The protection from the AstraZeneca vaccine outweighs the risk of rare but serious VITT
Risk vs. benefit analyses for Canadians from NACI’s April 23 statement support the decision to offer the AstraZeneca vaccine to those 30+, particularly in areas of high to moderate levels of COVID-19 spread, since the risks of waiting for an mRNA vaccine are greater than the risks of VITT.
I turned this Twitter thread into a blog post to make it more accessible to those not on Twitter. What you’re reading below is a more or less verbatim collection of those tweets.
On Friday April 23, NACI came out with the following updated recommendations for the AstraZeneca vaccine:
At this time and based on current evidence, NACI recommends the AstraZeneca COVID-19 vaccine may be offered to individuals 30 years of age and older without contraindications if the individual does not wish to wait for an mRNA vaccine and if the following conditions are met:
A benefit-risk analysis determines that the benefit of earlier vaccination with the AstraZeneca COVID-19 vaccine outweighs the risk of the individual getting COVID-19 while waiting for an mRNA COVID-19 vaccine;
The individual provides informed consent once the benefits and risks of VITT compared to COVID-19 are clearly outlined, including how long the individual will have to wait for an mRNA vaccine and what public health measures the individual is able to take to minimize their exposure to the COVID-19 virus; and
The individual will have to wait a long time in order to receive an mRNA vaccine.
Health Canada has maintained its authorization for the AstraZeneca vaccine for those 18+, though it did recently update the product information to make clear the potential rare risks of VITT.
Let’s dive into the updated NACI recommendations for the AstraZeneca vaccine….
The information provided below is based on Appendix E in their recommendations on the use of COVID-19 vaccines document. It analyzes the benefits of the AstraZeneca vaccine (in terms of preventing ICU admissions & deaths from COVID) vs the risks of waiting for a later mRNA vaccine.
Read the full document here: canada.ca/content/dam/ph…
QUICK SUMMARY: if you’re in AB or ON, or anywhere with high COVD cases (>30 new daily COVID cases per 100k people), the protections the AstraZeneca vaccine offer outweigh the rare but real risks — at any age.
This also seems true for those 30+ in areas with medium COVID levels (7.5 new daily COVID cases per 100k people).
Though do note that some of this discussion is moot because there are currently limitations to supply & as such provinces may not change eligibility criteria.
Before we dive in, please note that I’m trained as a scientist, but in neuroscience/molecular genetics — fields completely unrelated to this topic.
The only expertise I bring to this, besides a love of data, is based on my work as a science communicator, particularly one specialized in vaccine confidence
There are lots of hot takes on this right now… anyone not clearly stating the caveats to their expertise in relation to this particular multi-dimensional subject is someone whose advice you may wanna take with a grain of salt.
How NACI did these risk vs benefit analyses
Now let’s dig into NACI’s analyses: they assumed risk of VITT (Vaccine-induced Immune Thrombotic Thrombocytopenia) is equal across age & gender given lack of conclusive data to suggest otherwise. Because it’s rare, not enough cases worldwide to be sure trends aren’t confounded by rollout.
The reason they stratify risks by age in their analyses is because it is clear that the likelihood of severe outcomes from COVID-19 increase with age (see Table 19).
Though that’s just looking at hospitalization, ICU admission & death from COVID… not long COVID aka PASC.
What about underlying conditions? Gonna reference the brilliant Dr. @MPaiMD here
Here she states that *based on what we know so far* there are not many underlying factors that seem to make someone at higher risk of developing VITT after AstraZeneca:
They do a few different analyses, some assuming risk of VITT is 1 in 100,000 doses of AstraZeneca given, others assuming 1 in 250,000 doses.
I’ll show the ones assuming 1 in 100,000 to play it safe, since that’s closer to what our fave Dr. @MPaiMD says:
They also assumed one dose of the AstraZeneca vaccine has an effectiveness of 80% for preventing severe COVID outcomes.
This is fair based on findings from Scottish study that it can reduce risk of hospitalization by as much as 94%.
Here are some of their other assumptions:
I don’t love how NACI’s statement gives the impression the AstraZeneca vaccine is inferior to the others (it is equally effective at preventing hospitalizations and deaths from COVID, even with 1 dose).
But I do like how their analyses factor in the risk of each week of waiting for an mRNA vaccine:
Risk vs. benefit conclusions for areas with “high” COVID levels
If you’re in Ontario or Alberta, the conclusion from all NACI risk/benefit analyses are the same: take the 1st vaccine you can.
Both provinces are currently in “high” category of COVID levels with >30 new daily cases per 100k.
