Monday, June 17, 2024

The AstraZeneca vaccine saved millions of lives – it should not be demonised

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Last week, the pharmaceutical company AstraZeneca announced that its coronavirus vaccine was being discontinued, and would therefore no longer be manufactured or supplied. This has been accompanied by scary news headlines about its capacity to cause “rare blood clots” – but these are needlessly scary headlines. Fundamentally, the situation is this: there isn’t a new “smoking gun”; the AZ vaccine was one of the first and cheapest vaccines; it saved millions of lives globally; and there are better vaccines out there now, adapted to new variants.

The AZ vaccine was one of the first approved at the end of 2020, cheaper than Pfizer, and – importantly – easier to distribute and administer in lower resource settings as it didn’t require super-low temperatures for storage. AstraZeneca also offered a guarantee of 300 million doses of the vaccine on a non-profit basis to make it more accessible for low- and middle-income countries.

In most countries it was first rolled out in older adults, as they were the most at-risk population. As it was rolled out in younger adults, a very rare, serious, side effect was noticed: it could cause deadly blood clots in the first few weeks after the first dose.

Vaccine-induced immune thrombocytopenia and thrombosis (VITT) was a new, rare condition that affected approximately one in 50,000 AZ vaccine recipients under 50 years old, and one in 100,000 for those aged 50 and older. Crucially, this was spotted quickly and studied. Established protocols for vaccine monitoring did their job. It should also be emphasised that no long-term effects have been reported from the vaccine (monitoring of side effects is ongoing long after an initial vaccine roll-out) and the rare occurrences of VITT all happened shortly after vaccination.

Many countries then (in mid-2021) restricted the AZ vaccine to older populations, as long as their populations had access to other vaccines, such as the Pfizer mRNA vaccine. The AZ vaccine remained far safer than catching Covid, even if only considering the likelihood of getting blood clots. For instance, a Covid infection was 190 times more likely to result in a blood clot in the vein and about nine times more likely to result in low blood-platelet count.

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Because the AZ vaccine was affordable and relatively easy to roll out at scale, it was widely used in that first crucial vaccine year of 2021, particularly in lower-income countries. Its efficacy has been widely studied and it reduced severe infections by 90 per cent. Imperial College estimated that all Covid vaccines saved about 20 million lives by the end of 2021. A further study by vaccine type estimated that the AZ vaccine in particular saved more than six million lives by the end of 2021.

In terms of updating the vaccine for new variants, the way the AZ vaccine is made means this takes longer and is harder to do than mRNA vaccines such as Pfizer and Moderna. Specifically, the AZ vaccine is an example of a viral vector-based vaccine. We have several different ways that we create vaccines and this is one that was already being used for diseases such as Ebola. These vaccines use a modified virus which we call the vector to deliver genetic code for, in this case, the coronavirus spike protein to our cells. The viral vector is usually a harmless virus we are unlikely to have had prior exposure to, such as a monkey adenovirus (which is a mild cold virus).

The vector is weakened so it cannot replicate in our cells and cause a disease. The viral vector will infect our cells and instruct them to make large amounts of antigens, which then trigger an immune response. In effect, the vaccine is triggering what usually happens naturally with a viral infection. This has a huge benefit because these types of vaccines are incredibly good at stimulating our immune cells (specifically, our T and B cells). The T cells can target virally infected cells for destruction and the B cells can make antibodies, and both cell types will also generate memory cells.

Although this is a well-established technology to make vaccines, it is complex to manufacture and this is one reason it is not very amenable to be tweaked for an ever-evolving virus. Another reason is that prior exposure to the viral vector can reduce its effectiveness if used several times; there is a chance your immune response will recognise the vector and destroy it before it gets a chance to deliver its vaccine package, making the AZ vaccine less suitable for regular boosters.

In the context of Covid-19, where we have had a constant shifting set of variants and subvariants, there was always a finite lifetime for this vaccine. Since 2023, organisations such as the World Health Organisation (WHO) have no longer recommended targeting the original coronavirus spike protein for vaccine production. The current recommendations are for a further shift to target the JN.1 variants.

All this means that, three years on, there are better vaccines out there that are cheaper, (even) safer and quicker to update than the AZ vaccine. It is simply no longer needed and has been retired. There is nothing about this that is nefarious and, most importantly, it does not negate the vital role it played in 2021 in saving millions of lives across the world.

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