Explainer | What will Covid-19 vaccines achieve? And how much of the population needs to be vaccinated?

Mass Covid-19 vaccinations aim to achieve herd immunity by stopping the virus spreading. But what portion of a population need immunising to reach it?

The answer to that question depends on how well vaccination stops transmission of the virus, and whether new, more transmissible strains of the virus emerge

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The first Pfizer/BioNTech Covid-19 vaccines for Belgium are defrosted at a hospital. As vaccination against the virus is rolled out around the world, governments will be calculating how much of their people need to receive the vaccine to achieve herd immunity and allow a return to normality. Photo: Fred Sierakowski/Pool via Reuters

Published: 12:33pm, 28 Dec 2020Updated: 1:06pm, 28 Dec 2020

The aim of the vaccination campaign against Covid-19 is herd immunity – the point at which so few people are susceptible to infection that the virus runs out of places to go.

In the early days of the pandemic, epidemiologists estimated that would require inoculating about two-thirds of the US population. Now many of those same experts say that figure is almost certainly too low.

“If you really want true herd immunity, where you get a blanket of protection over the country … you want about 75 to 85 per cent of the country to get vaccinated,” Dr Anthony Fauci, the nation’s top infectious-disease official, told a reporter last week. “I would say even closer to 85 per cent.”

The shift reflects a deeper understanding of how the virus spreads – that it jumps from one person to another more easily than once thought.

The question of how many people must be vaccinated is of crucial importance to governments everywhere as the world embarks on the biggest inoculation campaign in decades.

The goal of vaccination isn’t just to protect the individual who receives it but also to drape a fire blanket over a large enough portion of the population that the fire begins running out of fuel.