What Exactly Is Herd Immunity?

What Exactly Is Herd Immunity?

Herd Immunity. Population Immunity. Herd Protection. You must be hearing these terminologies ever since the hunt for a solution to control the ongoing Covid-19 pandemic started. Let’s try to understand the basics of herd immunity, its threshold, and how vaccination has worked in the past.

Crux of the Matter

Population Immunity
It is a concept seen by epidemiologists, in which a population can be protected from a certain virus if a threshold of vaccination is reached.

Alternatively, it can also be achieved when individuals become immune by contracting and then recovering from an infection naturally. This provides indirect protection to those who are not immune to the disease yet.

Can You Give An Example?
Yes. For polio, if 80% of a population is immune to the virus, 4 out of every 5 people who encounter a sick person won’t fall ill. Thus, the community spread of the disease would decline and then disappear.

Does Herd Immunity Threshold Vary?
Yes. Herd immunity for high threshold disease like measles required about 92 – 95% of a population to be vaccinated. For a low threshold disease like influenza, 33 – 44% were vaccinated.

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How Do Vaccines Function?
Vaccines train our immune systems for the time when we are exposed to the actual disease, by creating Y shaped proteins called antibodies, which fight off the virus, without making us sick. Vaccines can either be prophylactic (to prevent the effects of a future infection by a natural pathogen) or therapeutic (to fight a disease that has already occurred like cancer).

How Can We Not Fall Sick?
A vaccine isn’t harmful to our body as it just contains an agent, which is either the weakened or killed forms of the microbe/its toxins/its proteins.

Herd Protection From Once Common Diseases
Before vaccines were developed, kids with weakened immune systems could still get infected. Diseases like Measles and Chickenpox are now rare in communities with high vaccination coverage. 

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Where It Doesn’t Work?
After viruses mutate and create new variations, the antibodies from a previous infection provide short-time protection. Also, herd immunity applies only to contagious diseases like flu and not infectious diseases like Tetanus.

  • The modern word “immunity” comes from the Latin immunis, meaning exemption from military service, tax payments, or other public services. The term “immunes”, is also found in the epic poem “Pharsalia” written around 60 B.C. by the poet Marcus Annaeus Lucanus to describe a North African tribe’s resistance to snake venom.
  • Variolation used to immunize individuals against smallpox (Variola) with material taken from a patient or a recently variolated individual, in the hope that a mild, but protective, infection would result. Material from infected patients was inserted in people through skin scratches which resulted in immunity against smallpox. The method was first used in China, India, and the Middle East before it was introduced into England and North America in the 1720s.
  • Pulse Polio is an immunization campaign established in India to eliminate polio by vaccinating all children under the age of five years. Tamil Nadu was the first Indian state to become 100% polio-free through the pulse strategy.

What Is Herd Immunity?

What Is Herd Immunity?

With recent studies showing more prevalence of the Covid-19 virus than the official records, experts have pointed to the possibility of herd immunity springing up soon. Let us understand what herd immunity is and how it has fared in several nations.

Crux of the Matter

Herd Immunity
The spread of a disease decreases or stops if a sufficient number of people develop the requisite antibodies. These antibodies are formed either through infection and then recovery, or through vaccination. After a sufficient number is achieved, the disease becomes unable to spread through these people which breaks the chain of the disease spread. The infection then becomes sporadic or eventually stops.

India Heading For Herd Immunity?
Sero Survey
A sero survey was recently conducted on 21,387 people in Delhi. It was found that 23% of the tested people had developed antibodies against Covid-19 through exposure to the virus (vaccine against Covid-19 not available yet).

The population of Delhi s estimated to be 1.9 crore, and 1.25 lakh cases were reported as of 22nd July. However, according to the sero survey, the actual number of cases in the city can be around 40 lakh, which is much higher than the official number.

Thyrocare, a private pathology lab recently conducted a survey of 60,000 samples, collected from 600 different Pin Codes across India. The study showed that 15% of the total population of India, or more than 20 crore Indians have developed antibodies against Covid-19 after contracting the disease at some point.

Expert’s Opinion
Jayaprakash Muliyil, chairman of the Scientific Advisory Committee of the National Institute of Epidemiology, expressed positive opinion over the recent surveys. He claimed that herd immunity would be achieved once 50-60% of the people get infected and recover, and that the immunity lasts “for a long time, maybe even for a lifetime”.

Cons Of Herd Immunity
Completely unchecked spread of Covid-19 will shoot up the mortality rate. Notably, several reports are claiming that immunity against Covid-19 is temporary, which might increase the risk of another wave of a pandemic. There is also a conflict over the required percentage, with experts’ claims ranging from 50% to 75%.

Herd Immunity Models Across The Globe
Sweden did not impose a lockdown, and marked masks and social distancing as voluntary measures. The government allowed schools, restaurants, gyms, etc to remain open, and claimed that the country would have better immunity when the “second wave hits”.

While the government closed restaurants and bars, people were allowed to stay outside without any necessary work. The government also “advised” social distancing rather than making it mandatory.

While the country fared better than Sweden with 356 deaths per million, overall it fared worse than neighbouring countries like Germany, even with a higher percentage of young population.

The government initially allowed all schools and major social events to remain functional. Prime Minister Boris Johnson claimed that evidence exists to show how lockdowns do not have any major impact on controlling the spread of Covid-19. The government also claimed that “it’s not possible to stop everybody getting it [Covid-19] and it’s also actually not desirable because you want some immunity in the population”.

The government applied lockdown within a week of the initial declaration, shutting all non-essential businesses and major events. The death rate of the country reached 670 per million, placing it amongst the top 5 of the world. In comparision, US has a death rate of 442/million, whereas India has death rate of 22/million.

  • Sweden has one of the most highly developed welfare states in the world. According to a 2012 OECD report, the country had the second-highest public social spending as a percentage of its GDP after France, and the third-highest total social spending at 30.2% of its GDP, after France and Belgium.
  • To date, two diseases have been eradicated using herd immunity and vaccination: rinderpest and smallpox. Eradication efforts that rely on herd immunity are currently underway for poliomyelitis, though civil unrest and distrust of modern medicine have made this difficult.
  • Ring vaccination is a strategy to inhibit the spread of a disease by vaccinating only those who are most likely to be infected. Herd immunity is integral to successful implementation of ring vaccination.