Over the last few months, both doctors and patients have asked me several times if it is safe for people with the Factor V Leiden mutation to have the Pfizer COVID-19 vaccine. At first, I couldn’t understand why this question would come up, as there is no evidence that the Pfizer vaccine has any risk of thrombosis. But I presume the concern has come from publicity about the Astra-Zeneca vaccine, which has been associated with a very rare type of thrombosis.
This needs some further explanation.
Why does the Astra Zeneca vaccine cause clots?
All COVID-19 vaccines are designed to deliver a short snippet of the COVID virus genetic material into the body to stimulate the immune system to produce COVID-19 antibodies. The genetic material is packaged in different ways. In the case of the Astra-Zeneca vaccine, the genetic parcel is delivered in another virus – an Adenovirus. The surface structure of the adenovirus allows another protein – platelet factor 4 – to bind to it. In rare cases, the immune system confuses the platelet factor 4 with the virus and produces antibodies to platelet factor 4. This can trigger a blood clot, causing the blood platelets to clump together. This can be a severe condition.
The Pfizer vaccine does not cause blood clots.
The Pfizer vaccine is packaged entirely differently, and it cannot cause the same problem as the Astra Zeneca vaccine. The vaccine has no effect on platelets and does not cause clots.
How can we be sure the vaccine is safe?
The best way to know is to look at extensive population studies. For example, a study from the UK looked at the incidence of blood clots in over 29 million people who had received a COVID vaccine; 9 million received the Pfizer vaccine. There was no increased risk of venous thrombosis in those receiving the Pfizer vaccine. But on the other hand, the incidence of venous thrombosis in people with COVID infection was eight times higher than usual.
Does Factor V Leiden increase the risk of thrombosis?
The is no evidence that people with Factor V Leiden have an increased risk of thrombosis from the Pfizer vaccine. Therefore, it is entirely safe for you to be vaccinated if you have the factor V Leiden mutation.
What if I get COVID?
Venous thrombosis is a complication of COVID. This seems to be of most concern in people with a severe infection who need hospital admission. Around 10% of people in hospital with severe disease have a blood clot, and the incidence is even higher in intensive care. Therefore, all people in hospital with COVID should be given a blood thinner, and most guidelines recommend using a dose higher than is typically used to prevent blood clots. Also, a recent study has shown that continuing a blood thinner after discharge from hospital can reduce the risk of deep vein thrombosis.
If you have mild disease and do not need hospital admission, you do not need a blood thinner. The present advice is
- If you are already on a blood thinner – continue on the same treatment.
- If you are not taking a blood thinner – You do not need to start a blood thinner unless you are admitted to hospital.
- If you have Factor V Leiden and have never had a blood clot – You do not need to start a blood thinner unless you are admitted to hospital.
- If you have factor V Leiden and have had a blood clot, but you are no longer on a blood thinner – The current advice is that you should discuss treatment with your health care professional.
- If you are not badly affected by COVID you do not need a blood thinner.
- if you have other risk factors, such as being confined to bed for several days, it would be reasonable to restart a blood thinner for a short peiord until you are fully mobile.
The advice above is based on International Guidelines however, these are changing all the time. Most of the advice comes from studies carried out early in the COVID pandemic. These may not be the same for people with Omicron, as it appears to be a less severe condition.
If you are concerned, you should talk to your own doctor before changing your treatment.