Vaccine efficacy versus effectiveness

For months, we’ve been hearing that COVID-19 vaccines are up to 95 per cent effective, but what does that figure actually represent?

First, it’s important to understand the difference between vaccine efficacy and vaccine effectiveness, even though the terms are sometimes used interchangeably.

According to Gavi, a partnership between the World Health Organisation, UNICEF, the World Bank and the Bill and Melinda Gates Foundation which is co-leading the COVAX program to distribute COVID-19 vaccines in developing nations, “efficacy” refers to the degree to which a vaccine prevents disease in controlled circumstances such as a clinical trial, while effectiveness measures real-world results.

“Although a vaccine that has high efficacy — such as Moderna’s COVID-19 vaccine with 94.5 per cent efficacy and Pfizer’s with 90 per cent efficacy — would be expected to be highly effective in the real world, it is unlikely to translate into the same effectiveness in practice,” a Gavi statement reads.

Recent updates from Pfizer and AstraZeneca — the drug manufacturers responsible for the COVID-19 jabs available in Australia — suggest that the two vaccines have efficacy rates of 91.3 per cent and 76 per cent respectively, with real-world data compiled by Israel’s Ministry of Health showing the Pfizer vaccine to be 97 per cent effective.

But what, exactly, are the vaccines effective at doing?

Pfizer itself measures vaccine efficacy in terms of how well the jab works to prevent “symptomatic disease, severe/critical disease and death”.

As David Spiegelhalter, the chair of the Winton Centre for Risk and Evidence Communication at Cambridge University, and Anthony Masters, a statistical ambassador for the Royal Statistical Society, outlined in The Guardian, a 90 per cent efficacy rate does not mean an individual has a 10 per cent chance of contracting COVID-19.

“Imagine 100 people are ill with COVID-19,” the experts explained. “‘Ninety per cent efficacy’ means if only they’d had the vaccine, on average only 10 would have got ill.

“Vaccine efficacy is the relative reduction in the risk: whatever your risk was before, it is reduced by 90 per cent if you get vaccinated. There is a lot of confusion about this number: it does not mean there is a 10 per cent chance of getting COVID-19 if vaccinated — that chance will be massively lower than 10 per cent.”

While that is likely to placate concerns around the toll of COVID-19 on health at an individual level, it doesn’t mean that a healthy vaccinated person cannot spread the disease.

Efficacy figures refer to how good the vaccines are at preventing illness, rather than how good they are at stopping you from being infected with COVID-19 altogether.