JNJ COVID-19 vaccine and blood clots – still safe and effective

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Like the earlier story about the AstraZeneca COVID-19 vaccine and blood clots, the JNJ (Johnson and Johnson) vaccine is being examined for a potential link to an extremely rare blood clotting disorder. The US Food and Drug Administration and the CDC have placed a “pause” on the use of the vaccine. They have not banned it. They have not recalled it.

Right at the top, I want to state that the JNJ COVID-19 vaccine is safe and effective by any rational measurement. My daughters and their significant others all received the vaccine over the weekend after I recommended it, despite the blood clots issue.

I want to review what is happening here. I hope that it helps, in some small way, to allay the fears from this issue.

What is the JNJ COVID-19 vaccine?

The JNJ COVID-19 vaccine is very different than the Pfizer and Moderna mRNA vaccines but is quite similar to the AstraZeneca version, which has also experienced potential blood clots issues. But the JNU vaccine still induces an adaptive immune memory response to the S-protein of the SARS-CoV-2 virus that causes COVID-19.

The JNJ COVID-19 vaccine, known as the JNJ-78436735, utilizes a recombined adenovirus vector, human Ad26.COV2.S., which expresses the S-subunit of the SARS-CoV-2 virus to induce an immune response. Basically, the adenovirus vector “carries” the genes for the S-subunit to the cell which will reproduce the protein, then inducing the immune response.

Adenovirus-based vaccines have been investigated for several decades. In fact, JNJ received approval for an Ebola adenovirus vaccine in July 2020, so the technology did not suddenly appear just for COVID-19. However, like the mRNA vaccines, these adenovirus vaccines, also used by AstraZeneca, can be quickly developed to deliver the most important antigen on the SARS-CoV-2 virus, which is the S-protein.

There is an advantage to adenovirus-based vaccines – they are much less fragile than mRNA vaccines because they are based on DNA which is more rugged than RNA.

Once the Johnson and Johnson COVID-19 vaccine is injected into the arm, the adenoviruses enter cells and moves to the nucleus, where the cell’s genes (DNA) are located.

The adenovirus then injects its DNA into the nucleus. The adenovirus is engineered so it can’t make copies of itself, but the gene for the coronavirus spike protein can be read by the cell and copied into a molecule called messenger RNA, or mRNA.

At this point, it is similar to the mRNA vaccines.

Normally, during the process called transcription, RNA polymerase makes a copy of a gene from its DNA to a corresponding mRNA fragment whenever required by the cell. In other words, the mRNA sequences in the cell usually correspond directly to the DNA sequences in our genes. These mRNA sequences “carry” that genetic message to a ribosome for translation, where tRNA triplets, which code for one amino acid, attach to the appropriate mRNA triplet, adding one amino acid to the protein chain. 

As in DNA, genetic information in mRNA is contained in the sequence of nucleotides, which are arranged into codons consisting of three ribonucleotides each. Each codon codes for a specific amino acid, except the stop codons, which terminate protein synthesis.

Like with mRNA vaccines, the adenovirus does not change the genetic code of any of the 50 trillion cells that are in a human. All that happens is that the adenovirus injects DNA that is coded for the S-protein and the cell produces mRNA from that DNA that then causes the ribosomes to produce the S-protein.

Those S-proteins migrate to the surface of the cell which are then recognized by the immune system as foreign invaders. The immune system then remembers those antigens – when the actual SARS-CoV-2 virus attacks, the immune system is ready to attack.

The Johnson and Johnson COVID-19 vaccine also has one additional advantage over the mRNA vaccines – the adenovirus itself provokes the immune system to activate immune cells that are nearby. This leads to the immune system reacting more strongly to the spike proteins.

Photo by Joel Muniz on Unsplash

JNJ COVID-19 vaccine and blood clots

As I discussed with the AstraZeneca vaccine, the European Medicines Agency, the FDA for the European Union, believes that there is a plausible link between the AstraZeneca vaccine causes rare but sometimes deadly blood clots in a tiny number of those vaccinated. The differences between the AstraZeneca and JNJ vaccines are minor – the AstraZeneca vaccine uses a chimpanzee adenovirus, whereas the JNJ uses a human adenovirus.

Like I mentioned above, the EMA believes that a rare condition called cerebral venous sinus thrombosis (CVST), a clot that stops blood from draining from the brain. Regulators have said it is occurring among those who have received the AstraZeneca COVID-19 vaccine at a rate above what they’d expect to see in the normal population.

The key thing to note is that a patient that presents with CVST must be treated differently than individuals with other types of thrombosis. For example, thrombolytics (clot-busting drugs) are not indicated for these patients as they may cause a hemorrhage into the brain.

The EMA is reporting a total of 169 cases of CVST among the 34 million people given the AstraZeneca vaccine across Europe as of 4 April 2021. Additionally, there have been 52 other cases of rare thromboses. The EMA reported that they based its scientific review on an initial 62 cases and 18 deaths up until March 22, but continued reports did not change their assessment.

In the USA, six cases of CVST have occurred among women below the age of 50 and appeared between one and two weeks after vaccination. This is out of around 7 million doses of the JNJ vaccine.

To put this into some perspective, let’s do the basic math. The risk of these uncommon blood clots is around 1 per 1 million people who received the JNJ COVID-19 vaccine. On the other hand, the risk of death from COVID-19 is around 1,000-2,000 per 1 million who contract the disease, which is around 200X greater risk than the risk of thrombosis from the vaccine. (See Note 1)

But there’s more. We do not have information of confounding risk factors – each of these six women may have another risk factor from blood clots to smoking to some other issue.

The risk of CVST in the general unvaccinated population in the USA is around 0.5 to 1 in 1 million, so the observed risk of these types of clots in those vaccinated with the JNJ COVID-19 vaccine is much higher.

Yes, the SARS-CoV-2 virus seems to cause thrombi in many patients, which might lead one to believe that there is some biological plausibility to the blood clot issue. We don’t appear to observe these thrombosis events with the Pfizer, Moderna, and JNJ vaccines, so it is possible that either there is an ingredient in the AstraZeneca or JNJ vaccines that leads to this increased risk.

Or it may be possible that the AstraZeneca vaccine codes for the spike protein in such a manner that it mimics the clotting issues observed with a SARS-CoV-2 infection.

We just don’t know, so, typical of science, we will do research until we figure it out.

Photo by Markus Winkler on Unsplash


Let me be very clear:

  • The risk of this type of blood clot post-vaccination appears to be higher than what is observed in the general population.
  • The CDC and FDA only “paused” recommending the use of the vaccine. They did this to set recommendations to healthcare practitioners so that they know what to do if they observe symptoms of CVST in patients. They have not recalled the vaccine.
  • The mRNA vaccines from Pfizer and Moderna have not shown this type of reaction.

If the JNJ COVID-19 vaccine is available to you, get it, it works and will potentially save your lives. If you have sustained, sharp pain in your abdomen, chest, or legs, or shortness of breath, see a physician, just in case you are experiencing blood clots.

Just remember this – every medicine and every medical procedure entails risk. The risks of CVST from the JNJ (or AstraZeneca) vaccines are small compared to the risk of the disease. However, it is a risk, and I’m not one of those pro-vaxxers that ignore risks, it’s just that the benefits of any of the COVID-19 vaccines far outweigh the risks.


  1. My calculated numbers for these risks are based on the infamous back of the napkin analyses. Once peer-reviewed papers are published, we will know precise risk difference between vaccinated unvaccinated groups.


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Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!