Herd Immunity in Covid-19 Pandemic

Immunity is the Body Shield

What is Immunity?

Immunity is the body’s ability to resist infection or in other words the state of the body which learns from the experience of past infection(s), how to deal more efficiently with subsequent infections.
In biological definition, immunity is the capability of a multicellular organism to resist harmful microorganisms.
It is basically a system consists of different body components(cells and molecules ) that assemble to combat foreign body infection.
In 1798, Edward Jenner (Father of Immunology) following the suggestion of a dairymaid inoculated a eight years old boy with blister fluid collected from cowpox and protected him from severe small pox.
This technique is called vaccination. vacca means cow in Latin.This technique was used extensively to eradicate small pox from the world.
Our immune system is essential for our survival. Immune system prevent the attacks from Bacteria,Virus,Parasite & more external pathogens.Once a pathogen is detected, the body defense mechanism become active to combat it & to destroy it.
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Immunology of COVID-19

The Coronavirus disease-2019 (Covid-19), caused by Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has affected millions of peoples worldwide.
It is an Infectious disease.Most people infected with the COVID-19 virus will experience mild to moderate respiratory illness,fever,sneezing,loss of taste or smell and most of them recover without requiring special treatment.Older people,lower immunity system and those with medical problems like cardiovascular disease, diabetes, chronic respiratory disease, and cancer are more likely to develop serious illness.
The recent emergence & rapid spread of SARS-CoV-2 and resulting coronavirus disease 2019 was declared Pandemic by the World Health Organization(WHO) on March 11,2020.China reported the threat to WHO on December 31, 2019.At first the pathogen was identified as a Betacoronavirus with high sequence homology to bat coronaviruses using Angiotensin-converting enzyme 2(ACE2) receptor as the dominant cell invading mechanism.SARS-CoV-2 is also closely related to SARS(SARS-CoV-1 more lethal than SARS-CoV-2) & Middle Eastern Respiratory Syndrome(MERS) that were responsible for zoonotic epidemic and local outbreaks in 2003 & 2012 respectively.

Immunity agains COVID-19:

Innate immunity works as the first line of antiviral defense here and it is essential for immunity to all viruses. Till date, the explore of the specific innate immune response to SARS-CoV-2 is extremely limited.
However, the virus-host interactions involving SARS-CoV-2 are likely to recapitulate many of those involving other Corona Viruses, given the shared sequence homology among Corona Viruses and the conserved mechanisms of innate immune signaling.
It is a RNA virus,in case of RNA viruses, the initial pathways are initiated through the engagement of Pattern recognition receptors (PRRs) by viral single-stranded RNA (ssRNA) and double-stranded RNA (dsRNA).It occurs by receptors like- cytosolic RIG-I like receptors (RLRs) and extracellular and endosomal Toll-like receptors (TLRs).
As the PRR activated,the downstream signaling cascades trigger the secretion of cytokines(Immune components). Among all cytokines,for antiviral defense type I/III interferons (IFNs) are considered the most important.Along with these other cytokines, such as Proinflammatory tumor necrosis factor alpha (TNF-a), and interleukin-1 (IL-1), IL-6, and IL-18 are also released. Altogether, they induce antiviral defense programs in target cells and establish the adaptive immune response. If present early and properly localized, IFN-I can effectively limit CoV infection.
Early evidence demonstrated that SARS-CoV-2 is sensitive to IFN-I/III pre-treatment in vitro, which is to a greater degree than SARS-CoV-1.The specific IFN-stimulated genes (ISGs) which mediate these protective effects are still being explored in details.
Lymphocyte antigen-6 complex locus E (LY6E) has been shown to interfere with SARS-CoV-2 spike (S) protein-mediated membrane fusion.
Likely, the IFN induced transmembrane family (IFITM) proteins inhibit SARS-CoV-2 entry, as demonstrated for SARS-CoV-1.

Evasion of Innate Immunity by Coronaviruses:

Though our body has some extend of Innate immunity,Corona Viruses can mutate themselves to evade the immunity.

