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  • Writer's pictureEdited by: Atif Zafar, MD

What is Herd Immunity and Why All Americans Need to Know About It in The Era of COVID19.

As Corona-virus surges again in the US, has launched a Public Health Awareness & Education Initiative for the Americans.

As a physician, I can’t even count the number of times I was asked on whether the lock-down of the entire country, paralyzing everyone and everything, was, is and will be an effective strategy or is this a fear-driven-hoax? There is no doubt that many people disliked this lock-down concept or were confused whether the lock-down makes sense. There was and continues to be astounding confusion. The following article is compiled by physicians, with input from infectious disease specialists and public health experts, incorporated in this relatively short piece. The hope is that our general public will get a neutral scientific perspective to an otherwise politicized and confused issue at hand.

Let us begin with the basics.

What Is Herd Immunity?

Herd Immunity also sometimes called herd effect, community immunity, population immunity, or social immunity refers to a situation where a large group of the population becomes immune to an infectious disease thereby stopping the disease from spreading. The eventual outcome of herd immunity is protecting those who are not immune.

There are two ways it can be achieved:

· A sufficient portion from a population contracts a disease and develops immunity against it (naturally acquired active immunity)

· A sufficient portion from a population is vaccinated in order to impart immunity against the disease (artificially acquired active immunity)

For instance, if 80% population in a community is immune to a contagious disease, eight out of every ten people who come in contact with an infected person will not get the sick and neither will be able to transmit it to the 2 vulnerable ones.

Generally, depending upon how contagious the disease is, 70 to 90% population needs to be immune in order to achieve herd immunity. For example, measles is a highly contagious disease. Likewise, SARS-CoV-2, when compared to SARS-CoV-1, is more contagious but less deadly.

Has Herd Immunity Helped Us in the Past?

Herd immunity has been used to contain a number of diseases that were highly infectious in the past. Some examples include polio, measles, mumps, rubella, and chickenpox that used to be very common once upon a time but now rarely exist. Thanks to vaccines that helped us achieve herd immunity. The outbreaks of these diseases are now rare in the US and usually confined to communities with insufficient vaccine coverage and thus lacking herd immunity.

For diseases without a vaccine, immunocompetent adults usually survive the infection and become immune. It can, though, still infect kids and older populations with a weakened immune system, therefore, staying in the community and causing morbidity (disability) and mortality (death).

However, it should be kept in mind that viruses and bacteria are continuously evolving and mutating and even if you are immune to one strain of a virus, another one can cause disease. This is the reason you are asked by health professionals to get yourself vaccinated as soon as the flu season arrives. It prevents an outbreak of a new strain thus protecting the vulnerable population and significantly cutting down the mortality rate. In this way, herd immunity does not only protect you but also the susceptible population whose compromised immune system does not allow them to get vaccinated, for example, very young kids, HIV positive patients, pregnant women in some cases, and those with a history of a severe allergic reaction.

Can We Achieve Herd Immunity with COVID-19 – Without Causing Serious Harm?

Considering there is no vaccine for SARS-CoV-2 yet, the only way to achieve herd immunity is to allow a large number of populations from around the world to become infected and subsequently develop immunity against it. However, there are serious challenges associated with this task. Keeping an unpredictable virus under control while the herd immunity is developing in a specific society is and will continue to create a burden on healthcare systems around the world.

Unless a town, city or a country can do a step-wise approach where the least crowded areas and least vulnerable people are gradually asked to resume normalcy (while keeping masks, sanitization and social distancing as safety measures), while having a mechanism to measure the outcome for the next 15-18 days. Only a controlled, calculated approach would provide a fool proof way to gradually open the society. But there is more to it.

According to the data available regarding the infectiousness of the virus, at least 60%-70% of the population must be immune in order to create herd-immunity, protecting the vulnerable 30%. In addition, there is no data providing information on whether or not you can develop COVID-19 more than once. Just like every other pathogen, SARS-CoV-2 (another name of COVID19) also has the potential to mutate resulting in the formation of a new strain that can start an outbreak all over again. All these reasons make the case for a slow, controlled and calculated approach of reopening saner than any other strategy.

