What is herd immunity?
Infectious diseases can strike at any
time. Some of them cause relatively
minor interruptions to daily life and
often can resolve of their own accord
when the body’s immune system
mounts a successful defense. Other
diseases can cause serious, even
life-threatening symptoms or spread
rapidly, which makes it essential for
medical professionals to help slow
down or stop the transmission.
What is herd immunity?
Herd immunity is a term that often
arises in relation to infectious diseases.
Herd immunity has taken on
renewed significance as the world has
been battling COVID-19, the novel
coronavirus introduced in late 2019.
Herd immunity, which is sometimes
called “community immunity,” “population
immunity” or “social immunity,”
refers to the indirect protection
from infectious diseases that occurs
when a large percentage of the population
has become immune to that
disease. If enough people are resistant
to the cause of a disease, whether it
is a bacteria or virus, that disease has
nowhere to go and the spread stalls,
according to WebMD.
How does herd immunity occur?
There are two ways that herd immunity
can occur. The first is when
resistance develops naturally when
the body is exposed to the virus or
bacteria. At this point, the immune
system will produce antibodies to
fight off the infection. After recovery,
these antibodies are still circulating,
and should exposure to the same disease
occur again, the body can defend
against another infection.
Another way that herd immunity
occurs is through vaccination. The
Centers for Disease Control and Prevention
states that when the majority
of people are vaccinated, it creates
the same disease lockdown — fewer
people get sick and fewer germs are
able to spread from person to person.
What’s more, even those who are not
vaccinated, such as newborn babies or
those who may not be able to get vaccinated
due to chronic illnesses, will
get some level of protection because
the disease will not be spreading as
readily within their communities.
When is herd immunity reached?
When enough of a population is
immune to a pathogen it prevents
further spread and herd immunity is
apparent. Diseases are different and
herd immunity is reached based on
the pathogen’s reproduction number,
or R0. Essentially this boils down
to how contagious the pathogen in
question is. WebMD says the R0 tells
the average number of people that a
single person with the virus can infect
if those people aren’t already immune.
The higher the R0, the greater number
of people will need to be resistant to
reach herd immunity. Measles, which
is very contagious at an R0 of 12 to
18, requires 93 to 95 percent of the
population to be immune for herd
immunity to be reached. The World
Health Organization estimates the
R0 for COVID-19 to be between 2
and 3. This means between 40 and 70
percent of the population will need to
be immune to halt the spread.
In the case of COVID-19, it’s still
unclear whether anyone can get
reinfected, and whether antibodies
produced for one strain can fend off
another strain of this novel coronavirus.
This reinfection mystery is
what makes herd immunity — both
through a vaccine or through natural
exposure — challenging for epidemiologists
in relation to COVID-19.
Herd immunity is an important
factor in disease prevention. Getting
the facts about this phenomenon is
important.
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