As we discussed cytokines represent a major role to stop viral infections,Corona Viruses have discovered their own protective mechanisms to inhibit IFN-I induction and signaling. Early studies have shown that SARS-CoV-1 suppresses IFN release in vitro and in vivo.
SARS-CoV-2 likely achieves a similar protection, due to lack of robust type I/III IFN signatures from infected cell lines, primary bronchial cells, and a ferret model.
There are multiple ways to evade immunity of a compromised person for Corona Viruses, with viral factors antagonizing each step of the pathway from PRR sensing and cytokine secretion to IFN signal transduction.

The scientific responses are extraordinary in this specific context. Here I got an publication about the Covid-19 immunity. You can download it from HERE.

What is Herd Immunity?

Pathogens can travel quickly through a community vai different vectors or inanimate objects and can make a lot of people sick. If enough people get the infection, it can lead to an outbreak. Corona is a great example of infectious disease.
But when a large part of population are vaccinated against a certain disease, the pathogens can’t travel as easily from person to person or person to animal or animal to person or animal to animal— and the entire community is less likely to get the disease.
That means even people who can’t get immunity by natural or by vaccine will have some protection from getting sick. And if a person become sick, there will be less chance of an outbreak of the disease because it’s harder for the pathogen to spread to the immunized persons. Sometimes, the disease becomes rare and, it’s wiped out from the community.
So, Herd immunity, or community immunity, is when a large part of the population of an area is immune to a specific disease. Then the group as a whole will have the protection against that specific disease.
Herd immunity is important for those peoples who can’t get vaccine for some reasons like serious allergies and those with weakened or failing immune systems (like people who suffers from cancer, Type 1 diabetes,HIV or AIDS or other health conditions).
It can be achieved by two way-

Naturally developed immunity
Vaccination

a) Naturally developed immunity : If most of the peoples naturally raise a certain level of Immunity then it can be considered as community immunity which will protect others too.This is what stopped the Zika virus outbreak in Brazil.
b)Vaccination: Vaccinating a large number of people will develop a Herd immunity.This is what stopped polio in the United States & Other countries.

When does a community reach Herd Immunity?

A community reach it’s Herd Immunity level when a certain percentage of people have achieved Immunity agains a specific disease.This calculation is based on the Basic reproduction number(basic reproductive number /basic reproduction ratio/basic reproductive rate), denoted $R_{0}$
This $R_{0}$ tells the the average number of people that a single person with the virus can infect if those people aren’t already immune.The higher the $R_{0}$ , the more people need to be resistant to reach the level of herd immunity.The most important uses of $R_{0}$ are determining if an emerging infectious disease can spread in a population and determining what proportion of the population should be immunized through vaccination to eradicate a disease. In commonly used infection models, when $R_{0}>1$ the infection will be able to start spreading in a population, but not if $R_{0}<1$ . Generally, the larger the value of $R_{0}$ , the harder it is to control the epidemic. See Full list

SARS: 0.19–1.08

MERS: 0.3-0.8

COVID-19: 0.5-0.6

Herd Immunity in Covid-19 Pandemic:

The main obstacle to herd immunity in COVID-19 is that the virus is novel(or new). That means that it hasn’t infected humans before and & there is no existing immunity to combat. Everyone is in threat.
SARS-CoV-2 virus is a highly transmissible virus, researchers says it will need at least 60 to 70% of the population to have immunity to really break the chain of transmission. The hope is that the infected peoples are developing immunity against it who are capable to do this. Another potential barrier is that we don’t know how strong the immune protection is or how long it will last in people who’ve had COVID-19.So if we allow this to occur naturally, it will take long time, along with that we have to face a collateral damage too.
So, we have to choose the second option of Developing vaccine to combat Covid-19 which will effectively protect us from getting infection.
Until the Vaccine developed, we must protect ourselves from getting infection. We should follow WHO recommendations of the public health and social measures, the physical distancing, wearing a mask when you’re in crowded settings, washing your hands frequently etc. These will not create herd immunity but will protect individuals.
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