i.e. Pfizer-BioNTech, Moderna)
  2. Virus Vectors (i.e. Janssen/Johnson & Johnson, AstraZeneca)
  3. Inactivated (i.e. flu vaccines)
  4. Live-Attenuated Virus (i.e. measles, mumps, rubella vaccine)
  5. Protein Subunit (i.e. Hepatitis B vaccines)

Both the Janseen/Johnson & Johnson COVID-19 vaccine and the AstraZeneca COVID-19 vaccine use adenovirus vectors. There are two additional COVID-19 vaccines that use adenoviral vectors: CanSino Biologicals and Sputnik V.

What is Viral Vector Technology?

The CDC provides a good explanation of viral vector technology for patients:

  1. First, the vector (not the virus that causes COVID-19, but a different, harmless virus) enters a cell in the body and then use the cell’s machinery to produce a harmless piece of the virus that causes COVID-19. This piece is known as a spike protein and it is only found on the surface of the virus that causes COVID-19.
  2. Next, the cell displays the spike protein on its surface, and the immune system recognizes it doesn’t belong there. This triggers the immune system to begin producing antibodies and activating other immune cells to fight off what it thinks is an infection.
  3. At the end of the process, the body learns protection against future infection with the virus that causes COVID-19.

Adenovirus Vaccine Technology and Adverse Effects

In addition to swelling and pain at the injection site, which is common to many vaccines, adenovirus-based vaccine adverse effects include fever, pneumonia, diarrhea, transient neutropenia and lymphopenia, fatigue, headaches, liver damage, and fasting hyperglycemia. Rare but grave adverse reactions include neuropathies such as Bell’s palsy, Guillain-Barré syndrome, gait disturbance, and transverse myelitis (an inflammatory condition in the spinal cord).

Additionally, some health officials now postulate that there may be a link between the adenovirus vectors and a rare blood clotting condition known as “vaccine-induced immune thrombotic thrombocytopenia” (VITT).

Janseen/Johnson & Johnson COVID-19 Vaccine and Hematologic Events

On April 13, 2021, the FDA and CDC initiated a pause of vaccine distribution "out of an abundance of caution." After reviewing data, there were six reports of women between the ages of 18-48 who experienced rare blood clots in combination with thrombocytopenia (low platelets). The six reports were found out of more than 6.8 million doses given. Of the six cases, one woman died; another was in critical condition.

According to the joint statement issued by the CDC and FDA, symptoms occurred 6-13 days after vaccination. The type of clot was a cerebral venous sinus thrombosis (CVST) seen in combination with thrombocytopenia.

In the general population, CSVT affects about 5 people in 1 million adults, children and newborns each year. Overall, about 3 out of 300,000 children and teens up to age 18 will have this type of stroke.

Side Effects of University of Oxford/AstraZeneca COVID-19 Vaccine

Vaxzevria, the COVID-19 vaccine developed by the University of Oxford and AstraZeneca, causes some of the mild-to-moderate side effects as the other coronavirus vaccinations (fatigue, chills, headache, etc.). However, there are also multiple cases of thrombosis post vaccination, including cases of cerebral venous sinus thrombosis.

In early March 2021, the Danish Health Authority put the Vaxzevria vaccination on hold following reports of severe cases of blood clots. The European Medical Union reported that 30 cases of thrombosis had occurred following Vaxzevria administration to around 5 million people in the European Economic Area. This included one fatal case in Denmark.

What should I tell my patients who received the vaccine?

First, remind worried patients that six reports of blood clotting and low platelets were found out of more than 6.8 million people who received the Janseen/Johnson & Johnson vaccine; meaning, the occurrence is very rare.

For patients who received the Janseen/Johnson & Johnson vaccine in the past 1-4 weeks, they should notify a healthcare provider immediately or call 911 if they experience any of the following symptoms:

  • Severe headache
  • Blurred vision
  • Abdominal pain
  • Leg pain
  • Loss of control or movement in an extremity
  • Shortness of breath
  • Fainting or loss of consciousness
  • Seizure

Last, you can refer patients to the CDC website, which provides up-to-date information regarding the safety and efficacy of COVID-19 vaccines.


  1. Cerebral Venous Sinus Thrombosis (CVST), John Hopkins Medicine, retrieved April 14, 2021.
  2. Molecular Therapy. 2020 Nov;28(11):2303-2304.
This article was written by Jami Cooley

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