Herd Immunity is gaining a lot of attention these days, as a potentially viable strategy to curb the highly contagious COVID-19 pandemic.

But do you correctly understand what Herd Immunity is? How does it work? Is it viable for COVID-19? What is the best way to achieve Herd Immunity against it – Natural infection or Vaccination?

LET’S FIND OUT!

CONTENTS

I. Herd Immunity: The concept

What is Herd Immunity / Community Immunity?

How is Community Immunity related to how contagious an infection is?

Does it protect you for life?

II. Herd Immunity & COVID-19 

Is Herd Immunity possible against COVID-19?

What is the best way to do so?

III. Where do Transplant recipients stand in this whole chorus about Herd Immunity & why?

I. HERD IMMUNITY: THE CONCEPT

WHAT IS HERD IMMUNITY?

Herd Immunity is a real-world concept in “community medicine”. It is a form of immune protection that is possible only in cases of “contagious” infections that show human to human transmission. Both Natural Infection & Vaccination can help you contribute towards this kind of mass immune protection.

This method has the potential to protect both the ones who receive the vaccine or contract the infection naturally, as well as those who are medically unfit to receive any of these forms of direct immunization.

However, there is a catch.

For effective Herd Immunity to kick in, the majority of the local population must be immunized simultaneously / about the same time.

Let us explain.

As the illustration above explains, when a majority section of a population simultaneously gains immunity for a certain contagious infection, either with specific vaccination or natural infection, it helps by:

🍁 Effectively “breaking the chain of transmission” of infection

&

🍁 “Significantly diminishing” the chances of the infection to “find” a vulnerable human host.

Such simultaneous mass immunity in the majority population acts as an “immunity shield”. This indirectly “protects” the minority section of the population that maybe medically unfit to receive the vaccine or survive a natural infection. Patients with a weakened immune system due to a certain medicine or a medical condition fall under this category.

WHAT EXACTLY DO YOU MEAN BY “MAJORITY” OF THE POPULATION? IS THERE A NUMBER TO IT?

Yes. There is a number. And this number varies with how contagious the infecting germ / microbe concerned, is.

Welcome to the concept of Herd Immunity Threshold.

You know how in case of a war, the more vicious an invading army is, the more robust & leak-proof the barrier systems of the defending army must be? After all, this is what significantly lessens the chances of the invaders finding potential loopholes to “infiltrate” a system, right?

It’s the same in case of Herd Immunity.

The aim here is to block the infection concerned, from finding a vulnerable human host. So, the more “contagious” the infection, the greater “the share of population that must be simultaneously immunized”.

In this context, the minimum percentage of a population that must attain simultaneous immunization for adequate mass immunity, is called “Herd Immunity Threshold” or “HIT”.

So, long things short, greater the Contagiousness of an infection (R₀), more is the HIT necessary for an effective Herd Immunity against the infection.

Presenting below, the HIT & contagiousness (R₀) correlation of some very well known infectious diseases:

Information source: [References 4-8]

DOES HERD IMMUNITY PROTECT YOU FOR LIFE?

Herd immunity stays effective only for the duration for which “antibodies” (specific immune proteins”) or other immune defences against the germ circulate in the bloodstream of the vaccinated/infected population.

Such duration of “antibody circulation” in blood can vary with:

♦️The infecting germ (microbe)

♦️The mode of acquiring immunity (natural infection or vaccination).

For instance, Herd Immunity can last for:

♦️15-20 days for a natural flu epidemic

♦️1 year if the majority of the local population gets a Flu Vaccine about the same time of the year.

OR

♦️Even lifelong, as with the large scale vaccination drive for Small Pox  undertaken by respective healthcare systems worldwide.

Antibodies from Small Pox Vaccine circulate in the bloodstream for life. Plus, a huge percentage of the local populations received the vaccine about the same time. As a direct result, community immunity from the vaccine kicked in effectively, and Small Pox now stands officially eradicated.

♦️ Another rather well-known instance of successful, lifelong mass immunity is that against Polio virus. Polio is a disease that spreads by drinking water or taking food contaminated with traces of human faecal waste carrying Polio viruses from infected patients. In case of this disease, mass immunization programmes with Oral Polio Vaccine came as a saviour.

In one of our previous discussions, we explained how Vaccines are prepared with “weakened or dead microbes” such that it trains your immune system to identify & combat an infection, but does not cause an infection itself.

So, children who received the oral vaccine attained direct immune coverage. In addition, because this is an oral vaccine, these children eliminated traces of these weakened vaccine virus in their saliva & faeces. Although unintended, people who came in contact with water contaminated with such vaccine-based weakened polio viruses, got immunized too. In addition, of course, members who couldn’t receive the vaccine for medical reasons ended up being protected by the Herd Immunity shield.

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II. HERD IMMUNITY & COVID-19

IS HERD IMMUNITY POSSIBLE WITH COVID-19?

It is no news that COVID-19 is a highly contagious disease. In other words it has high R₀. So as a concept, Herd Immunity is certainly not unthinkable. But this is a completely new viral strain to the research world & has the scientific community intrigued in more ways than one.

Whether this can become a practical reality with COVID-19 depends on two things:

🍁 Whether simultaneous “community” immunity is a possibility.

&

🍁 If doing so is a safe & achievable option.

SO WHAT IS THE BEST WAY TO ATTAIN HERD IMMUNITY AGAINST COVID-19?

Natural Infection or Vaccination? Now that’s a raging topic these days, isn’t it?

