Of COVID-19: Immunity, Vaccines, Herd Immunity

Kausik Datta
6 min readJul 15, 2020


A quick primer to make sense of the encouraging news about vaccines

Today is the 15ᵗʰ of July. For more than half a year, humanity in the entire world has been witnessing the ravages of a deadly pandemic, caused by a respiratory virus that belongs to the ‘Coronavirus’ family and is named SARS-CoV-2. Currently, we are practically defenceless against the disease, termed COVID-19, caused by this virus. In clinical studies, limited effects, in overall small number of patients under special conditions, have been seen with an antiviral medication (remdesivir) and the use of an anti-inflammatory medicine (dexamethasone), while a few other medicines fell off the wayside when rigorously tested for efficacy against COVID-19.

But that situation may change. All hope is not lost.

Scientists worldwide are making concerted efforts to produce one or more vaccines that are safe and will protect adequately against COVID-19 in both people infected with the virus and those at risk for the disease. As of today, there are more than 150-odd vaccines to protect against COVID-19 in development, and 23 have entered the phases of human clinical trials. Under greatly accelerated studies and fast-tracked regulatory approvals, we may have at least one such vaccine by 2021.

But how does a vaccine work?

To explain what a vaccine is and how it works to protect us, allow me to take a step back and start with our bodies, which truly are marvels of nature. Throughout our lives, from even before we emerge from our mothers’ wombs, we endure continuous attacks upon our persons by microscopic forms of life — ‘microbes’ or colloquially bugs — and other substances from outside. These can make us fall ill. And yet, under ordinary circumstances, most of us don’t.

How does the body achieve this tremendous feat?

There are diligent blood cells that defend our internal environment and keep us healthy. Here is how that ‘immunity’ (that is, resistance to microbe-borne diseases) works.

An Ode to Cellular Immunity

We drink, we dine, we breathe, to live at ease.
With every bit of water, food, and air
we also take in bugs that bring disease
And yet we live in health without a care.

Of roaming cells, the blood carries a bunch
with fancy names; they have their roles defined:
the “Monocytes,” they grab the bug and munch,
then spit the bits for “Lymphocytes” to find.

The job demands they learn and hone the tools
that help discern these bits benign from foe;
so off they go to special army schools
in spleen and thymus, organs where they grow.

A foe that harms, they help expel or kill;
their quick recall’s another vital skill.

Blood Smear showing a monocyte (M) and a lymphocyte (L); photo by Magdalena Wiklund via Flickr used under CC-BY-ND-2.0 (annotation added)

Together these monocytes, lymphocytes, and other roaming cells in the blood comprise the immune system, our body’s interior defence force.

Unchallenged, a bug invader may cause disease, but these cellular defenders valiantly fight it off. During this skirmish, the lymphocytes make enhanced copies of themselves, which retain the memory of the event. If the same or a similar bug gains entry a next time, these memory cells wake up and respond to those irritants in a quicker, stronger manner. As nature intended.

It is indeed a testament to human ingenuity that we have studied this natural process and found a way to harness it to our advantage.

In nature, not only are these minuscule foes plentiful, but they also come in various shapes, they may wear a variety of outer coverings that look and feel different from each other (say, a woolen overcoat versus a muumuu) and they may have different abilities to cause disease. To fight them all-bacteria, parasites, viruses-sometimes our immune system needs a little assistance; this assistance comes via a useful contrivance called, you guessed it. . .


Designed by scientists, a vaccine contains a gemisch of substances, such as bits of the outer coverings of some bugs or whole dead bugs or live bugs which scientists have rendered harmless. Known as antigens, these components are engaged by the defenders when the vaccine is injected into a fleshy part of the body, like the upper arm or the buttocks. There is no skirmish, because the vaccine cannot cause fulminant disease, but the lymphocytes learn about the antigen’s shape and signature in intimate detail and then commit that knowledge to immune memory.

Woman receiving an intramuscular immunization from a nurse, while her daughter looks on; photo in public domain, via CDC

A vaccine thus mimics nature and creates protective immunity against a bug without the pain of disease.

Neat, no?

Scientists have worked out exactly how much of each vaccine component to put together for maximum efficiency. An added component — called adjuvant — in some vaccines aids the defenders in recognizing and remembering the antigens.

So, how would a COVID-19 vaccine work?

Like many virulent viruses, coronavirus possesses a machinery for commandeering its victims’ cells and coercing them to compile proteins it needs. One vaccine approach has artificially replicated a key part of that machinery, a tiny molecule called mRNA (“em-arr-en-eh”); introduced in the recipient’s body, this molecule compels their cells into cooking up a protein of coronavirus origin. Other approaches are using either a lab-made coronavirus protein or a weakened, inactive coronavirus.

Neither these proteins nor the impaired virus can cause disease on their own; however, when administered as vaccine, they do act as antigens to create immune memory and help the intrepid defenders organize and prepare in case the real virus shows up.

What about ‘Herd Immunity’?

Glad you asked. The somewhat-odd term herd immunity has become politically charged these days. But what does it mean?

Immunity for each individual, which I described above, functions to maintain the health of said individual; more importantly, if it successfully manages to kill or contain the pathogen, the disease cannot be transmitted to another individual. Herd immunity, on the other hand, is a population-scale term. Within a group of people, when more and more people become individually immune to the disease and cannot spread to others, the overall level of immunity of the group or herd goes up. When the herd immunity reaches a sufficiently high level, the spread of the disease is checked, which, most importantly, protects other vulnerable people within the group — people who have lower immunity due to age, diseases such as HIV/AIDS, medically induced immune suppression for transplants or cancer, and so forth.

As one can imagine, people who are infected by a bug and then recover become naturally immune, thereby contributing to herd immunity. However, under certain circumstances, natural infection dynamics (which, in turn, may be heavily influenced by the natural characteristics of the pathogen) may not simply be enough in quantity or rapidity to generate adequate herd immunity in the required time.

To take the example of SARS-CoV-2, the rapid, wildfire-like spread of the virus within communities, as well as the inflammatory nature of the disease it causes with deadly consequences, makes it difficult for sufficient number of people in a community to gain sufficient individual immunity in time to raise the level of herd immunity. Under these conditions, immunization from outside — achievable via vaccination — is the only tried-and-tested way to protect the maximum number of people. A COVID-19 vaccination would reduce the number of people vulnerable to the disease, making it difficult for the virus spread and protecting those who cannot (or will not consent to) be vaccinated, as well as those whose underlying medical conditions may prevent an efficient response to vaccination.

This is how immunization programs work, even when a particular vaccine may sometimes not be 100 per cent effective against a given virus.

To sum up, vaccines are humankind’s greatest tool against microbial diseases. Employing our natural immune processes, vaccines protect us and save lives. That said, until we have an effective vaccine, our best bet is to wear a mask in public, practise social distancing and avoid crowded places, and maintain good hand hygiene; all these techniques help reduce the risk of virus transmission.


Originally published at https://inscientioveritas.org on July 15, 2020.



Kausik Datta

Wannabe storyteller in science. Graduate of John Hopkins Science Writing MA program.