Deciphering Vaccine Efficacy: A Closer Look at the 92% Effective Claim

When you hear that a COVID-19 vaccine is 92% effective, it's important to understand what this really means. This figure does not necessarily reflect the real-world effectiveness of the vaccine, especially when you consider anecdotes like the almost perfect 100% vaccination rate on Gibraltar and subsequent widespread infection. Here, we delve into the nuances of vaccine efficacy, absolute and relative risk reduction, and why the 92% figure can be misleading.

The Meager Impression of 92% Efficacy

First and foremost, let's dissect the claim that a 92% effective vaccine means it offers substantial protection in real-world scenarios. The case of Gibraltar provides a stark example. Despite near-perfect vaccination rates, almost everyone still contracted the virus, often multiple times. This discrepancy highlights the gap between theoretical figures and practical outcomes.

The 92% efficacy reported is often a relative risk reduction (RRR) figure, comparing the risk of infection in a vaccinated group to an unvaccinated group during clinical trials. This measure can be significantly different from the absolute risk reduction (ARR), which provides a direct comparison of infection rates in the two groups.

Absolute vs. Relative Risk Reduction

Understand the difference between absolute risk reduction (ARR) and relative risk reduction (RRR). ARR gives the actual reduction in risk in a population, whereas RRR is a ratio indicating how much the risk is reduced in the treatment group compared to the control group. The RRR of 92% in a clinical trial means there is a 92% reduction in the relative risk, which is not always what we need to know in a public health context.

The Disappointing Reality Behind 92% Efficacy

Let's break down the numbers. The Appendix 1 of the Pfizer-BioNTech vaccine efficacy study (P3 trial) indicates that out of 43,000 participants, only 170 reported any symptoms of infection. This means the vaccine had a LESS THAN 1% population-level impact on symptomatic infection risk reduction. This is far from the level of protection you might expect from a 92% efficacious vaccine.

The reality is that vaccine efficacy (VE) primarily reflects the calculated probability of infection in the treatment group as compared to the placebo group. It is often misinterpreted as the percentage of individuals who will not become infected when exposed to the virus. This is not accurate, and can lead to the misunderstanding that the vaccine offers 95% protection or other similar claims that are not supported by the data.

Misinterpretation and Misinformation

There is often a miscommunication of the efficacy figures, with RRR figures being the only ones conveyed, skirting around the ARR which would provide a clearer picture of the vaccine's true effectiveness. This can lead to the spread of misinformation and create public perception that a vaccine is far more effective than it actually is.

Take the example of water being 95% effective against infection, as shown in a study by Prof. Fenton. Such tactics can bias results and mislead the public on the true efficacy of interventions.

In conclusion, while a 92% efficacy rate sounds impressive, it is crucial to scrutinize the underlying figures and understand the context in which this figure was derived. Real-world effectiveness can differ significantly, and the true impact on public health may be much lower than the reported efficacy rates suggest.