AZ vaccine and clots – number crunching time

13th April 2021, Dr Chee L Khoo

Every time the government overpromise and underdeliver, we are the ones that have to deal with the public whose expectations have been led unnecessarily high for political reasons. With the issue of clots relating to the AZ vaccine on the front page over the last week or so, patients are now utterly confused and look to their GP for advice as to where and when to proceed. Many who should be having the vaccine have cancelled their appointments and it’s now up to GPs to discuss the issues with them. It’s all a balance of the risk and benefit consideration with individual patients but guess what, the advice always ends with “please see your GP to discuss your situation”.

So, what is the “real” situation with the risk and benefit calculations with our patients in Phase 1A and 1B. We explored the full details of the “clotting” issue with the AZ vaccine a week ago before the change in direction of the vaccine roll out. There is a suspicion that the AZ vaccine is linked to thrombosis with thrombocytopenia syndrome (TTS) and there is some evidence towards that effect but naturally, these are early days yet. The situation will continued to be monitored worldwide and no doubt, we will be informed in due course.

Unfortunately, media release after media release and commentaries after commentaries over the last week has completely confused doctors and patients alike. Now, patients of all ages are hesitant about covid-19 vaccinations. Many patients (and colleagues) prefer to “wait”. Not sure what they are waiting for. Some are waiting for the Pfizer vaccine since the government ordered another 20 million Pfizer vaccines in addition to the 20 million on order but under delivered. Truly, not sure what the addition 20 million Pfizer vaccines will do since we only got a paltry 1 million doses came. It is likely that the new (or old) Pfizer vaccine order will get here towards the end of the year if not, in the new year.

Let’s look at the numbers. Covid-19 infections have an average mortality rate of 3.2 per 100. The average risk of getting TTS from AZ vaccine is 1:250,000 to 1:300,000 vaccine administered. The mortality of TTS for now is 25% which means the mortality of TTS from AZ vaccine in about 1 in 1 million vaccine administered.

So, how should we proceed with the chaos? These are the three questions we need to ask:


Is the patient over 50 years old? If yes, they can have the AZ vaccine as the risk of TTS is in patients under 50 years old. If this patient is Phase 1A and Phase 1B then, I would go ahead and offer the patient the vaccine making sure that they are aware of the TTS issue.


Is the patient under 50 years old? If yes, is the patient in Phase 1A and Phase 1B? If they are in either one of the phases, then they are either at higher risk of exposure to the virus or they are at higher risk of severe disease if they contract the disease. They should have the AZ vaccine if they cannot get access to the Pfizer vaccine soon.


Is the patient is under 50 years old but not in Phase 1A and Phase 1B? They will be in Phase 2 which under current projection, will be unlikely to begin till towards the end of May if not June 2021. Let’s talk about that when the time comes. Hopefully, we will have more data on safety or risk by then.

That’s how I will be approaching this chaos.