What do vaccines do? What is the difference between vaccine efficacy and effectiveness? And, what about Omicron? An epidemiologist answers six questions.
A health care worker prepares a dose of Moderna COVID-19 vaccine during a vaccination drive for local people in Banda Aceh, Indonesia, 30 November 2021.The pandemic has brought many tricky terms and ideas from epidemiology into everyone’s lives. Two particularly complicated concepts are vaccine efficacy and effectiveness. These are not the same thing. And as time goes on and new variants like Omicron emerge, they are changing, too.
Effectiveness, on the other hand, describes how well a vaccine performs in the real world. It is calculated the same way, by comparing illness among vaccinated and unvaccinated people. Vaccines with moderate or even low efficacy can work very well at a population level. Likewise, vaccines with high efficacy in clinical trials, like coronavirus vaccines, may have lower effectiveness and a small impact if there isn’t high vaccine uptake in the population.
Imagine conducting a vaccine trial. You randomise 1,000 people to receive the vaccine in one group. You randomise another 1,000 to be given a placebo in the other group. Say 2.5 per cent of people in the vaccinated group get COVID-19 compared with 50 per cent in the unvaccinated group. That means the vaccine has 95 per cent efficacy. We determine that because /50%=.95. So 95 per cent indicates the reduction in the proportion of disease among the vaccinated group.
The preliminary data about Omicron and vaccines is coming in quickly and is revealing lower vaccine effectiveness. Best estimates suggest vaccines are around 30-40 per cent effective at preventing infections and 70 per cent effective at preventing severe disease. A woman receives Pfizer vaccine jab from a healthcare worker in Katlehong, east of Johannesburg, South Africa Friday, 1 October 2021.Initial data reinforces that a third dose would help boost immune response and protection against omicron, with estimates of 70-75 per cent effectiveness.
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