The extremely small risk of side effect is better than not being vaccinated

The Institute's Professor Catherine Bennett, Chair in Epidemiology at Deakin University, and La Trobe University's Associate Professor in Epidemiology Hassan Vally shed some light on the COVID-19 vaccine rollout.

Written by Professor Catherine Bennett and Associate Professor Hassan Vally (La Trobe University)

The global rollout of COVID-19 vaccines is under the microscope to a degree that is unparalleled in any vaccine or medicinal rollout. All eyes are on the world’s collective experiences in rolling out these vaccines, as they should be, as we amass safety and effectiveness data on the millions of people being vaccinated. We all also individually have a vested interest in this as we contemplate our own vaccine choices, and the impact that vaccines will have on our health, and the liberties afforded by good vaccine coverage. The challenge in this is that it can be hard to make sense of the numbers and to apply them to our own circumstances.  

The reason it is so important to continue to monitor vaccine tolerance and side effects as we roll out the vaccine in the real word is to identify any risks that weren’t able to be picked up in clinical trials, even though they are based on tens of thousands of volunteers. Adverse events that are extremely rare may not be seen until the full roll out when millions receive the vaccine, including certain subgroups that may have been excluded from the trials.

It is important to highlight that all of the data we have to date support that the AstraZeneca vaccine is a very safe and effective vaccine. What we learned over the last couple of weeks, however, is that a very small proportion of people who received their first jab experienced a serious adverse reaction due to an unusual type of blood clotting syndrome. The risk of this happening is estimated to be somewhere between 4 and 6 per million doses of vaccine delivered. This risk is so very low that it is hard to be precise on how common it is. Although we are not able to identify a causal mechanism for this syndrome, or identify particular subgroups who might be at risk of this serious reaction, the pattern across different countries suggests this is most likely caused by the vaccine.

What changed this week is that more recent data has allowed us to see more clearly that these cases are clustering in younger adults, suggesting that the risk for older adults is much less than the four per million seen at the whole population level, and only one quarter of the risk in adults under 20.. And whilst the risk is still very low even in younger adults, we have the chance to reduce even this small risk further by switching the preferred vaccine for these age groups. It does not change the already very low risk to others, and reduces the chances of us seeing even the small number of cases in our younger adults that have been experienced overseas. So people over 50 can be reassured that the case reviews have not shown this same small risk in their age group and should be even more confident knowing millions of people have been monitored for side effects.   

The decisions made about vaccine strategy this week weigh up the potential risks and benefits. These differ across age groups. We know older adults are more at risk of adverse outcomes from COVID-19 but, conversely, we now know that younger adults are at a slightly increased, although still very small, risk of serious side effects to the AstraZeneca vaccine. If we had no other vaccine choices, then this very rare side effect would likely never have led to any of our adult population being excluded from receiving the vaccine, but because we secured an additional 10million doses of Pfizer in February, and another 20 million overnight, we have the capacity to reduce this risk even further in the age groups where these cases have tended to cluster overseas.

A key advantage in not having to make emergency approvals of vaccines was the opportunity we then had to observe the real word roll out of the very vaccines we were to later use. Even then, it took millions of doses to identify the possible association, and then many more to identify which groups in the population might be vulnerable. Australia has vaccinated more than 500,000 people with AstraZeneca now, and thankfully we had this heads up before our first case in a 44 year old male in Melbourne detected a couple of weeks ago. This meant we already had the guidance in place for doctors so that diagnosis and treatment could be as optimal as possible for that patient.

All of this is actually a sign our safety monitoring and international data sharing is working. But as soon as you mention the words risk and serious adverse events in the same sentence, you run a real risk that this might damage public confidence in the vaccines. It’s important to understand that the Australian government has taken an extremely cautious approach and has taken steps this week to make an extremely small risk of having a bad reaction to the AZ vaccine even smaller. The challenge now is to manage how these changes will impact the vaccine rollout and particularly the time it takes to have vaccines available to all age groups. Any delays that may be caused by these changes are worth it if it means we can prevent rare serious adverse reactions, and can look forward to the additional benefits that will come of having a vaccinated Australia.

An earlier version of this article was originally published in The Age as Extremely small risk of side effect, better than not being vaccinated