What the polio vaccine can tell us about AstraZeneca
The recent move by many provinces to pause first doses of the AstraZeneca vaccine has felt like a punch in the gut for many Canadians. Heralded as the “GenXeneca” vaccine a few weeks ago, it has quickly become a political hot potato, with experts and governments debating if the first available COVID-19 vaccine really is the best vaccine anymore.
The move reflects a growing awareness of the seriousness of the vaccine-related blood clots caused by AstraZeneca, which occur at a rate of around one in 60,000 vaccinations. But it is also a response to the ongoing struggle to get a reliable supply of AstraZeneca into Canada, when there is ample supply of the safer Pfizer and Moderna vaccines.
Understandably, Canadians who received a first dose of AstraZeneca have a lot of fear and anger, both about the safety of the vaccine but also about the uncertainty of what comes next. What many may not realize is that Canadians were in a very similar situation 60 years ago.
The polio epidemic of the early 1950s was every parent’s worst nightmare. Polio was a childhood disease where children could go to bed with flulike symptoms and wake up paralyzed. Up to 15 per cent of kids who had polio died.
When a polio vaccine became available in 1954, parents jumped at the chance to vaccinate their kids. The Salk vaccine came first. It was an injectable vaccine that contained inactivated poliovirus, meaning it did not contain a live virus. It was remarkably effective at preventing polio and was widely used.
In 1962, a new polio vaccine emerged. The Sabin vaccine used a weakened live virus, but had the advantage of being given by mouth. Like the Salk vaccine, the Sabin vaccine was also highly effective. With the oral dosing advantage for kids, the Sabin vaccine quickly replaced the injectable Salk vaccine as the preferred option.
The oral Sabin vaccine continued to be recommended for 30 years, until the 1990s, when a new concern emerged — a rare but serious adverse reaction called vaccine-associated paralytic poliomyelitis, or “VAPP.” It turned out that the live poliovirus used in oral Sabin vaccine was causing paralysis in one in one million children after vaccination.
With little polio circulating in society, the risks of paralysis from the oral Sabin vaccine were difficult to justify. In 2000, almost 40 years after Canada started recommending the oral Sabin vaccine, the recommendation shifted back to the injectable Salk vaccine.
Like Sabin’s oral polio vaccine, the AstraZeneca vaccine is very effective at preventing serious disease and death. AstraZeneca’s vaccine is largely credited with driving COVID-19 down in the U.K., allowing the lifting of many pandemic restrictions. The oral Sabin vaccine was also easier to give to kids than the injectable Salk vaccine. Similarly, AstraZeneca is much easier to use in pharmacies and primary care offices compared to the more fragile Pfizer and Moderna vaccines.
But both the Sabin and AstraZeneca vaccines have an Achilles heel — a rare but serious side effect. As cases of infection fall and when there are safer vaccine options widely available, preferred vaccines emerge. When the recommendations changed for the polio vaccine in 2000, kids who had been partially vaccinated with Sabin’s oral polio vaccine had to complete their vaccine series with Salk’s safer injectable vaccine. A similar option will likely be given to Canadians who are in limbo after having received a single dose of AstraZeneca.
The shift from the injectable Salk vaccine to the oral Sabin vaccine and back again played out over almost four decades. Health-care recommendations change with new evidence. That is the nature of scientific progress. And the research was slower in those years. Now, in our modern era, these same changes are happening at a dizzying pace, and under an intense media spotlight. For an exhausted public trying to find the path out of the pandemic, each change can feel like a setback rather than a step forward.
The AstraZeneca vaccine is an excellent vaccine. In the weeks and months ahead it will likely continue to play an important role in Canada’s vaccination strategy — most likely for second doses, where the risks appear to be lower.
Canada will eventually move past it entirely in favour of the safer mRNA vaccines. But it can be reassuring to know that shifts in vaccination policy are not unique to COVID-19, and are not a signal that previous recommendations were wrong. Rather, they show us how much we are learning about COVID-19 and that we are still on the right path out of this pandemic.