Herd immunity: the concept... and misconceptions around it

Updated: July 8, 2020


We are in the midst of COVID-19 pandemic and you must have been hearing a lot about herd immunity. This expression is often used as a likely preventive solution against the disease. Some are heard saying, now that COVID is spreading fast, no amount of social distancing will help, and so let herd immunity take over. It is said that when that is done, the virus will not be able to infect people.

Well, if you think of herd immunity on these lines, you are getting a sense of it but are too much off the mark. For making a clear inference about how the COVID-19 might eventually pan in different populations and how safe the non-infected people would remain, let us delve deeper into the subject.

Herd immunity is not a form of immunity. It refers to a situation when so many people in a population have developed immunity that the virus does not find its next prey, and thus does not spread to the people who have not been infected yet.

So, what happens is that when a very large number of people in a population have immunity against a disease, they act as a barrier against spread of the disease: when the virus or bacterium reaches them, it does not multiply in them and their coming in contact with others does not spread the disease.

Let's rephrase that. When a population has more and more people who have immunity, the spread of the disease slows down because all those who come in contact with an infected person do not contribute to multiplication of the disease. When the number of persons with immunity reaches a high level, the chances of that infected person spreading the disease to other susceptible persons become extremely low and it puts a stop to spread of the disease. Even if the disease spreads to some, it does not become an outbreak.

More immune people in population means
less chance of infection to others.

It is clear that herd immunity does not mean that if almost all in the herd (population) have immunity, the rest will automatically become immune to the disease. Depending upon a number of factors, their probability of getting the disease will be much less as compared to those in a population in which there is no herd immunity.

Herd immunity is especially helpful to very young children, the aged, people suffering from immune deficiency, those already suffering from a major disease and those who cannot be vaccinated because the immune ones reduce their chances of coming in contact with the infection.

What makes people immune, in the first place?

Immunity to an infectious disease develops when a person is attacked by the disease causing organism (virus, in the case of COVID-19) and the person's defence mechanism is able to fight back the disease with the help of special proteins called antibodies.

Some immunity comes to the child from her mother. Blood plasma transfusion from recovered persons also induces immunity. This type of immunity (=passive immunity) wanes over time and the person must develop own immunity when the borrowed antibodies expire.

Immunity can also be built by introducing attenuated (=weakened) or dead parts of the pathogen into humans through a vaccine. The human body reacts to these weakened or dead organisms and develops antibodies so that when the real organism attacks that person, he already has antibodies against that pathogen.

Generally speaking, once a person is vaccinated, he does not have that disease for a long time, sometimes throughout his life. However, vaccines are not 100% effective. Moreover, vaccines can be ineffective when an organism of a slightly changed structure (mutated) attacks a vaccinated individual.

Another fact to be noted is that vaccination is not equal to immunization. There are many reasons why a vaccine may not produce antibodies enough to fight an actual infection; these include age of the person, effectiveness of the vaccine, specificity and type of pathogen, the way the vaccine is administered, etc. In fact, some people - including experts - argue that the harm due to inefficacy, side-effects and complacency related with vaccination outweighs its benefits. However, let us go with the mainstream thinking that vaccination results, to a large extent, in immunization.

Let us now face some questions regarding the relevance of herd immunity to COVID-19.

Is herd immunity really a solution for COVID-19?

More than offering a solution, it is a fait accompli - something that will sure happen one day whether we work for it or not.

An understanding of herd immunity does guide us that when a vaccine is available, we need not vaccinate every individual but a certain percentage of population. With this understanding, we save a lot of money and time by rationing the resources. It might tell us when the restrictions to movement and mixing up can be taken back because the infection would keep falling on its own - but such a decison may not be desirable.

It is also to be noted that for determining the level of vaccination required, there cannot be a precise assessment because different groups of people keep getting together and separating. Methods have to be devised, including sample testing, to ascertain the quantum of vaccination required in different population.Thus, the level of vaccination required will differ from city to city and between urban and rural populations.

