Can first Covid-19 vaccines bring ‘herd immunity’?
November 19 2020 01:35 AM

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% for its models, while German Chancellor Angela Merkel said this month that Covid-19 restrictions in Germany could be lifted if 60% to 70% of the population acquired immunity, either via a Covid-19 vaccine or through infection.
World Health Organisation experts have also pointed to a 65%-70% 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% of a population have all been vaccinated, there will be so little virus in the community that the 2% 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.
For instance, herd immunity from highly contagious measles, with an estimated R value of 12 or higher, will kick in only if 92% or more within a group are immune.
For a seasonal flu strain that could have an R value of 1.3, the threshold would be just 23%.
“The problem is that for now we don’t know exactly how fast the virus spreads without any precautions and with the normal travel and social activities we had a year ago,” said Winfried Pickl, professor of immunology at the Medical University of Vienna.
With so many countries still operating in far from normal circumstances, the assumption should be that the Covid-19 R value would be “closer to 4 than to 2”, he said, since even with semi- or full lockdown measures the R value is around 1.5.
Additionally, anything less than 100% vaccine efficacy – such as the 90% or so suggested in early data on the Pfizer-BioNTech and Moderna Covid-19 shots – would require a matching rise in percentage of coverage to reach herd threshold.

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