COVID-19 Vaccines and Blood Clots

Aaron S. Weinberg MD, MPhil
April 21, 2021
7 mins

The COVID-19 pandemic has been a world-changing event. Tragically, it has resulted in the loss of more than three million lives worldwide, including 567,000 deaths in the United States alone (as of this writing) — which is almost twice the number of Americans who died in World War II. However, out of great tragedy have come tremendous medical advancements, including the development of multiple successful vaccines in a short period of time. These vaccines have proven to be effective in preventing COVID-19 disease, and new data even suggests general infection protection. Typically, it can take many years of development for novel therapies to come to market. This breakthrough was rapid, but no steps were missed during the vaccine development process. Each vaccine underwent the usual three-phase clinical trials and enrolled tens of thousands of patients. The data was scrutinized by third-party unbiased review boards for both safety and efficacy. Part of the rapidity was due to the fact that these trials were performed during a pandemic, so there was access to an abundance of participants and cases, enabling researchers to reach statistical significance at remarkable speeds. 

Today, 84 million people, or about 25 percent of the total U.S. population, have been vaccinated. And with any new medication or vaccine, the medical community and regulatory bodies keep close track of any new side effects or medical problems that may emerge but that were not appreciated during the clinical trials. Almost every treatment on the market for common medical conditions has been associated with some degree of adverse events. COVID-19 vaccines have been proven to have an extremely low number of possible associated adverse outcomes, making them extremely safe on the spectrum of therapies.

Deciding to Suspend Use of Some Vaccines

There has been recent news regarding two of the COVID-19 vaccines, those from AstraZeneca and Johnson & Jonson, and their potential association with an extremely rare increased risk of blood clots. The two vaccines have a similar design and differ from the mRNA vaccines (from Pfizer and Moderna), in that they use an inactive virus to deliver genetic code, which then uses the body’s own cellular machinery to read the code and produce the COVID-19 outer surface spike protein. The immune system recognizes the spike protein as foreign and builds an immune response against it, so when a person encounters a live COVID-19 virus, their body is primed to fight it. 

The concern about blood clots must be viewed alongside the overall clinical data, as well as community incidence of the disease in general. Blood clots in the legs (deep vein thrombosis or DVT) and blood clots in the lungs (pulmonary embolism or PE) are common — there are more than one million cases per year in the United States, and this condition is the third-highest cardiovascular cause of death, behind heart attacks and strokes. (It’s important to note that multiple studies have shown that COVID-19 disease is associated with a significantly increased risk of DVT and PE; both have happened at alarming rates in COVID-19 patients and have even been fatal.) In the phase 3 clinical trial data, there were 15 cases of blood clots in the vaccine group and 10 cases in the placebo group, which was on par with what would be expected in the general population and was too small of a number to determine whether the five additional cases in the vaccine group were due to a causal association with vaccination or due to chance alone. In fact, real-world data post-vaccination seems to indicate that there does not appear to be an association between the vaccine and traditional thromboembolic disorders (DVT and PE) in healthy patients overall, since the total rate of thromboembolic events following vaccination is actually lower right now than what is expected when we consider the usual rate of such blood clots in the general population. 

What is catching the attention of authorities is not an increased risk of typical DVT or PE blood clots but rather a very rare clinical syndrome that may be occurring in a select few patients. This syndrome results in blood clots in more unusual locations of the body, such as the veins in the brain (cerebral venous thrombosis) and the veins of the abdomen. These patients also tend to have low levels of platelets, which are the cells that help the body form clots and are noticeably lower because they are being consumed by the clotting process. In the past, we have seen a similar syndrome associated with a commonly used blood thinner known as heparin: select patients may develop paradoxical blood clotting and low blood platelet levels; this is known as heparin-induced thrombocytopenia (or HIT). The cause is believed to be a type of autoimmune response wherein the body, for some reason, produces antibodies when exposed to a certain medication or treatment. This then triggers a clotting cascade, blood clots throughout the body, and a subsequent drop in platelets.

Extremely Low Risk Levels

It must be stressed that this very rare event happens only among a small minority of people who are genetically susceptible. Among 34 million AstraZeneca vaccine recipients in the United Kingdom and Europe, there were 169 cases of cerebral venous sinus thrombosis and 53 cases of splanchnic vein thrombosis, which reflect rates that are higher than what would be expected for these rare types of thromboses in the general community but still low in overall incidence in the vaccinated population. The Johnson & Johnson blood clotting cases are also extremely rare, with six cases reported out of more than 6.8 million vaccines given to date. The data also seems to suggest that those who are most susceptible tend to be female and under 60 years of age, and that the problem arises within 14 days of receiving the vaccine. 

While any adverse event is taken very seriously, especially if it results in loss of life, it must be emphasized that this is a very rare condition, and the Johnson & Johnson vaccines that have been given to date have likely, according to statistical modeling, saved 120,000 lives that may have otherwise been lost due to COVID-19 infection. Another point to emphasize is that if someone experiences such adverse events, it’s important to recognize the signs and symptoms early, as the condition is potentially treatable with medical therapies. People should be aware that if they experience unabating severe headaches, new neurologic symptoms, seizures, abdominal pain, petechial rash, leg pain or swelling, chest pain, or shortness of breath, they should seek immediate medical attention. If you have received the Johnson & Johnson vaccine already, it is extremely unlikely that you will experience a problem, even if you are within the 14-day post-vaccine window. This should put minds at ease — there is nothing in particular that must be done unless you develop one of the aforementioned symptoms. 

Looking to the Future

The medical community is still gathering data and monitoring the situation, and has determined that while a causal relationship is plausible, it has yet to be fully confirmed. In addition, because of the rarity of these events and the potential severity of COVID-19, the European Medicines Agency and the World Health Organization have concluded that the overall benefits of the vaccine continue to outweigh the very small risk. Some countries have chosen to take a very cautious approach until more data is available, and have either suspended the use of the vaccine or limited it to patients over a certain age who may be at lower risk of such a syndrome. The United States has decided to temporarily pause administering the Johnson & Johnson vaccine until data can be thoroughly assessed and vetted. 

To use a sports analogy, we are currently in the fourth quarter of the pandemic. Cases may be decreasing in some areas, and we may be getting somewhat ahead of the curve, but now is not the time to abandon a defensive approach. This means wearing masks, practicing safe preventive measures, and continuing the race against time to vaccinate yourself and your loved ones before the virus has a chance to further mutate and negate all the scientific progress that has been made. 

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Carbon Health’s medical content is reviewed and approved by healthcare professionals before it is published. But note that our knowledge and understanding of COVID-19 are developing and changing very rapidly; if you have questions or concerns about COVID-19 precautions, treatments, and vaccinations, please talk to your healthcare provider.


Aaron S. Weinberg MD, MPhil

Aaron S. Weinberg, MD, MPhil, is Director of Program Development at Carbon Health and triple board-certified in Pulmonary, Critical Care, and Internal Medicine.


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