In small populations, it is being suggested that the entire population be tested for antibodies to find the proportion of people immune to COVID-19. It will be an expensive [even if samples are taken and not the entire population is tested], untested exercise that can be followed only by small and rich nations. It also ignores the 'background immunity' and a number of other factors that determine whether the population has the strength to fight the virus or not [discussed at the end of this post]. Maybe, if the virus does not mutate fast and people do not lose their immunity fast, this might measure the herd immunity of the population and generate specific recommendations to reduce the impact of COVID-19. Or it may not! (The previous sentence is proving prophetic: in a study in Spain, it has been found that despite wide-spread infection, only 5% of the population has antibodies against the virus and the recommendation is: do not let your guard down. Reference to the study can be seen at pt 4, Section C below).

What is the percentage of population that should develop immunity, before it is called herd immunity?

The minimum percentage or threshold for being called herd immunity differs among infectious diseases. In most cases, it is 60%-80% or so. If the disease is highly infectious, more people should have immunity before the herd immunity comes into effect. For example, measles needs 92%-95% immunity saturation before the population develops herd immunity. In the case of COVID-19, it looks to be in the range of 60%-70%.

Have we developed herd immunity against other viruses?

Yes. Human population has achieved sort of herd immunity against major fast-spreading infections. Take the example of chicken pox. Most people in most countries have been vaccinated against it or have developed immunity by having recovered from it. However, the virus has not been eradicated. So, a small number of children do get chicken pox because they were not vaccinated/ have an immune deficiency issue. Since most populations have developed herd immunity against small pox,  the disease can not spread fast from these infected children to all other susceptible children in that city/ region/ country to lead to an epidemic.

As per published research, herd immunity has helped checking many diseases at global level. It helped in eradication of small pox from the earth.

Once a population achieves herd immunity for the first time, is it safe from future epidemics from the same virus?

No, especially in the case of fast-mutating viruses (=their genetic structure changing fast). If the virus causing that disease has mutated, the disease will spread even among those immune to the earlier strain of virus.

Herd immunity can also be broken if a large number of infected people suddenly enter a population that earlier had herd immunity. Similarly, if a large number of susceptible persons (who have not been vaccinated or recovered from the disease) enter the population, it would compromise the herd immunity of that population.

Vaccinated or recovered persons' immunity can go down over time, and healthy people's immunity can suffer due to health issues and age. Such people then do not act as barrier to the spread of the disease.

It should be a better strategy to help develop herd immunity quickly rather than delay it by social distancing, etc. No?

No way. The coronavirus behind COVID-19 is a fast spreading virus and is not as safe as common cold virus; it directly or indirectly kills 2%-4% of those it infects. That means, in dense populations or among highly susceptible groups, it might kill a very large number of patients, and fast, before the herd immunity takes shape.

A few countries have tried a lax approach towards COVID-19 and have suffered. In Sweden, they experimented with letting people carry on with their work without a lockdown. Though Sweden is a sparsely populated country, has very good health infrastructure and most of its population lives in separate apartments (because of small families and no joint family system), the experiment seems to have failed. As of early June '20, the death rate in Sweden is much more than its neighbouring countries. While the graph of infections and deaths is climbing down from a peak in many nations in Europe, the graph of Sweden is on a constant rise. Since even a fraction of the population has not been infected and recovered to reach the herd immunity threshold, it is likely that the graph will keep rising for a long time.

Of course, Swedish government's reason for not imposing lockdown was NOT to develop herd immunity but to keep the economy running. Yet, it shows that letting people mingle so that the population gains from herd immunity will not work. Sweden can afford this but densely populated countries such as India cannot - here, the number of people getting infected without social distancing could rise by millions every week.

In Brazil and Pakistan, the laxity is leading to fast rise in infections and deaths. Things might get worse in the coming days.

On the other hand, countries that enforced lockdown strictly from the beginning have been able to tackle the epidemic better. Among less populated countries, New Zealand, is a fine example of checking COVID; among highly populated countries, India's example can be cited. A couple of studies are reported to have estimated that India would be having over 3 million corona +ve cases  rather than about 200 thousand that it had by end of May '20, if it had not implemented a lengthy lockdown.

