Is AstraZenica vaccine really safe or not?

by Adjunct Associate Professor Priscilla Robinson, Department of Public Health, La Trobe University

The news about the AstraZenica (AZ) vaccine has been full of ‘It’s safe, but look someone just had a dangerous clot! It’s safe but look someone just died!’ kinds of headlines. We know that many people are really confused and a little worried about the AZ COVID19 vaccine. Why is the advice so confusing? Is it safe or not? Well, here’s an explanation. 

Last year we all had to understand about r-values (how many people one infected person go to infect), k-values (clusters of cases), what was meant by social distancing, and why masks and handwashing are important (they still are, by the way!). This year we are dealing with vaccine clinical trials, herd immunity, protective antibodies, and vaccine schedules. So here is something else for you to get to grips with – the Precautionary Principle (or PP for short).

The PP is about preventing permanent damage caused by threats to people, animals, or any other part of our ecosystem. The PP insists that anything which threatens to be irreversibly damaging must be avoided until scientific investigation proves it to be safe, and that where appropriate and available, safe alternatives should be used. It is important to remember that whilst apparent links must be scientifically investigated, an association is not proof of cause. For example, research has shown that the more people who wear seatbelts the fewer the number of astronauts who die in space accidents – they really are associated, but clearly they are unrelated!

The PP was originally used as a means of protecting healthy people and their environments from the ill-effects of contamination of various kinds, such as manufacturing waste pouring into rivers. Some people refer to administrative caution as ‘the nanny state in action’, but most people are unaware of these regulations unless something goes wrong. When a possible harm is identified, authorities call a ‘pause’.

Pauses are really a way of buying time to investigate further. Sometimes they happen for a short time and sometimes they are permanent. The PP has been used in various ways, ensuring that the air we breathe, the water we drink, and the food we eat are safe.  There are so many examples it is hard to know which to mention here but some obvious ones include not allowing lead in paint and petrol anymore because of the harm to children’s brains, food suspected of being contaminated is recalled to prevent food poisoning, and cooling tower owners have to stop any lurking legionella bacteria from getting into our lungs by preventing spray drift from escaping. That’s why when clots happen in people who have recently been vaccinated investigations always happen to see if they are associated with the vaccine or not.

Previous articles have described the four trial stages of vaccine development to ensure safety. Phase 4 COVID vaccine monitoring systems are well established worldwide. Worldwide there have now been about 850 million doses of various COVID vaccines given, and about one quarter of these were AZ. Because so many people have been vaccinated a few very rare events have shown up. The European Medicine Agency provides regular information about all of the vaccines, available to the public and used by most countries (including Australia) as a part of Phase 4 safety monitoring (if you want to see them, they are not secret – go to and look for the latest scientific and technical reports).  

The PP is being applied to thinking about the AZ vaccine (as well as one or two other coronavirus vaccines which are not being used in Australia). The much-publicised very rare and very specific kind of blood clot (called cerebral venous sinus thrombosis, or CVST for short) and similar problems – less than 100 worldwide – are reported as happening mainly in a particular group of people – mainly young-ish women. That is less than 100 CVSTs in the 200 million or so doses of AZ vaccine, or about one in 2,000,000 doses. Importantly, clots happen about eight in every 200,000 people who get COVID19 disease. This compares with about one in 2,000 women on the pill. There is even a risk of clots in young people who have no risk factors at all of one in 10,000. 

In Australia the PP has been applied to make sure that some younger people do not now have the AZ vaccine, but have the alternative Pfizer one instead. Importantly, this is not to say that the AZ vaccine is actually unsafe – the risk of a major event from the vaccine definitely remains much, much lower even in the group of people identified with a higher risk, than from actually having COVID19 – but as there is an alternative which does not seem to have the same effect then it will be given to those people instead. This is the PP in action! It’s considered sensible to be safe rather than sorry, and we are very lucky that there is an effective alternative for the group of people who might, even if the likelihood is very, very, very small, have major side effects. There are of course the usual range of general side effects such as a sore arm, a headache, and feeling a bit off-colour for a day or two, but major side effects are terribly rare. The vaccines are now making a big difference to the number of people who have died or have long-lasting side effects from actual COVID 19.   

Phase 4 monitoring will continue, and the PP applied where appropriate.  And of course, we can expect that a few coincidental events will happen, and given panicky press coverage. 

Worldwide evidence shows that the COVID vaccines being used in Australia are safe and effective, that the risk of blood clots is very low, and vaccination is very much safer than COVID19 disease. And of course we have impressive safely monitoring systems in Australia to watch for problems, so that we can be confident about the safety of all COVID19 vaccines, including AZ.