ANALYSIS REALITYCHECK
UK’s action plan to tackle coronavirus, but is “a
natural by-product of an epidemic”.
So where did the 60 per cent figure come from?
And will we eventually experience the positive
effects of herd immunity?
The ease with which a disease spreads is measured
using the ‘reproduction number’, R0, which is the
average number of people expected to catch the
disease from a single infected person. For COVID-19,
R0 is estimated to be around 2.5. Flu has an R0 of 1.3,
while measles has an R0 as high as 18. The proportion
of the population needed to become immune to
achieve herd immunity (the ‘herd immunity
threshold’) is calculated by 1 - (1/R0). For coronavirus,
this gives the threshold of 60 per cent that was quoted
by Va lla nce.
Crucially, a disease’s infectiousness isn’t set in
stone. Measures can be taken to make it harder for a
disease to spread, reducing the threshold. A few days
after Vallance’s comments, Boris Johnson introduced
new guidelines that help to do this, including
stopping non-essential contact and travel.
“All the measures that we have put in place will
reduce R0,” says Prof John Edmunds at the London
School of Hygiene and Tropical Medicine. Edmunds
GETTY IMAGESis a member of the Scientific Pandemic Influenza
Group on Modelling, which is providing advice to the
government. “Exactly by how much is hard to say at
this time, as we have never tried these measures
before. But overall, we would expect them to reduce
R0 to a low level – even to below 1.”
If this number is kept low for long enough, then
fewer people will need to become immune to reach
the herd immunity threshold, and the epidemic will
slow. But how long it would take to reach this point is
tough to predict. Edmunds notes that, even once herd
immunity is reached, the epidemic won’t suddenly
stop, as there will still be infectious people generating
new cases. “One other important strategy is to lower
the number of infectious individuals so it is low at
the point when the epidemic peaks,” he says.
There are currently three big unknowns when it
comes to the coronavirus, says Dr Jeremy Rossman, a
virologist at the University of Kent. Each of these will
affect the disease’s future infectiousness. First, we
don’t know whether the coronavirus will track our
seasons. If the coronavirus prefers colder weather,
like flu, then the epidemic could gain a second wind
during the winter. Second, we don’t know how much
this coronavirus will mutate. “Changes to the virus
could make it better at evading our immune systems,
or more easily transmitted,” says Rossman. And
third, we don’t know how long those who are infected
are subsequently immune for. Some other viruses in
the coronavirus family, such as those that cause the
common cold, only provide immunity for around
three months. There’s evidence for short-term
immunity with COVID-19, says Rossman, “but we
don’t have the data yet to know whether it provides
long-term immunity.” Rossman estimates that we’d
need to be immune for at least a year, possibly two, in
order to build up herd immunity. If this isn’t the case,
he says, our best hope for herd immunity might be a
vaccine, although this is still believed to be at least a
year away. For now, he says that the best actions that
governments can take are to keep the public
informed, keep testing people, and reduce the spread
by clamping down on social contact.
In fact, Rossman still thinks there’s a chance that
we could control the epidemic before we need the
protection of herd immunity. “China’s a great
example: the virus didn’t create herd immunity, but
they’ve managed to contain the outbreak and stop
any new cases from spreading,” he says. “If we can do
it there, we can do it around the world. It’ll be
difficult, but I think it’s possible.”by JAMES LLOYD
James is staff writer at BBC Science Focus.WHAT IS HERD
IMMUNITY?
Herd immunity is what
happens when enough people
are immune to a disease (in
this case, COVID-19) for the
chains of transmission to be
broken. As more people
become immune, infected
people are less able to pass on
the disease, and the spread of
the disease slows down. This
provides an indirect form of
protection for those who are
not immune. Usually,
individuals become immune
through vaccination. But
there isn’t yet a vaccine for
coronavirus, so herd
immunity could only
currently be achieved if
enough people contract the
disease and recover,
developing an immunity to it
in the process.