Let us assume that we abandon the current protective measure i-e social distancing, frequent sanitization and disinfection, and the use of protective masks, the infectivity of the virus suggests that it will be able to infect the proposed proportion of the population in a matter of few months. However, we must understand that there will be repercussions associated with the abandoning of preventive/protective measures:

One, our health system gets overwhelmed trying to manage critical cases:

And two, once a rapid and rabid spread of the virus initiates, it is hard to predict whether it will “behave” and if we will be able to control it from spreading to the vulnerable population. Meaning, controlling things when they are unmanageable. Folks with severe asthma or advanced COPD (chronic obstructive pulmonary disease, commonly called smoker’s lung), heart conditions, diabetes & other immunocompromised states will naturally get exposed causing more critical illnesses in this population. In summary, abandoning the precautionary measures can be a hit or miss depending on various factors (proportion of older population in that region, burden of morbid illness in that group, intermingling at airports, train stations and night clubs, etc.) in that specific cohort.

Comparing herd immunity in flu virus compared to COVID19:

When discussing and comparing the regular flu virus with the COVID19, we need to be aware that the current mortality rate of COVID-19 is higher compared to the common flu, more so in the older and vulnerable population (considering the differences in how CDC measures these numbers for the two viral illnesses)

Are there countries or states that can implement herd immunity?

Herd immunity can be given consideration in countries whose healthcare system will be able to efficiently manage those in need of ventilation and critical care but for the United States, where the population to ventilator ratio is less than many developed countries, it seems to be out of the question. In the next article, we will specifically discuss the case example of Sweden and assessing how this concept of herd immunity when applied in real life, can play out.

Other factors to consider:

Before we wrap up this discussion, we should be wary of some more issues. There are still so many unknowns that need to be explored with regards to this novel coronavirus. We do not yet know the far-reaching impacts of the virus and the disabilities it can potentially cause leading to life-long health issues. For example, folks who we are calling asymptomatic COVID19 carriers at this time, may end up having some cognitive or psychological symptoms a few months later. Probably and hopefully not. However, in the public health arena, these factors must be considered when government decisions are being made. Moreover, the 1918 flu pandemic with high mortality should serve as a lesson, that just trying to get over with it is not the best idea.

If I am healthy, why shouldn’t I just get infected and get over with it?

As hard as social distancing and protective mask usage appears to be right now, this is the only choice to slow down the impact from COVID19 in a society or country. Yes, many of us will get infected eventually, so you may be asking if many of us will get infected anyway, why not just get it over with? Here is the answer: a slow, gradual progression of the disease in a society can be handled in a better way by the healthcare system and the public health system compared to a rapid and rabid penetration of that same infection. Let’s assume, one of us has a new diagnosis of cancer. Would you rather choose to have an aggressive, invasive, infiltrative, widespread cancer in your body or would rather prefer a non-aggressive, slowly progressing type of a cancer? For once, think of your body as our country (for non-nationalists, consider the world) and your cancer as a group of people getting infected. Need me to explain more?


The WHO had predicted a second wave which we have started to see in various parts of the country and the rest of the world. There is an anticipated third wave of the COVID-19 as well. The lockdown of the high-risk areas, such as gyms, underpasses, tramways, nightclubs will have to be manipulated based on the numbers in that region. While keeping up with precautionary (masks, sanitization, social distancing) measures – especially in nursing homes, hospitals, airports, centralized transportation areas. This alone will allow us to keep the infection at a lower level in the community ensuring that healthcare facilities are available to those who need them the most at any certain point. In simple words, our analysis favors the idea of gradual reopening with emphasis on protecting the vulnerable population.

For questions and comments, email me at takes no responsibility of the content and its implications.


Atif Zafar, MD is a physician, entrepreneur, and the author of the book "Why Doctors Need To Be Leaders". He can be reached at

The physician-leadership book is available on Amazon:


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