Why we’re talking about . . .

The meaning of Covid herd immunity

UCL modelling suggests population immunity will come as early as next week for the UK

Modelling by scientists at University College London (UCL) has predicted that the UK will pass the threshold for herd immunity on Monday, just as shops and restaurants are due to reopen.

The results will place “more pressure on the Government to move faster in releasing restrictions”, says the The Daily Telegraph. However, Health Secretary Matt Hancock this morning suggested that ministers will be sticking to the current roadmap out of lockdown, reports The Times.

What does herd immunity mean?

Also known as “population immunity”, it is the indirect protection from a disease acquired when a population is mostly immune to it through vaccination or from previous infection, explains the World Health Organization (WHO). The aim is to break the chains of transmission and also protect those who are unable to get vaccinated, because of an allergic reaction or age, for example.

How is it achieved?

Immunity can be achieved through natural infection as well as through vaccination. However, WHO’s chief scientist, Dr Soumya Swaminathan, warns that if you allow it to happen naturally “it will take a long time” and “more importantly, it’s going to do a lot of collateral damage”.

How many people need to be immune?

The percentage of a population needed for herd immunity varies depending on the disease; for instance, measles requires about 95% of a population to be vaccinated, says WHO. The proportion for Covid-19 has not been pinpointed to an exact percentage, but most estimates have placed the threshold at 60% to 70%, reports Nature.

So how far has the UK come?

Government data up to Tuesday 6 April, shows that nearly 32 million people have been given their first vaccination, amounting to 60.2% of the adult population. Oxford University tracking suggests this is about 46.71% of the whole population.

The modelling by UCL, which also takes into account people who have natural protection, predicts that the population’s immunity will hit 73.4% on 12 April, “enough to tip the country into herd immunity”, says The Telegraph.

Professor Karl Friston, of UCL, told the newspaper: “The herd immunity estimates surprised me. However, they are unremarkable when one considers that over 50 per cent of adults have been vaccinated, around 42 per cent of people have now been exposed to the virus and about 10 per cent have pre-existing immunity.”

This differs from the latest infection survey by the Office for National Statistics, which bases its estimates on blood test results taken from a randomly selected subsample of individuals aged 16 years and over. The data published on 30 March estimated that around 54.7% of the population in England would have tested positive for antibodies against the coronavirus, although nearly four weeks have passed since it collected the samples. The proportion was slightly less in the devolved nations.

Does that mean the pandemic is nearly over?

Whatever date the UK hits the threshold for herd immunity, Nature warns that there are other factors to consider.

One big issue is the globally uneven distribution of vaccines. “No community is an island, and the landscape of immunity that surrounds a community really matters,” says Shweta Bansal, a mathematical biologist at Georgetown University in Washington DC. 

She tells the journal that geographic clustering of infection “is going to make the path to herd immunity a lot less of a straight line, and essentially means we’ll be playing a game of whack-a-mole with COVID outbreaks”.

The longer it takes to prevent transmission, the more time new variants of the virus have to emerge, explains Nature. On top of this, natural immunity to Covid-19 appears to wane over time “so that needs to be factored in to calculations”.

An analysis by McKinsey, updated last month, concludes: “Even when herd immunity is achieved, ongoing monitoring, potential revaccination, and treatment of isolated cases will still be needed to control the risk of Covid-19. But these would fall into the category of ‘normal’ infectious disease management – not the society-altering interventions we have all lived through this year.”

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