Here’s how to make sense of all the vaccine data

Effectiveness is real-world performance. We only get this data once the vaccine starts being administered, as it is now around the world. “Now you’re going to have people who would not have met the eligibility of the trials: older people, pregnant women, people with underlying health conditions, who are getting the vaccine. The chances are the effectiveness would be different from a very controlled trial,” says Professor Hoffmann. “Probably less, by how much, we just don’t know.“

So what does 95 per cent efficacy mean? That 95 out of every 100 people will be protected?

Not quite, says Professor Hoffmann.

“That’s what it’s often interpreted as. It actually means there was a 95 per cent reduction in new cases of COVID-19 in the vaccine group, compared to the placebo group.”

That’s a complex idea, so let’s break it down. In Pfizer’s phase 3 trial, 18,198 people got the vaccine and 18,325 got a placebo. There were eight cases of COVID-19 in the vaccinated group and 162 in the placebo group. Scientists can then calculate the infection risk for people given a placebo: 0.88 per cent. For people given the vaccine: 0.04 per cent. That allows scientists to calculate the difference between the groups in the risk of being infected with COVID-19: about 0.8 per cent. In this trial, the vaccine had a relative risk reduction of catching COVID-19 by 95 per cent.

Following along so far? Now let’s get even more complex: do these vaccines prevent serious illness and death?|

“In terms of preventing death, it works, 100 per cent of the time. In terms of preventing severe illness, it works, 100 per cent of the time,” Australia’s Chief Medical Officer Professor Paul Kelly has said – referring to AstraZeneca’s vaccine. This is true – to an extent. In all phase 3 trials so far, there have been zero cases of serious illness or death among participants given the vaccine.

That’s great news – but it does not prove the vaccines prevent serious illness or death.

That’s because the primary goal of the vaccine trials was to show they prevented illness of any severity. Therefore, there were not high enough numbers to prove they stop serious illness or death.

So why are they being given out? Because scientists can very strongly infer they will prevent serious illness or death. The vaccines prevent infections of COVID-19 with symptoms. On a cellular level, they create a strong immune response. We have not seen a single case of serious illness or death among people vaccinated in these trials.

“If you put data from the trials together, a picture is clearly emerging,” says Professor Greg Dore, a leading virologist based at the Kirby Institute.

“There is still some uncertainty, in terms of the overall effect. But, these vaccines do appear particularly effective at the prevention of hospitalisation and severe illness. All the trials are pointing in the same direction. The picture that is emerging is very very encouraging.”

Receive Examine, a new science newsletter, in your inbox every week. Sign up for The Age here, The Sydney Morning Herald here; Brisbane Times here; and WAtoday here.