16 April 2022 | New Scientist | 17News
A RECORD percentage of covid-
tests for people in England and
Wales are positive. Here’s what
you need to know about how
lateral flow tests work, why
symptoms may linger even
if a person is no longer testing
positive, and how long you
can test positive after you have
recovered from your symptoms.What is the current advice in the UK
if you have covid-19?
Although people in England no
longer legally have to self-isolate
if they have covid-19 symptoms
or test positive, it remains UK
government advice that they
should try to do so for at least
five days, although they can be
infectious for up to 10 days and
so should avoid contact with
people who are at higher risk
for that period.
“The focus of this new phase
[of the pandemic] is on protecting
those who are most at risk from
the virus,” said a spokesperson
for the UK Health Security
Agency (UKHSA) in a statement.
In Scotland, Wales and Northern
Ireland, people should only stop
isolating before 10 days if they
have two negative results from
a lateral flow test (LFT) over
two days.
In England, people are no longer
advised to take LFTs to check when
they become negative, and the
tests are no longer free for the
general population, although they
can be bought at pharmacies. “The
fact that legally it’s not enforced
anymore doesn’t mean that
people shouldn’t still be careful,”
says Christopher Chiu at Imperial
College London.How have the rules in England
changed for children?
The UKHSA no longer advises
people under 18 to get tested for
covid-19, unless it is on a doctor’sadvice. For those who do have a
positive test, the recommended
self-isolation period has been cut
to three days after the day the test
was taken.
“There is some evidence that
children have a shorter duration
of illness compared to adults,”
said UKHSA head Jenny Harries
in a statement. “Ideally, children
would return to school as soon
as they turn lateral flow negative,”
says Iain Buchan at the University
of Liverpool in the UK. “But atsome point, political decisions
need to be taken, over cost and
priorities. Prioritising children’s
education and social development
is important.”How do I count how long I have
been infected?
The first day someone experiences
symptoms or tests positive is
counted as day zero. Someone
trying to self-isolate until day five
would actually need to stay home
for six days.Does the intensity of a line on an
LFT reveal anything?
Lateral flow tests aren’t approved
to be used in this way, but people
usually see the line on their test
changing in intensity from faint to
dark, and back to faint again, over
the course of their infection. Some
studies do show that the intensity
of the line correlates with the
amount of virus particles present
in the person’s nasal fluids.
“These tests actually are
very quantifiable based on the
darkness of the line,” says Michael
Mina at eMed, a US testing firm.But even if the line is faint,
there must still be replicating
virus present in someone’s
body in order to be making
enough protein that it gives
a positive result.Why might I still have symptoms
even though I test negative?
Some symptoms may continue
after someone is no longer
infectious.
“In general, people have a cough
for a long time,” says Al Edwards
at the University of Reading in
the UK. “There are two reasons for
having a cough. One is because
you’re infected, and that’s causing
damage, and that makes you
cough. The other reason forhaving a cough is because your
respiratory tract gets damaged by
the viral infection and it’s healing.”Can you have false positives from
LFTs after a covid-19 infection?
Schools in the UK are being told
by health services that children
may have a positive LFT for up to
three months after their infection.
This is “theoretically possible,
but it would be very unusual”,
says Buchan, who ran the first
mass community study of LFTs
in Liverpool.
LFTs test for virus protein, which
is produced by replicating virus,
and so are less likely to give a false
positive than PCR tests. PCRs
detect the virus’s genetic material,
fragments of which can remain
for several weeks after there is
no viable virus left. ❚BriefingDOMI
NIKA^ ZARZY
CK
A/NURPH
OTO/GET
TY
IMAGESHow LFTs really work
As England changes its guidance around testing for covid-19,
Clare Wilson looks at how lateral flow testing works“There is some evidence
that children have a
shorter duration of illness
compared to adults”A woman takes a throat
swab for an at-home
covid-19 testA positive result
on a home lateral
flow testRICHARDHARDING/ALAMY