Vaccines represent remarkable, life-preserving advancements, and the immunizations developed for COVID-19 are no exception. While any medical intervention carries potential adverse effects, extensive scientific investigations confirm that the COVID-19 vaccines are both secure and efficacious.
Despite this, as frequently reported in the media, a significant portion of Europe has temporarily suspended the deployment of one particular vaccine formulation. Instead of administering millions of doses of the AstraZeneca/Oxford (AZ) vaccine against COVID-19, a dozen nations have opted to halt vaccinations due to apprehension regarding potential side effects.
Consequently, I aim to elucidate the circumstances surrounding these events and explain why you likely need not be concerned, even if you have already received the AZ vaccine.
The Mechanism of Vaccination Programs
The fundamental principle governing vaccine rollout initiatives is straightforward. Common and severe side effects are identified and typically excluded during clinical trials, while prevalent and moderate reactions, such as headaches or localized pain at the injection site, are documented.
However, even with comprehensive studies involving tens of thousands of participants, identifying events that occur with extreme rarity is practically impossible. If an adverse event manifests in only one out of every 200,000 individuals receiving the vaccine, a clinical trial would necessitate millions of participants to achieve statistical certainty in detecting such occurrences.
This presents a challenge, as vaccinations are administered on a broad population scale. An occurrence affecting one in 200,000 individuals, while exceptionally infrequent, will inevitably manifest dozens of times when inoculating 80 million people. Crucially, for many European nations, vaccinating a substantial portion of their populace with utmost expediency is paramount, given the widespread COVID-19 outbreaks across the continent.
To address this, an ingenious strategy is employed: the establishment of surveillance systems that monitor individuals within the general population who have received the vaccine. This facilitates the detection of any emerging risk signals within the vast cohort of individuals vaccinated after the drug has been approved for public use.
In the case of the AZ vaccine, some countries have observed a very small yet potentially significant elevation in the incidence of a specific type of blood clot within their monitoring data. This has prompted a temporary cessation of vaccine distribution to allow for further investigation.
The prudence of this decision is contingent upon several factors.
The Rationale Behind Pausing Rollouts
The primary inquiry naturally revolves around the precise risk indicator that has engendered concern in Germany, Norway, Spain, and other nations.
According to statements from the regulatory bodies and involved national health authorities, the concern pertains to a rare but serious form of thrombosis in the brain, known as cerebral venous sinus thrombosis. These instances have predominantly occurred in younger individuals, typically between the ages of 20 and 55, which contributes to the basis for the apprehension.
The German federal agency for medicinal products and health products has disclosed its findings, indicating that a total of seven cases of these specific blood clots were identified among 1.6 million vaccinated individuals aged 20-55, when the expected number by chance alone would have been one.
Now, addressing the critical question: what are the implications of this data?
It is beneficial to quantify the observed rate. Assuming all seven identified clots are attributable to the vaccine, with 1.6 million inoculations administered, this translates to approximately one clot per 230,000 doses, or a rate of 0.00044 percent.
While the relative increase in risk may appear alarming, it corresponds to an absolute risk increase of 0.00038 percent for vaccinated individuals, a figure less substantial than suggested by prevailing headlines.
We can also draw a comparison with the risks associated with COVID-19 itself. Even individuals in their early twenties are not entirely impervious to the disease, and although their mortality risk is considerably lower than that of the elderly, it approximates one fatality per 16,000 infections.
Therefore, if the vaccine is indeed implicated in causing these potentially fatal blood clots, the risk of succumbing to COVID-19 for a 20-year-old is roughly 15 times greater than the risk of experiencing a clot. Again, given that both risks are exceedingly low, the absolute difference remains minuscule, approximately 0.004 percent.
It is important to emphasize the conditional “if.” This remains a subject of active investigation. Adverse event reporting systems are designed to flag potential risks, not to definitively establish causality. The ongoing inquiry is precisely intended to ascertain such causal links.
Furthermore, it is indeed improbable that the vaccine is the direct cause of these blood clots. The reason? The data from Germany, while substantial at 1.6 million individuals, does not represent the entirety of AZ vaccine administrations. The United Kingdom, having administered over 10 million doses of the AZ vaccine, reports through its surveillance system that no comparable increase in risk has been observed.
A thorough examination of the reports from the MHRA, the UK’s medical regulatory authority, reveals three instances of this specific type of clot following vaccination in the UK. This number is actually lower than what would be statistically anticipated by chance. When the data from both the UK and Germany are aggregated, the overall detected risk of blood clots diminishes entirely.
The Concluding Assessment
What are the implications of these findings for individuals eligible for vaccination?
Firstly, the individual-level risks are exceedingly low. Even if a causal link is eventually substantiated—a possibility that cannot be dismissed—the likelihood of experiencing a blood clot induced by the AZ vaccine, based on current evidence, is lower than the risks of drowning in a bathtub or being struck by lightning.
While the risk of blood clots may hold significance at a population level, this is distinct from the ordinary risks faced by individuals.
Conversely, the potential for mortality from COVID-19 is not negligible, even for a healthy 20-year-old. For individuals aged 50 and above, the risk becomes considerably more concerning. This has been consistently demonstrated, including through my own research, highlighting the severe nature of the COVID-19 illness.
The decision by European nations to suspend the AZ vaccine rollout is, understandably, their prerogative. However, it presents a degree of ambiguity. Although a potential signal of risk may exist, this signal is quite faint and certainly does not exceed the risks associated with contracting the disease itself.
If presented with the opportunity to receive the AZ vaccine, what would be my personal course of action? While I cannot speak for others, and the assessment of risk versus benefit is a personal endeavor, as a healthcare professional in Australia, I am scheduled to receive the AZ vaccine within the next two months.
I eagerly anticipate this appointment.
Gideon Meyerowitz-Katz is an epidemiologist specializing in chronic disease research in Sydney, Australia. He maintains a regular health blog that delves into science communication, public health discourse, and the interpretation of new scientific studies.
