While we’ve lost the plot over blood clots, the truth is that we’re more likely to get it from the virus. But it’s not that simple.



A study has found people are eight times more likely to get blood clots from COVID-19 than from the AstraZeneca vaccine.

Not that probability is likely to dissuade someone who has decided that what someone said on YouTube is more sound medical advice than that from someone who had qualifications, experience and expertise in immunology.

IFL Science has reported that scientists at the University of Oxford compared the risk of rare blood clotting in the brain, known as cerebral venous thrombosis (CVT), following COVID-19 to the risk after vaccination (including mRNA vaccines, such as Pfizer and Modern, and the AstraZeneca-Oxford vaccine). 

“The key finding was that the risk of CVT from COVID-19 is about eight times greater compared to the AstraZeneca-Oxford vaccine, while the risk of a CVT from COVID-19 is about 10 times greater compared to the mRNA vaccines,” it said.

And that really ought to do it, right? 

In this study of over 500,000 COVID-19 patients, CVT occurred in 39 in 1 million patients. In over 480,000 people receiving a COVID-19 mRNA vaccine (Pfizer or Moderna), CVT occurred in 4 in 1 million. CVT was reported in about 5 in 1 million people after the first dose of the AZ-Oxford COVID-19 vaccine.

In Australia, per the Department of Health, the AstraZeneca vaccine has been provisionally approved by the Therapeutic Goods Administration (TGA) for people 18 years and older. Having said that, the Australian Technical Advisory Group on Immunisation (ATAGI) more recently recommended that AstraZeneca is preferred for adults over 50 years old and the Pfizer (Comirnaty) is preferred for adults aged under 50 years.

This recommendation is based on the increasing risk of severe outcomes from COVID-19 in older adults (and hence a higher benefit from vaccination), and a potentially increased risk of thrombosis with thrombocytopenia following AstraZeneca vaccine in those under 50 years.

The “potentially increased risk” is five in every million. That’s some odds. Talk about probability (“the extent to which an event is likely to occur, measured by the ratio of the favourable cases to the whole number of cases possible”).

Let’s look at some other examples of probability, shall we?

Poker players are less likely to receive high-ranking hands, such as a full house (probability 17/100 or 0.17%) or royal flush (probability 77/500000 or 0.000154%), than they are to play low-ranking hands, such as one pair (42/100 or 42%) or three-of-a-kind (2.87/100 or 2.87%). Yet, many continue to go all in.

Winning the lottery pretty much the sole option left for many stuck in a dead-end job, or in need of paying off their mortgage. You’re unlikely to win, but the odds aren’t, strictly speaking, zero. Lottoland has broken down the probability of winning across a range of lotteries, showing that the odds of winning any prize in Oz Lotto is 1 in 55, but if you’re keen to score the jackpot, the odds skyrocket to 1 in 45,379,620.

The odds of winning any Powerball prize is 1 in 25, but the odds of winning the jackpot is 1 in 292,201,338. Yet people still put their hard-earned into these things, often to their own detriment. 

As C-3PO noted, the odds of successfully navigating an asteroid field is approximately 3720-1. But Han managed it, so there’s that.





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