AstraZeneca ‘pause’ doesn’t add up

Sir, – It is well-known that Covid causes many clots particularly in the lungs which is why those who contract the virus get shortness of breath.

Thousands of people all over the world take common medications that carry clot risks and yet they still use those medications, while patients are just warned of the possibility of clots developing.

All medicines and drugs including vaccines carry risk of adverse effects, yet they are widely used when the benefits outweigh adverse reaction risks. Zero risk is not achievable in the same manner as scientific measurements cannot reach 100 per cent accuracy.

The principle of acceptable risk is applied in toxicology to quantify the benefits relative to adverse effects; it is this principle not the precautionary approach that is used in the introduction of new drugs and other chemicals to world markets and is widely used by WHO when assessing the dangers of air pollutants, for example.


Basically, the principle states that a risk range of 1 in a million (10-6) to 1 in a 100,000 (10-5) is an acceptable level of risk of an adverse reaction. This level of risk is usually categorised as negligible. Given that the UK has administered more than 11 million doses of the AstraZeneca vaccine and Israel almost the same number, it may be seen that a few poorly-described instances of clotting claimed to be related to the vaccine by Austria and Norway mainly are well within the acceptable risk range.

The decision to suspend the AstraZeneca vaccine by Ireland is ill-informed and puts many of us at relatively high risk of coronavirus even though the adverse reaction is very low, in fact, negligible. – Yours, etc,

PROF JAMES HEFFRON, Emeritus Professor in Biochemical Toxicology, University College Cork.