Ludwig Burger and Kate Kelland –
Governments and officials are voicing hopes that COVID-19 vaccines could bring “herd immunity”, with some calculating that immunising just two-thirds of a population could halt the pandemic disease and help protect whole communities or nations.
But the concept comes with caveats and big demands of what vaccines might be capable of preventing. Some experts say such expectations are misplaced.
For a start, figuring out what’s needed to achieve herd immunity with COVID-19 vaccines involves a range of factors, several of which are unknown.
What is the rate of the spread of the COVID-19-causing virus? Will the first vaccines deployed be able to stop transmission of the virus, or just stop people getting ill? How many people in a population will accept a vaccine? Will vaccines offer the same protection to everyone? “Herd immunity is sometimes wrongly understood as individual protection,” said Josep Jansa, an expert in health emergency preparedness and response at the Stockholm-based European Centre for Disease Prevention and Control (ECDC).
“It’s inappropriate to think ‘I will not be affected myself because there is herd immunity’. Herd immunity refers to community protection, not to how an individual is protected.”
The ECDC uses an estimated herd immunity threshold of 67 per cent for its models, while Chancellor Angela Merkel said this month that COVID-19 restrictions in Germany could be lifted if 60 per cent to 70 per cent of the population acquired immunity, either via a COVID-19 vaccine or through infection.
World Health Organization experts have also pointed to a 65-70 per cent vaccine coverage rate as a way to reach population immunity through vaccination.
“The idea of herd community is to protect the vulnerable,” said Eleanor Riley, a professor of immunology and infectious disease at the University of Edinburgh.
“And the idea behind it is that if, say, 98 per cent of a population have all been vaccinated, there will be so little virus in the community that the
2 per cent will be protected. That’s the point of it.”
Central to the public health calculations on this concept for COVID-19 is the reproduction rate, or R value, of the virus that causes it. This is a measure of how many other people an average infected person passes a pathogen on to in “normal”, or restriction-free, circumstances.
Assuming complete vaccine efficacy, herd immunity percentage thresholds for infectious diseases are calculated by dividing 1 by the R value, deducting the result from 1, and multiplying
by 100. — Reuters