A research paper quoted at the end of this article gives a statistical basis to the reasoning that not having any social distancing and other measures to check spread of the disease would lead to overall higher level of infections than when these measures are in place. This gives another reason why the thinking that herd immunity protect the society is flawed.

A nation-wide study in Spain (details at the end of this article) has found that infection is not producing enough immunity in the population and the immunity once developed too seems short-lived, giving a clear message that social distancing measures and efforts towards finding a drug/ vaccine must continue in right earnest.

How fast can we achieve herd immunity in the case of COVID-19 if there is no vaccine?

Building natural herd immunity level will take a very long time. In New York and London, only about 19% and 18% of their population had acquired immunity by the end of May '20, after two full months of  rampaging infection. From this level, it will take years to reach the threshold of 70%. 

However, there is a hypothesis that herd immunity may develop among different populations much earlier than getting 70% population infected (and recovered) and vaccinated. [For details, see the discussion in B section of this article.]

So, herd immunity is what we want to achieve in the population, and is not a preventive solution to COVID-19? Say that again.

Yes, that is it.

The spread of COVID-19 in its present form needs to be moderated, especially in densely populated areas and where medical facilities are not good enough, through sanitization and social distancing. If the virus itself becomes weak due to mutation in the coming months (which is not too likely) and medicines are discovered to effectively treat patients, these help to that extent. If not, we will have to wait for vaccines to come out, and then carry out mass vaccination. The vaccination will have to be to the extent that the population is saturated with recovered and vaccinated persons to the extent of about 70%: call it herd immunity if that helps!


Immunity beyond herd immunity: let's discuss finer points

Let us discuss how immunity against infectious diseases works in individuals and societies, and what is the scenario with regard to COVID-19 in India.

Immunity is not a one-off affair. Except when it is introduced through vaccination, it can develop and then dwindle; it can be partial; it may be influenced by other factors in a dynamic way. The immunity, when it develops, may not last long as happens in the case of tetanus. For COVID-19, the possibility is that it might last 2-3 years, going by the closely related disease, SARS.

How a human body reacts to attack by a virus is multifarious. In case of respiratory diseases like COVID-19, the virus starts with attaching itself to the nasal or oral mucous layers. It has been studied that the mucous secreted by the walls of these air channels itself contains chemicals that neutralize or reduce the damaging power of microbes. By the time the infection reaches lower respiratory track and lungs, the body has had enough time to prepare itself against the disease. The resistance created by the body is often enough to weaken the virus and let the infection die, even though the body did not have immunity in pure technical term (i.e. it had not created antibodies). In such a case, practically the individual showed immunity to the virus, but may be susceptible to further attacks.

There are many factors that build resistance among Indian people towards infectious diseases, more than Europeans and Americans. The major ones are:
  • Exposure to infections due to hygienic conditions and habits. The constant microbial pressure on the human body makes it more resistance to infections than when living in highly sanitized conditions.
  • Indians' food habits. Indian food usually contains turmeric and spices. These have a number of anti-microbial and therapeutic properties, including strengthening the immune system and reducing the severity of symptoms.
  • Lack of medical facilities and the common man's attitude to avoid visiting a doctor/ hospital may indirectly help build immunity in the society though these might be very risky at the individual level. Since most people have a level of resistance, they may have a low level of disease - many times without being diagnosed - and will recover even without what can technically be called 'immunity'.
  • A large proportion of the population lives in open air and breathes fresh air. In western countries, people are used to living/ passing a lot of their time in air-conditioned spaces.
On the other hand, some factors go against Indian population, e.g.
  • High population density. When people live close together, share toilets and bathrooms, are in crowds in markets and public transport, and have frequent religious congregations, there are more chances of quick spread of pathogen.
  • Poor hygiene, poor health infrastructure and high likelihood of not getting treated for common diseases (which are mentioned above as factors that might increase immunity), together with poor diets can make people perpetually sick with minor and major diseases. Such people can lose their overall body strength and immunity against diseases. In fact, it has been established that when people with some type of diseases get COVID-19 infection, complications arise and probability of mortality becomes high.
It is notable that the factors mentioned above decide the overall level of 'background' immunity present in a population. Please also note that all populations have some people who have a high level of inbuilt immunity. They may constitute 40-50% or even more of the population; this is why the human race survives. If this level of immunity were not there, we all would be dying with immune deficiency-led diseases in millions every month.

