Millions of people across the world have been infected with
SARS-CoV-2, the virus that causes COVID-19. Countries are also now embarking on
campaigns to control the virus and protect their most vulnerable citizens.
One of the biggest questions remaining is whether vaccination and/or prior
infection with SARS-CoV-2 offers lasting protection against this deadly virus.
The good news is that immunology is at last revealing some clues.
To understand whether immunity is possible – and why this
has even been questioned – it is important to consider the nature of SARS-CoV-2.
It is a betacoronavirus, and several betacoronaviruses already circulate widely
in humans – they are most familiar to us as a cause of the common cold.
However, immunity to cold-causing viruses is not long-lasting, leading many
researchers to question whether longer term immunity to SARS-CoV-2 is possible.
However, studies considering the closely related
betacoronaviruses that cause the diseases Mers and Sars
offer a glimmer of hope. With these viruses, immunity has proved more durable.
Could this be true for immunity to SARS-CoV-2 too?
The first of the body’s immune cells to respond to an
infection are designed to attack the invading substances to try to control the
infection’s spread and limit the damage done. The immune cells that respond
later that are responsible for immunity are known as lymphocytes, which include
cells and T cells. Lymphocytes need time to learn to identify the threat
that they are facing, but once trained they can be rapidly deployed to seek and
destroy the virus.
Our T cells and B cells work together to combat infection,
but they have quite different functions that enable them to deal with a huge
variety of threats. B cells make antibodies that neutralise infections. T cells
are broadly divided into two types – T helper cells and cytotoxic T cells.
Cytotoxic T cells directly kill viruses and cells that viruses have infected. T
helper cells support the functioning of B cells and cytotoxic T cells.
Collectively these are known as “effector” cells.
Studies have now demonstrated the critical role
that these effector cells play in the fight against COVID-19. Once the
infection is gone, these cells should then die off in order to avoid causing
excessive damage in the body.
But some effector cells persist. In an early piece
of research yet to be reviewed by other scientists, functional T cells have
been detected six months after infection. Similarly, even patients who have had
mild COVID-19 have
detectable antibodies six to nine months after infection. However,
antibodies do wane over time, so these antibodies against SARS-CoV-2 could
Such discoveries raise real optimism about protection from
reinfection. But what happens if or when effector lymphocyte levels finally
drop off? Well, our immune system has another trick up its sleeve to protect us
for the long term, even after people’s effector cells and antibody levels have
fallen. Once lymphocytes have been trained to deal with a virus, a pool of the
cells remember it and are kept for the future. These “memory” cells can then be
rapidly deployed if the threat is encountered again.
Memory cells are incredibly powerful tools for our immune
system and can be very long-lived, with studies showing memory B cells for
smallpox persisting at least 60 years after vaccination
and for Spanish flu at least 90 years after the 1918 pandemic. In order to
understand whether long-term immunity to SARS-CoV-2 is possible, it’s therefore
critical to consider not just effector cells but all types of memory cells – B,
T helper and cytotoxic T memory cells.
Fortunately, memory cells can be identified by specific
structures and proteins that they express on their surfaces, enabling
researchers to distinguish them from effector cells. Now that COVID-19 has been
with us for a year, researchers are becoming able to make great leaps in understanding
about memory responses to COVID-19. Evidence is emerging of memory
T cell responses lasting six to nine months after infection, and a recent
preprint study (yet to be reviewed by other scientists) has also identified
what appear to be memory B
Studies have also been investigating whether prior exposure
to the virus confers protection, with research
showing that in the UK’s second wave, previously infected health workers were
either completely protected from reinfection or were asymptomatic if they
picked up the virus again. Such observational studies give real hope for the
durability and potential of protective immunity.
We still have much to learn about the immunology of
COVID-19, but the pace of research is astounding, and the more we learn, the
more we are empowered to beat this virus. Our immune system is incredibly
powerful, and these studies showing persistent immune responses nine months
after infection are real cause for celebration. They give us confidence that,
with vaccination, we have a real chance to win the war against COVID-19.
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hearing from academic experts. January 11, 2021
* Professor in Biomedical Sciences, University
of Manchester .
–Disclosure statement: Sheena Cruickshank does not work for, consult, own shares in or
receive funding from any company or organisation that would benefit from this
article, and has disclosed no relevant affiliations beyond their academic
appointment. University of Manchester provides funding as a member of
The Conversation UK.