In a recent interview with Radio New Zealand, a vaccine expert claimed that the risk of blood clot was 165,000 times higher after having covid-19, compared to the risk after having the AstraZeneca jab. This claim illuminates several misunderstandings of the nature of the SARS-CoV-2 virus, the true nature of the side effects that are worrying health officials overseas and the influence of misleading claims on social media.
Even though New Zealand is currently using a different vaccine, the emergence of blood clot reactions to some covid-19 vaccines has worried those who have been saying the vaccines are safe and effective.
In response they have tried to do something they refused to do with SARS-CoV-2; provide people with realistic data about the small risk posed.
To make the vaccine-related blood clots seem comparatively small, Dr Helen Petousis-Harris recently claimed that the risk of covid-19 blood clots was high.
She said the risk of clot from the AstraZeneca vaccine is about 1/1,000,000 against risk of clotting from covid-19 which is 165,000/1,000,000.
The frequencies of 165,000/1,000,000 are hard to understand until we start wiping off a few confusing zeros and end up with 16.5/100 or 16.5%.
Dr Petousis-Harris claims that 1/6 people who have covid-19 infection have a clot; not just any clot, but the rare brain vein clot being experienced by covid-19 vaccine takers.
All Helen’s words are taken verbatim from numbers on an infographic image doing the rounds on social media.
The statistic of 1/6 people suffering rare clots after being infected with the covid-19 virus comes from a summary study of hospitalised patients which evaluated the risk of pulmonary embolus and deep vein thrombosis in patients hospitalised for covid-19. Over half the studies included in the summary were from patients in intensive care. Some studies screened all patients for clots. The average of all studies showed a weighted proportion of 16.5% for both deep vein (leg) and lung clots.
Despite widely held belief, over 95% of people who test positive for covid-19 do not need a hospital, so would not have appeared in the denominator of the 16.5% figure. A study from Iceland, one of the most tested nations on earth, showed that 5% of positive patients for covid-19 were hospitalised, and only 1% went to intensive care. This means that the 16.5% figure is a very skewed proportion of all patients with covid-19. Since only 1-5% of cases make it to intensive care or hospital, that 16.5% chance should be less than 1%.
We know also that many more people have caught the virus than the positive genetic (PCR) tests say, as shown by serological tests and other immune studies. T-cell tests show that even more have been exposed to the virus, compared to antibody studies. The incidence of blood clots following covid-19 infection is simply not known, but it must be at least an order of magnitude lower than presented by our vaccine expert. So now the claimed 16.5% chance of blood clots across the population is not even 1%; it is closer to 0.1%.
Now comes the worst part of this attempt to mislead people about the vaccine risk; we’re not even talking about the same type of blood clot.
The blood clots experienced by some vaccine takers is cerebral venous sinus thrombosis, a deadly and rare condition.
The blood clots that threaten about 0.1% of us who catch covid-19 is deep venous thrombosis, a comparatively common condition found across all manner of hospitalised patients. It is so common that in one autopsy case-series, 10% of deaths in hospital patients who had the post-mortem procedure were caused by venous thromboembolism.
The background rate of cerebral sinus thrombosis is estimated to be 1.32 per 100,000 person years.
In contrast, the background rate of deep venous thrombosis is estimated at 50/100,000 person years, about 38 times higher than for cerebral sinus clots. The risk of leg clots is very strongly age-related, with older people more affected.
A direct comparison of the rate of cerebral sinus thrombosis in covid-19 patients compared to those who have had covid-19 vaccines has been carried out. The rate of cerebral venous thrombosis was higher in the covid-19 group compared to the vaccinated, but by a factor of 6 rather than 165,000-fold higher, as claimed in the Radio NZ interview. The cerebral sinus thrombosis group after covid-19 was more likely to have heart disease than those who had had the virus without the clot. The covid-19 group only counted PCR positive individuals, which as mentioned, underestimates the spread of the virus. The rate of venous thrombosis in the vaccinated groups (both Pfizer and AstraZeneca) was about 4-5 per million people in the two weeks following the vaccine. The risk of the vaccine is clearly higher than baseline which is an annual statistic, even if it is lower than for people who have had covid-19.
The administrative bodies of several nations are rightly concerned about the incidence of a rare type of blood clot from the AstraZeneca vaccine. Concern is justified when one particular risk of taking the vaccine is higher or worse than the risk of not taking it.
The image carrying the numbers quoted by Dr Petousis-Harris has been shared over social media by New Zealand doctors. I am sure they were well-intentioned, but it is never justified to allay fears using false information. It is always wrong to misinform people, particularly over the risk to their health of a medical intervention.
I am severely disappointed that our national broadcaster has not questioned these statements. It concerns a vaccine New Zealand is not using. But what happens when it does? What happens if rare reactions and deaths are attributed to treatments used here? We must be able to count on our media, and taxpayer funded experts to look at data impartially.
The conversation they held with Dr Petousis-Harris revealed a hopelessly exaggerated view of the severity of covid-19 in the minds of our “experts”, doctors, and the governing elite.
I call on Dr Petousis-Harris and Radio NZ to check the numbers, issue a retraction and an apology.