In Ontario, that threshold is equivalent to approximately 4371 new daily cases.
In Alberta, that’s about 1311 new daily cases of COVID.
The following table from the NACI recommendations PDF convinced me that the benefits of the AstraZeneca vaccine outweigh the risks for any age in Ontario & Alberta (and any place experience “high” COVID levels.
Note that the grey boxes indicate that it’s safer to get the AstraZeneca vaccine than it is to wait for an mRNA vaccine, based on current risks of COVID levels and supply for mRNA vaccines.
Note that in this analysis they used rather conservative variables, meaning you can be fairly confident in the conclusion:
- a VITT risk of 1 in 100k doses
- low ICU admission rates from COVID (0.22 per 100,000 cases)
- and they assumed that every case of VITT has to be admitted to the ICU, which is not necessarily true (note that 4 out of the 5 reported cases of VITT in Canada thus far are recovering at home, presumably after being treated by emergency services in the hospital)
Nonetheless, even these conservative estimates (under-estimating the risk from COVID and over-stating the risk from VITT) suggest that:
For 30 to 39 year olds, the AstraZeneca vaccine could prevent 2.42 ICU admissions per 100k cases, compared to 1 per 100k vaccinated with the AstraZeneca vaccine who could get VITT.
For #GenXZeneca (age 40–49), the most conservative estimates from NACI suggest 1 AstraZeneca dose can prevent the 3.09 ICU admissions per 100k in high COVID areas, compare to 1 in 100k risk of VITT per dose.
If you recently got the AstraZeneca vaccine, you can feel good about that decision!
Risk vs. benefit conclusions for places with “moderate” COVID levels
If you’re somewhere with moderate levels of COVID spread (7.5 new daily cases per 100k people): the risk vs. benefit teeters depending on variables used.
But it often still tilts in favour of those 30+ getting the AstraZeneca vaccine vs. waiting for an mRNA vaccine.
For example, keeping VITT risk at 1 in 100k, and using slightly higher ICU admission rate from COVID (what they say is closer to recent overall rate):
1 dose of the AstraZeneca vaccine offers more protection against ICU admission from COVID for those 30+ than VITT risk, even at moderate COVID levels. Again, see grey boxes in this version of the analysis:
When you factor in that VITT is treatable (4/5 cases in Canada reportedly recovering at home), and that outcomes are probably better now that we’ve learned more about it… I think offering the AstraZeneca vaccine to those 30+ makes sense, even in areas with moderate COVID risk.
What did other countries decide about AstraZeneca?
Internationally, regulatory agencies in UK and Europe have reached similar conclusions as NACI. See this summary of the EMA’s conclusions this week on the AstraZeneca (aka Vaxzevria) vaccine from the wonderful @kakape
While NACI hasn’t made statements on the Janssen J&J vaccine yet, here’s the decision on that from the US. Note that both AstraZeneca and Janssen J&J use adenovirus vector, but unclear to what extent risks & mechanisms of clots following each vaccine are the same.
Learn more
Please again note that these are my interpretations of the NACI statement, as it currently stands April 24, 2021.
I am a scientist so I am comfortable reading and interpreting data, but this is not my area of trained expertise so I write this as a communicator, not an expert.
If you’d like to hear from a subject matter expert on clots, please check out the twitter thread below from the brilliant Dr. Menaka Pai. She’s the best (& lead author of @COVIDSciOntario’s summaries on this topic, which are also great)
And here’s a summary of what infectious disease expert Dr. Allison McGeer had to say about the benefits vs. risks of the AstraZeneca vaccine
(TL;DR — if you’re in Ontario or AB, take whatever vaccine you can ASAP).
Ultimately this discussion, even the media attention, are important because, as NACI says:
Healthcare professionals should be aware of VITT, including how to diagnose and treat the condition. There is additional guidance available from the Ontario COVID-19 Science Advisory Table and Thrombosis Canada.
Individuals who receive the AstraZeneca COVID-19 vaccine should monitor their health and immediately seek medical attention if they develop symptoms of VITT, including shortness of breath, chest or abdominal pain, leg swelling, severe headaches or blurred vision, and skin bruising or a skin rash. Quick diagnosis and treatment is critical to reduce the risk of negative outcomes.
For what it’s worth, it is not my job to convince anyone to take any vaccine ever.
But I do see it as my job as a science communicator to make navigating the evidence to help with this decision a little easier.
And regardless where you get your info from, always good to discuss your personal health concerns with your doctor since they know the full scope of your medical history 💗️
I’ll be recording video summaries of this information for Instagram, so if you prefer to learn by listening, keep an eye out on my account there!
