On Wednesday evening, Professor Philip Nolan, the NPHET scientist responsible for NPHET’s statistical model which predicted 700,000 cases of Covid-19’s Delta Variant in the event that the Government proceeded to re-open the economy as planned, composed a twitter thread outlining the figures.

Amidst that thread, he included one of the most incredible statements made by an Irish civil servant in years, claiming that even if 700,000 older people were vaccinated, the fact that the vaccine is 95% effective meant that 25,000 people remain vulnerable:

Why would this happen if everyone is vaccinated? Vaccines offer extraordinary protection, but not perfect. We have almost 500,000 people aged 70 and over; even if the vaccine is 95% effective in preventing severe disease, 25,000 people remain vulnerable. 26/36

— Professor Philip Nolan (@President_MU) June 30, 2021

The problem is that this is not what vaccine efficacy means, though naturally, to an average person, it very much sounds like that is what it means.

So what does it mean?

Well, when vaccines are being developed, they go through clinical trials, to determine how safe they are, and how effective they are. In terms of effectiveness, how do you measure that?

Well, you have two groups. A placebo group, which does not get the vaccine, and a second group, which does. For simplicity’s sake, let us imagine that both groups are 100,000 people.

Vaccine efficacy, very simply, is this: By what percentage are cases of the virus lower in the vaccinated group, compared to the unvaccinated group. Let us use round numbers to demonstrate:

In the first group, the unvaccinated group, let us imagine that over the course of the trial, 5,000 people, or 5% of the group, contract covid naturally. That is 5,000 cases amongst the unvaccinated.

But in the second group, the vaccinated group, only 250 people test positive for covid.

250 is 5% of 5000. So you have reduced cases by 95% in the vaccinated group.

That is what 95% vaccine efficacy means. It means the percentage of cases prevented, not the percentage of people protected.

The figures do not only refer to covid cases. We can also say, for example, that a vaccine is 95% effective at preventing severe cases, and hospitalisations. This works in the same way: You compare an unvaccinated group to a vaccinated group, and measure the reduction in severe cases, deaths, and hospitalisations between the two.

So, to the numbers Nolan cites: He says that out of 700,000 people vaccinated, 25,000 are at risk of severe illness, despite the fact that the vaccine is 95% effective against severe illness. That means that 25,000 people are at risk of suffering severe illness after being vaccinated. If the vaccine is 95% effective, that means that 25,000 is 5% of the figure that would be at risk if nobody was vaccinated. So, without the vaccine, Nolan is saying, half a million people would be at risk – or 71% of all those infected. That is nonsense, and does not align with any covid figure, anywhere in the world, at any time.

The bottom line here is this: Nolan either does not know what vaccine efficacy actually means, or he does know, and wrote a tweet which misconstrued what it means, in order to make the risks of the Delta variant seem greater than they are. It is hard to know which would be more concerning.