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 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 Organization 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."
REPRODUCTION RATE IS CRUCIAL
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.
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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.
Amesh Adalja, a scholar at the Johns Hopkins Center for Health Security, said a
good target for immunity in the United States would be for more than 70% of the
population to be inoculated, but added that the figure could go up if vaccines
are less effective.
STOP TRANSMISSION IN THE "HERD"?
Experts say another important factor is whether the COVID-19 vaccines a
government chooses to deploy can stop transmission of the virus.
Evidence so far suggests the first COVID-19 vaccines to come to market will at
least stop people developing the disease. But it cannot be ruled out that people
will still catch the SARS-CoV-2 virus and pass it on to others unnoticed.
"While protection against illness has a value for an individual, it will not
prevent circulation of virus and risk of disease in unvaccinated (people)," said
Penny Ward, a visiting professor in pharmaceutical medicine at King’s College
London.
Bodo Plachter, a professor and deputy director of the Institute of Virology at
Germany's Mainz University teaching hospital, said that respiratory infections
in particular can be hard to block completely with vaccines - although the shots
will go some way to reducing the amount of circulating virus.
"It may well be that vaccinated people will shed fewer viruses," he said. "But
it would be a mistake to assume vaccination alone can suppress a pandemic."
FOCUS ON PROTECTING THE VULNERABLE?
Edinburgh's Riley said this suggests that for now, pursuing an idea of herd
immunity through COVID-19 vaccination is fruitless.
A better approach, she said, could be to "turn herd immunity on its head", and
use the first limited supplies of vaccines to protect those most in need,
without worrying about the more robust members of the "herd" who can live
relatively happily with the virus.
"Let's forget about protecting the masses to protect the vulnerable," she said.
"Let's directly protect the vulnerable."
(Reporting by Kate Kelland in London and Ludwig Burger in Frankfurt, Additional
reporting by Carl O’Donnell in New York, Editing by Timothy Heritage)
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