COVID-19 is a completely new strain. Any information about the kind of “immune protection” it can afford after a natural infection or vaccination is completely dependent on the research findings of the time. This is amenable to change.

But based on what we know till now:

1. IMMUNE PROTECTION DURATION

A recent study by NHS UK has found that the circulation of natural “antibodies” specific to COVID-19 dips in your bloodstream latest by 6-8 weeks after a natural infection.

In case of the Vaccine under development at Oxford, researchers claim that in addition to the usual “antibody” protection, it should kick-in a separate category of long term immunity called Cell Mediated Immunity. If this is true, such a vaccine can afford immune protection upto 1.5 – 2 years.

2. ABILITY TO GET “IMMUNIZED SIMULTANEOUSLY”

A separate study has concluded that somewhere between 50-83% of the community needs to be immunized against COVID-19 for effective herd immunity. In other words, COVID-19 has a Herd Immunity Threshold (HIT) of over 50%. Now 50-60% of world is unlikely to contract natural infection simultaneously. That defeats the whole purpose.

On the other hand, when COVID-19 Vaccines become available, they are likely to be administered through timed Vaccination programmes. So it would be a more convenient way to cover 50% population in a short scheduled time frame. Herd immunity will potentially kick in much more effectively. (As in case of the Small Pox vaccine programme)

3. LACK OF A CURE & THE NEED FOR ONE

Most importantly, even if all 50% of the population decides to contract natural infection together, the healthcare systems will simply collapse because COVID-19 is new, specific medicines to cure COVID-19 aren’t available yet.

Even if you argue that majority could be Asymptomatic cases who only require Home Isolation, the fact that this immunity gained is only for as less as 2 months. Would the asymptomatic patients prefer to get infected with COVID-19 again after 2 months? That’s significant risk for little or no benefit.

Vaccines, on the other hand, are prepared in a way, that the recipients’ immune system is able to identify the specific microbe, but the body does not get a full blown infection or its complications. This is to help your immune system identify the “enemy” and be able to act aggressively in case of a “real invasion”. So there is no question of depending on a “cure” for the infection.

Moreover, the immunity obtained is now being claimed to afford protection for upto 1 year. That’s longer & safer than getting naturally infected! So, the possible future vaccine schedule would require you to get an annual booster shot at a designated time to retain your immune protection as well as the herd immunity without really infecting you.

That was a basic point-by-point comparison between Natural COVID-19 Infection & getting a COVID-19 Vaccine in terms of Herd Immunity.

COVID-19 Vaccines give you a better, long-lasting, infection-free, less painful & pocket-friendly deal to contribute towards Community Immunity as compared to Natural Infection. What’s more, vaccines also preclude the need for you to isolate yourself at home and swallow medication.

However, if you are cynical about the available COVID-19 vaccines, or maybe wish to wait to see for any potential long term side effects for your health profile before making a decision, you are welcome to decide for yourself.

Whatever be your decision, the general scientific & clinical consensus based on current evidence is that steps like masking, maintaining appropriate self-hygiene & physical distancing are proving effective measures in this fight against COVID-19. Do continue to follow these.

Choose wisely, people!!

NOTE:

WHERE DO TRANSPLANT PATIENTS STAND IN THIS WHOLE CHORUS ABOUT MASS IMMUNITY?

In all likelihood, transplant patients will have to remain in the “indirect immunity” group that benefits from more people getting a vaccine at a time. Let’s see why:

NATURAL INFECTION

Transplant patients are immuno-compromised. Getting infected is simply not an option. People like us, will always get aggressive symptoms, will need intensive healthcare facilities & are more likely to get critical.

VACCINE

Vaccines are of various types. Live-attenuated vaccines are NOT an option for those who are immuno-compromised. Inactivated vaccines need extensive testing on Transplant Patients before they can receive approval for clinical use.

Some mRNA vaccines may induce something called Cell-Mediated Immunity (CMI). This is brought about by a subset of immune cells called “Memory T-cells”. While CMI induces relatively long-term protection, fact is, transplant patients take medication to specifically suppress CMI. That is because CMI has a role in rejection of the transplanted organ. Even if the vaccine induces CMI specific to infection, the possibility of graft rejection is simply too high a risk to take.

Hence, arguably the best strategy for us Transplant Patients in terms of COVID-19 is to:

1️⃣ Proactively prevent infections

2️⃣ Scrutinize every aspect of any vaccine (the type & how it works) in consultation with respective healthcare providers

&

3️⃣ Choose health-logic as our preferred approach towards Immunity

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REFERENCES:

1. COVID-19 Immunity Duration | Medscape

2. COVID-19 Immunity Duration | NHS UK

3. “Immunity to COVID-19 could be lost in months” | The Guardian

4. “Smallpox: Disease, Prevention, and Intervention” | Training Course by CDC & WHO | Slide 17 

5. “Herd Immunity: Understanding COVID-19″| Immunity (journal) | Volume 52 Section 5 Page: 737-741

6. “Estimates of the reproduction number for seasonal, pandemic a d zoonotic influenza : A systematic review of the literature” | BMC Infectious Diseases  | Volume 14 Page 480

7. Different epidemic curves for Severe Acute Respiratory Syndrome (SARS) reveal similar impacts of control measures | American Journal of Epidemiology | Volume 160 Section 6 Page 509-16

8. Estimating the Reproduction Number of Ebola Virus (EBOV) during the 2014 Outbreak in West Africa | PLOS Current Volume 6