Could vaccines actually protect against pulmonary embolus?

My headline conclusion from yesterday’s post was that, compared to the general population, there was no increased risk of pulmonary embolism following the AstraZeneca vaccine: indeed, the published general population rate was about 6 times that following the AstraZeneca vaccine. More than one reader has commented that far from showing that the vaccine increased the risk of embolism, the reverse seemed to be the case and that the vaccine gave substantial protection against this disorder.  I do not believe protection is a logical conclusion and in this post I will explain why the data are problematic. 

Is there any plausibility that vaccines are protective?

  • The short answer – it does not make sense that vaccines provide short term protection against the occurrence of embolism  
    • Vaccines do not have any anticoagulant properties that would provide the protection
    • Vaccines do not have anything that would protect the walls of veins from clots
  • If the above are true, then what are the explanations for what would appear to be a false suggestion of a level of protection?
  • Thinking about the calculation I posted; there are number of possible sources of error inherent in the available data

1. Was the quoted background population rate of pulmonary embolism inaccurate?

  • I quoted the figure of 8/10,000 (800/million) provided in a  UK official guidance document on the prevention of this disorder*
  • The accuracy of this  figure is reinforced by other data I have examined**
  • Thus the estimate is as good as we can have

* ** 

2. What is the correct time interval following vaccination that should be considered relevant in counting a pulmonary embolism?

  • I made a guess at one month, to allow time for a clot to have developed in the legs and then pass to the lungs
  • From the publicly available data on AstraZeneca’s embolism cases though we don’t know the interval between vaccine and diagnosis
  • If all occurred within a week then the apparent reduction in incidence is only 1.5 times, compared to 6 times, background risk by allowing 4 weeks for cases to be included

3. Could there be under-reporting of the  number of cases of pulmonary embolism following vaccination?

  • The short answer is yes, but we ‘don’t know what we don’t know’
  • I would have expected that something serious like pulmonary embolism occurring within a short time after vaccination would have been reported
  • We might expect because of the publicity more cases will be reported in the future

4. Is like being compared with like?

  • The regulators like the European Medicines Agency will have done a similar task to my own calculation
  • Essentially two separate rates are being compared though the data are gathered in very different ways
  • Perhaps the biggest difference is how pulmonary embolism is diagnosed
    • The general population cases will include the hospital cases that will have been picked up incidentally, for example as an unexpected finding on an X-ray 
    • By contrast, cases of pulmonary embolism following vaccines, would not have been detected if they had been mild, with only minor symptoms
  • As mentioned yesterday many cases of pulmonary embolism occur as a complication of surgery and it is not easily possible to identify the rate of ‘underlying natural cases’
  • Probably not a major issue, but any analysis needs to take account of age, gender and other factors such as ethnicity which might affect the overall rates 


  • I do not think it is likely that any of the vaccines protect against pulmonary embolism – though calculations based on the limited available data could give that misleading impression
  • There are many ways in which feeding the limitations of the available data into these calculations could distort the results 
  • These comments only refer to the data on pulmonary embolism and do not address the other concern about thrombosis in the veins in the brain
  • None of the above explain what appears to be lower rates of embolism from the Pfizer vaccine 

If you would like to receive email notifications of new posts on this blog just click this link and enter your email at the bottom of the home page:

One reply on “Could vaccines actually protect against pulmonary embolus?”

In the UK, AstraZenica jabs are 12 weeks apart whereas I believe the gap between Pfizer jabs is slightly less and was 3 weeks at the beginning of the vaccination programme. Overseas, I believe countries have stuck religiously to Pfizer’s 3-week gap recommendation but have stretched the gap for AstraZenica’s vaccine, supported by the WHO. This may have an effect on outcomes of all sorts, not just the efficacy of a vaccine. In any event, the critical fact is that incidences are small compared with the risk of not having the vaccine and catching Covid.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s