The EconomistNovember 14th 2020 Science & technology 692
1ity. The story gets complicated, though,
when the virus is actually around. If some-
one tests positive for covid-19 in a pcrtest,
the best antigen tests will agree in more
than 90% of cases if the testing is happen-
ing within a week or so of the onset of
symptoms, a value called the sensitivity.
But the rate of agreement falls if the anti-
gen test is done at the beginning or end of
an infection, when the amount of virus
present in the nose and throat is consider-
ably lower. This means that diagnoses rely-
ing on antigen tests are unreliable during
those periods.
Fortunately, from a public-health point
of view this may not matter. The relation-
ship between viral load and contagious-
ness is not fully understood, but current
thinking is that higher loads make people
more contagious. Since those with higher
loads are most likely to show up as positive
in an antigen test and therefore be asked to
isolate themselves, the transmission-
breaking value of the new tests should not
be too badly compromised.
In theory, then, all of this sounds great.
But reality is messier. Even a highly accu-
rate test will produce fewer true positives
than false positives if the people being test-
ed are unlikely to be infected in the first
place (see chart). That would be the kind of
problem which arises with mass testing in
places that are not covid-19 hotspots. For
example, Britain’s Office for National Sta-
tistics estimates that on October 28th
0.82% of people in private households in
London were infected. If everyone in Lon-
don that day was given a test that has the
minimum “acceptable” accuracy for rapid
tests set by the who(80% sensitivity and
97% specificity) the number of those with
false-positive results will be 353% bigger
than those with true positive results.
This is why deciding whether to trust
the result of an imperfect rapid test—or, in-
deed, whether it is worth using the test at
all—depends on who is being tested, and
why. A positive result is more credible for
someone with symptoms, or who is a close
contact of an infected individual, and per-haps lives in an area with a high covid-19
rate. But testing people when there is no
obvious reason to believe they may be in-
fected is likely to be a waste. A positive re-
sult in that case will be suspect.Do try this at home
Doctors are used to making such decisions
when testing for things like cancer, sexual-
ly transmitted infections and so on. The
guidelines they employ draw on years of
research and practice. But for covid-19
things are new and changing rapidly. To
deal with that, some test developers are
pairing their products with “digital wrap-
arounds” such as apps in which such deci-
sion-making algorithms are fed up-to-date
data on things like trends in local covid-19
prevalence and the weight of various per-
sonal risk factors derived from various an-
alyses. Some of these apps issue a time-
limited bar code to those who test negative,
for use where proof of a negative test may
be required.
For now, rapid tests are licensed for use
only by medical professionals. The regula-
tory bar for stand-alone home tests is set
high. They must be 99% accurate and pass
extensive usability trials to ensure that
people employ them correctly. That would
be easier if the secretion being tested was
saliva, which is freely accessible, rather
than material found high in the nose or
deep in the throat. Saliva does work reli-
ably in some pcrtests but no one has yet
devised a good antigen test that uses it.
At the current pace of progress, though,
this may soon change. Bruce Tromberg of
America’s National Institutes of Health
(nih) thinks that a rapid over-the-counter
test could be available in America as early
as next summer. Rapid antigen tests are,
then, likely to become a big part of coun-
tries’ covid-19 testing strategies. In particu-
lar, they will be used for testing at home, in
doctors’ surgeries, and in remote places
where pcrlaboratories are not available.
They will be especially handy for mass test-
ing in places prone to outbreaks, such as
prisons and student dormitories.
As more rapid tests are developed and
demand for them increases, competition
and manufacturing at scale will make them
cheaper. Stand-alone antigen tests are now
available for as little as $5 apiece, but prices
may eventually drop nearer to $1, which is
the cost of a rapid test for malaria. Tests
that use small machines are about $10-20
each, plus a few hundred dollars for the de-
vice. A pcrtest now costs around $50, but
will be cheaper for automated large-scale
testing of samples that come in bulk on a
set schedule, such as samples from univer-
sities or workplaces.
Even though antigen tests are cheap,
however, some people worry that rich
countries will corner the market for them
until production has ramped up suffi-ciently, leaving poorer places with a short-
age. To avoid this, the Bill and Melinda
Gates Foundation, a big charity, has teamed
up with the whoto place an order for 120m
rapid tests which will go to 133 developing
countries over the next six months.
Dr Tromberg, who leads a project at the
nihwhich invests in new covid-19 testing
technologies that can be scaled up rapidly
to mass production, reckons the 22 pro-
ducts in his pipeline which are already at
the manufacturing stage will add 2.5m
tests a day by the end of this year—helping
raise America’s total to 6m-7m. Around the
world, several makers of rapid covid-19
tests have said they have the capacity to
make tests in the tens or hundreds of mil-
lions a year. This sounds plausible, given
that 400m malaria test kits are made each
year. But expanding into the billions is ter-
ra incognita. Though new production lines
can be built and existing ones put to work
around the clock, making tests requires
skilled workers, who are in limited supply.
Whether rapid tests change the course
of the pandemic and end the need for lock-
downs until a vaccine can likewise be made
and distributed at scale will depend on
whether those which are available are used
wisely. Eventually, such a vaccine will re-
duce the demand for tests dramatically.
But, for now, the world needs them. 7What is truth?Source:TheEconomistCovid-19testresults*,%*Fora testwith80%
sensitivity,97%specificity02550751000 25 50 75 100
Share of population with active infection, %True positive
As % of positive testsTrue negative
As % of negative testsE
xtraordinary claimsrequire extraor-
dinary evidence. So goes the dictum,
usually credited to Carl Sagan, a celebrated
astronomer, on the need for caution when
interpreting radical new ideas in science.
And there are few claims more extraordi-
nary than that of the discovery of life be-
yond Earth.
Jane Greaves of Cardiff University, in
Britain, has not actually made that claim.
But she came close to it when, in Septem-
ber, she and her colleagues published re-
search that appeared to show the existence
of a gas called phosphine in the clouds of
Venus. This substance, a compound of
phosphorus and hydrogen, should be able
to survive only briefly in an atmosphere
like that of Venus. But Dr Greaves’s team re-
ported that it actually seemed to be persis-
tent there, at a concentration of 20 parts per
billion. This turned heads because, on
Earth, the minuscule amounts of phos-
phine around have only two sources:
chemists and microbes. The former areIs there really phosphine on Venus?Planetary scienceQuestions of life