Another point worth mentioning is that the faster a population gets herd immunity, the better. If it takes a long time to reach the level of 60-70%, by that time, many erstwhile immune persons would lose the potency of their immunity and the pathogen might also mutate into a different one.

Please read the above para with my answer to whether herd immunity can really be achieved against COVID-19. The two concepts together lead to a conclusion that yes, herd immunity will be achieved in many populations without a very large number of people getting infected. Moreover, with this level of hidden/ background immunity, the mortality rate in many populations will not be too high.

Let us look carefully at the pic below. It has three graphs. The numbers given against each graph are hypothetical, but infections like COVID-19 follow these patterns in all populations. The differences among populations occur depending upon many factors such as (i) whether people observe social distancing and other ways of avoiding infection, (ii) proportion of old/ ailing people in the society, (iii) overall immunity levels of the population.

Please note that The last one - background immunity - is very important because if the base is high (say, 55% instead of 50% as given in the above pic), the disease will plateau sooner because only 5-15% more people will need to get immunity by recovering or vaccination, before the population gets herd immunity. If the base immunity is high, the population will also benefit because the herd immunity will have come fast, before changes occur in the virus or the induced immunity (that brought in due to recovery or vaccination) declines over time.


This section has material for further discussion, and also new inputs.

1. A research paper published in Science on 23rd June 2020 validates these arguments given in the present article: 
  • The actual required infection need not be 60% or so to reach levels of herd immunity; in fact, these need to be much less.
  • There are wide variations among populations that determine what level of immunity leads to herd protection.
The paper defines herd immunity as the level of population immunity at which even if all preventive measures are removed, the rate of infections will keep going down.  It gives a model in which population is divided into different cohorts based on age and activity, and comes out with the hypothesis that no or light level of preventive measures would lead to high level of infections while moderate level of preventive measures would dampen the infection curve. On the other hand, severe level of preventive measures would lead to a second wave of infections when the preventive measures are removed. 

2. America's Center for Disease Control (CDC) chief said on June 25, 2020 that, based on serological surveys, it appears that coronavirus may have infected ten times of the number of people reported to be positive. About 5-8% of the population might have already been infected so far. That takes the number of possible COVID infected persons in the US comes to about 24 million. 

That explains why the cases are still rising, though US has been very proactive in testing for coronavirus from the beginning. I is clear that the total percentage of people (infected and recovered + those with high levels of innate immunity) who can act as wall against spread of the virus is less than the herd immunity level, yet.

Moreover, as explained earlier, the distribution is very varied, with central and northern states having lesser infection as compared to peripheral states in east, west and south. The populations in which the infection levels are very low will catch up in the coming days.

Coronavirus infection in the US: 25th June, 2020 (CDC)

3. In a study published in the Lancet, it has been found that in Spain, one of the worst-affected European nation, infection has not led to significant immunity in the society to give it the protection of herd immunity. Only 5% of the population was found to have developed antibodies against the COVID virus. In addition, the study also found that a third of those who have developed antibodies against the virus (and thus can be said to have immunity) are asymptomatic.

The study concludes: Despite the high impact of COVID-19 in Spain, prevalence estimates remain low and are clearly insufficient to provide herd immunity. This cannot be achieved without accepting the collateral damage of many deaths in the susceptible population and overburdening of health systems. In this situation, social distance measures and efforts to identify and isolate new cases and their contacts are imperative for future epidemic control.

4. An explantion on how immunity is defined in this article: Immunity is defined in many ways, e.g passive or innate or active; natural or induced; caused by special blood cells or proteins; etc. It need not always be through antibodies or special proteins working against a pathogen. In this article, immunity is mostly used in the context of COVID-19 and refers to the capacity of human body to ward off infection from the time the virus attacks the upper respiratory tract.