Pretty scary stuff. We just don't know enough about COVID-19 to
predict its behavior.
Only Donald Trump can say that it will "magically go away".
Carl Jarvis
On 8/30/20, Miriam Vieni <miriamvieni@xxxxxxxxxxxxx> wrote:
Declining antibodies and immunity to COVID-19 - why the worry?
August 26, 2020 8.22am EDT
Author
Alexander (Sasha) Poltorak
Professor of Immunology, Tufts University
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Alexander (Sasha) Poltorak does not work for, consult, own shares in or
receive funding from any company or organization that would benefit from
this article, and has disclosed no relevant affiliations beyond their
academic appointment.
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Most people are aware that testing for antibodies in a person's blood can
show if someone has had a specific disease, such as COVID-19. Those
antibodies provide protection from getting the disease again.
But in a paper published in the New England Journal of Medicine,
researchers
found that antibody levels decline in individuals who have recovered from
COVID-19, dropping by half every 36 days. Does that mean people who have
recovered from COVID-19 have lost their immunity?
I am a geneticist interested in innate immune response - the part of the
immune system that we have at birth - and how the innate immune cells
"educate" antibody-producing cells about a pathogen and how to identify and
destroy it. As I'll explain, antibodies are important for immunity, but
they
aren't the only factor that counts.
Two arms of the immune system
The immune system is made up of two parts: innate immunity and adaptive -
or
acquired - immunity.
The innate immune system, which includes white blood cells called dendritic
cells, monocytes and neutrophils, is present at birth and responds
instantly
to invaders. This group of white blood cells bombard pathogens with
destructive chemicals and swallow and destroy viruses and bacteria. The
innate immune system provides an instantaneous reaction to a pathogen. The
problem is that it's a blunt instrument - it responds the same way to all
perceived threats.
The adaptive immune system, which is made up of B cells and T cells, must
learn about a pathogen and its characteristics from the innate immune
cells.
This system takes longer to kick in, but the up side is that it is very
specific and in many cases lasts a lifetime.
The immune system's memory
The history of pathogen exposure is carried in so-called memory T cells and
memory B cells. When an infection is defeated and gone, these cells reside
in the peripheral tissues of the body such as lymph nodes or spleen and
serve as a memory of the disease-causing virus. This immunological memory
is
responsible for the host defense and kicks into action in case of the
second
wave or attack of the pathogen.
It is normal for antibody levels to decline after a person has recovered
from a disease. But the New England Journal of Medicine paper raised
concerns because it suggests that we are losing our immunological memory -
which is as bad as losing a real memory.
Basophils, Neurophils, Eosinophils and Monocytes (left) make up the innate
immune system. B cells and T cells are part of the adaptive immune system.
Vitalii Dumma / Getty Images, CC BY-SA
What role do T cells play in immunity?
B cells and antibodies are only part of the immune response. T cells help B
cells produce antibodies - which are proteins that can bind to a specific
pathogen and destroy it.
The way this happens is that first the B cells swallow the virus and start
producing antibodies.
T cells cannot swallow the virus. But a type of white blood cell called an
antigen-presenting cell can. After it does, it "shows" different parts of
the virus to the T cells. The T cells then learn about the virus which they
can now seek and destroy.
T cells also stick to the B cells and send them the activation signals that
help B cells ramp up antibody production.
If antibodies decline, what does this mean for COVID-19 immunity?
It suggests that when there are fewer antibodies in the blood, there is a
greater chance that a number of individual virus particles, called virions,
will survive and escape destruction. Therefore, the remaining virions will
continue to proliferate and cause disease.
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What do declining antibody levels mean for establishing herd immunity?
Herd immunity refers to a population and occurs when a sufficiently high
number of people within a community are immune to the virus and incapable
of
transmitting it. That provides protection for those who are still
vulnerable. For example, if 60% of people are protected against COVID -
because they have survived the infection and carry antibodies - it might
protect (via less frequent interactions) the remaining 40% from getting
sick.
But the results in the New England Journal of Medicine suggest that people
with lower levels of antibody may still have the virus and may not have
symptoms of the disease.
That means that if these people with low antibody levels hang around
healthy, uninfected people, they present a danger to them because they can
transmit the virus.
When antibody levels fall, does immunity disappear?
In general, the answer is no. If the virus attempts to cause a second
infection, the memory B and T cells are able to recognize it, multiply
million of times and defend the body against the virus, preventing it from
triggering another full-blown infection.
The protection provided by memory T and B cells is the reason that
vaccine-based protection works.
However, there are exceptions. A lifelong vaccine against the flu does not
work because flu's genetic code changes rapidly, altering the appearance of
the flu, and therefore requires a new vaccine every season.
But with SARS-CoV-2, the problem as I see it, seems to be that those memory
T cells and B cells seem to be wiped out.
Antibodies are proteins and last for only between three and four weeks in
the blood circulation. To keep antibody levels high, B cells need to
replenish them with a fresh supply. But in COVID-19, the declining antibody
levels suggest that the cells that produce these antibodies are not present
in sufficient numbers, which would explain the drop in antibody levels.
Studies of how long immunity from COVID-19 last may shed more light, but
for
now we do not know the reason why.
