70 Science & technology The EconomistNovember 7th 2020
S
ince thebeginning of 2020 medical re-
searchers have been in hot pursuit of co-
vid-19. One of their most important goals is
to understand the immune response to
sars-cov-2, the virus that causes it. Find-
ing out what a good response looks like,
and how long it lasts, is crucial. The an-
swers will reveal whether people who have
recovered from the illness are protected
from a second infection, and also indicate
how difficult it will be to develop a vaccine.
There is much to worry about. Over the
past year, many reports have shown rapidly
waning levels of covid-specific antibodies
after the initial burst caused by an infec-
tion. Antibodies are parts of the immune
system that attack the virus directly. They
are expected to be involved in any long-
term protection against reinfection. If they
disappear quickly, that looks, on the face of
things, like bad news. Worriers also high-
light the facts that immunity to types of co-
ronavirus which cause the symptoms de-
scribed as “a cold” is short-lived, and that
there are already a number of proven cases
of reinfection with sars-cov-2.
Yet antibodies tell only part of the story.
Another important actor is the t-cell (pic-
tured above). Rather than attacking viruses
directly, t-cells attack infected cells, to
stop the virus reproducing. The balance of
importance of the antibody and t-cell arms
of the immune system varies with the ill-
ness in question. And, as far as this partic-ular infection is concerned, although al-
most all patients who catch sars-cov-2 are
thought to create t-cells in response, an
understanding of their significance has
been elusive.
This is largely because t-cells are harder
to measure than antibodies, and so are less
often studied. Shamez Ladhani, a consul-
tant epidemiologist with Public Health
England, a government health-protection
agency, who has worked on a new, long-
term investigation of these cells, says it
took nearly three weeks to count them in
the 100 patients his study looked at. The ef-
fort was worthwhile, though, because it
has shed new light on how long-lasting this
form of immunity to sars-cov-2 might be.To a T?
Dr Ladhani’s project is part of a wider effort
focused on health-care workers that Public
Health England began in March. Over
2,000 people have donated blood samples
every month since then. The 100 he and his
colleagues have studied are a subset of
these. In a paper just published as a pre-
print, but not yet peer reviewed, they say
that six months after infection all of these
patients, even those who had had only mild
symptoms, or none at all, still had detect-
able levels of t-cells directed against the vi-
rus. Though their antibodies might have
vanished, t-cells remained on the scene.
These findings bode well for the ideathat t-cells offer long-term protection
against reinfection, says Eleanor Riley, a
professor of immunology at the University
of Edinburgh. And Paul Moss, a haematolo-
gist at the University of Birmingham, says
that his experience with other viruses leads
him to expect the t-cell response Dr Lad-
hani has observed will last many more
months than the six it has persisted for so
far. In sars-cov-1, the name now given to
the virus that caused the original sarsout-
break, in 2002-03, this form of cellular im-
munity was found to last in some people
for over a decade. The long-lived nature of
such t-cell responses fits in with the obser-
vation that, so far, reinfections seem rare.
Dr Moss says the implication of the new
study is that those searching for vaccines
against covid-19 should give priority to the
production of t-cells. There is more good
news here. Two of the leading candidates—
one from a collaboration between Pfizer, an
American drug company, and BioNTech, a
German biotech firm, and the other from a
second joint effort, between AstraZeneca, a
British-Swedish drug company, and the
Jenner Institute of Oxford University—do
exactly this, while also stimulating the pro-
duction of antibodies. Speaking in July,
when the first results from the Astra-
Zeneca-Oxford vaccine were published,
Adrian Hill, the Jenner Institute’s boss, was
keen to highlight the “excellent” t-cell re-
sponse their vaccine created. “That is what
this vaccine type does,” he explained. “That
is what it is designed for.”
The significance of t-cells was obvious
to Pfizer and BioNTech as well. In July they
switched the focus of their efforts in re-
sponse to data suggesting that an experi-
mental vaccine they had hitherto regarded
as secondary was eliciting strong t-cell re-
sponses. The coming weeks are likely to
produce results concerning whether one or
more of these putative vaccines actually
provides protection against covid-19. That
the two leading candidates provoke strong
t-cell responses is grounds for optimism
that vaccinology is on the right track.
As for the vanishing antibodies, there
may be grounds for optimism here, too.
The fact that levels of these proteins wane
rapidly after infection, when they are no
longer needed, should not be so surprising.
It might not matter, either. The immune
system could well be primed to make them
in large numbers again, if and when it re-
encounters sars-cov-2. It is also worth re-
membering that, even if researchers can-
not detect antibodies, that does not mean
they are not there at all.
As winter approaches in the northern
hemisphere, the scientific fight against co-
vid-19 feels more urgent than ever. But,
while it is true that science has not yet
jammed the pandemic exit doors open,
there are now at least a few shafts of light
emerging around the doorway. 7The T-cell response to covid-19 lasts at least six months. That is good newsCovid-19Teed up