What are we to make of the governments of 11 nations (Ireland, Iceland, Netherlands, Denmark, Bulgaria, Norway, Thailand and most recently, France, Italy, Germany and Spain) suspending the rollout of the AstraZeneca COVID-19 vaccine following as yet unspecific reports of “blood clotting” or thrombosis in what is a reportedly small number of those being vaccinated in Norway?

There are some basic considerations that all reading these news reports need to keep in mind. Foremost here is that blood clotting is far from uncommon. Around 30,000 people develop blood clots (venous thromboembolism (VTE) – in the deep veins of the leg (deep vein thrombosis) or in the lungs (pulmonary embolism), every year in Australia, with about 5,000 dying as a result. Many of these develop post-operatively in hospitals and account for about 10% of all deaths in hospitals.

The risk factors for developing thrombosis include:

  • Older age
  • Smoking
  • Diabetes
  • High blood pressure (hypertension)
  • High cholesterol
  • Lack of activity and obesity
  • Poor diet
  • Family history of arterial thrombosis
  • Lack of movement, such as after surgery or on a long flight

All of these risk factors are of course very prevalent in most societies, which explains why the prevalence of thrombosis is also significant. Aging is perhaps the single most important risk factor: thrombosis  is very rare in young people (< 1 per 10 000 per year) but increases to around 1% per year in the elderly.

This means that, regardless of whether there is any COVID-19 vaccine program being rolled out in any nation, we should expect to see many cases of thrombosis. With the aged and those with chronic disease being prioritised in vaccine rollouts, we would therefore expect the incidence (new cases) of thrombosis to be considerably higher in these groups than in segments of the population who have been given lower priority in the rollout queue.  

But this may have nothing to do with the vaccine.

Epidemiologists investigating whether reports of possible rises or clusters in the incidence of diseases always look first to what the “expected” or background incidence is, and then statistically compare this with the current “observed” rate. If the observed rate is significantly higher than that which earlier data would suggest should be expected, and comparable ages are being considered, then focus quickly shifts to investigating possible plausible other causes.

I’ve yet to find any reports that this point has been reached in the Norwegian investigations.

Reasoning that just because symptoms of disease occur after exposure to some agent (drug, vaccine, contaminant etc), that one of these agent must have caused the symptoms, is known as the post hoc ergo propter hoc (after, therefore because of) fallacy.

During my career I’ve seen many examples of media and community alarm bells going off because of this reasoning. For example, when nicotine replacement therapy (NRT) first became available in the early 1990s, there were claims that some people using it were having heart attacks. It was not rocket science to point out that all who were using NRT were smokers, and that smoking is a major risk factor for cardiovascular disease and heart attack. Of course there would have always been smokers using NRT and trying to quit who were already at high risk of heart trouble, regardless of whether they were using NRT.

I used to start an early lecture to new public health students by saying that about 90% of drivers involved in serious road accidents in their commute to work, had eaten breakfast. Was it therefore reasonable and sensible to suspect that eating breakfast increased your chances of being injured on the road shortly afterwards? Obviously not.

Nearly all of the suspending nations have many advanced epidemiologists . These nations are also not known for imprudent policy shifts based on knee-jerk reasoning. Their decision to suspend the roll-out is therefore significant. We should expect a thorough analysis of the issues raised above and of each of the Norwegian cases.

[this blog was updated twice after publication to increase the number of suspending nations from 4 to 11]