68 TheEconomistNovember 14th 2020
1A
s news emergedthis week that an ex-
perimental vaccine against covid-19
has proved effective in late-stage clinical
trials, hopes that the pandemic’s days may
be numbered are running high (see Brief-
ing). But, even with the best of luck, it will
be months before a vaccine starts to make a
difference on the ground in those coun-
tries that get the first supplies of it, let
alone the ones at the back of the queue. In
the meantime, the pandemic juggernaut
rolls on.
To try to slow it, many countries are
starting to deploy tests which, at some cost
in accuracy, deliver their results much
more rapidly than the polymerase-chain-
reaction (pcr) tests that were common-
place at the pandemic’s beginning. These
rapid tests will allow greater numbers of
infected people than previously possible to
be detected and quarantined before they
can spread the contagion. They are there-
fore being used in increasing numbers to
screen people for the presence of sars-
cov-2, the virus that causes covid-19, in set-tings ranging from airports to nursing
homes. In Europe, indeed, they are some-
times used to blitz entire neighbourhoods,
cities and even small countries, like Slova-
kia. But will they change the course of the
pandemic?Smaller, faster, cheaper
pcrtests look for the genetic sequence of
the virus in nose and throat swabs. These
swabs have to be processed in laboratories
and require machines that take hours to
come up with a result. They are extremely
accurate. But the delay involved can hobble
test-and-trace systems.
Rapid tests, by contrast, are designed to
detect certain proteins that sars-cov-2
sheds when it replicates during an infec-
tion. These proteins, known as antigens,spur the immune system into making oth-
er proteins, called antibodies, that go on to
disable the virus. Antigen tests need no
laboratory backup and can report a result
in 15-20 minutes. They work by dipping the
swab into a vial containing a solution that
extracts the antigen of interest. A few drops
of the mix are then applied to a test strip
laced with antibodies that recognise that
antigen. The test strip displays the results
like a home pregnancy test.
The speed with which these tests have
been developed is impressive. More than
70 are now on the market in one part of the
world or another, according to a catalogue
compiled by the Foundation for Innovative
New Diagnostics (find), a charity in Gene-
va that supports the World Health Organi-
sation (who) with research on diagnostic
tools. So far, only two of them have been
granted provisional (“emergency use”) ap-
proval by the who, and seven by America’s
federal regulator, the Food and Drug Ad-
ministration. But more approvals are ex-
pected to be forthcoming in the weeks
ahead as find and other organisations
complete validation studies that test the
tests in the real-life conditions in which
they are likely to be used.
Early antigen tests were not terribly
good, but many of the newer ones are ex-
tremely accurate. If a pcrtest is negative, a
modern antigen test on the same individ-
ual will agree with that analysis more than
97% of the time, a value called its specific-Fast diagnosis for covid-19Test match
Cheap, rapid tests for sars-cov-2 are here. Will they be the stopgap needed
before a vaccine is deployed?Science & technology
69 LifeonVenus,redux
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