Herd Immunity

WRITTEN BY; PALLAVI VIVEKANAND KOLHAR CLASS OF 2025

This blog addresses about herd immunity, how to achieve it, challenges faced during COVID -19 and the importance of herd immunity microbiologically.

WHAT IS HERD IMMUNITY?

Herd immunity or community immunity is when majority part of public is immune to a specific disease. If majority of people are resistant to the cause of the disease such as corona virus, herd immunity saves patients who are immunologically weak and can’t resist by themselves.

HOW DO YOU ACHIEVE HERD IMMUNITY?

There are 2 ways to achieve herd immunity.

• You can build resistance organically when your body comes in contact with bacteria or virus. This makes antibodies to survive the infection. Your body will defend against another infection as well. This is how zika virus outbreak stopped in Brazil, two years after the outbreak began. 63% of the population were exposed to zika virus.

• By using effective vaccines which would induce herd immunity. This decreases disease prevalence and also decreases its mortality and morbidity. Herd immunity not only protects immunologically weak individuals but the community in a majority is protected.

WHAT WERE THE CHALLENGES TO DEVELOP HERD IMMUNITY FOR COVID -19?

Main obstacle to develop herd immunity was that the virus which caused the disease was novel, which meant nobody had gotten infected from this. Another potential barrier to researchers were determining how strong the immune system should be to survive this virus and how long it lasts.

According to WHO, herd immunity against COVID-19 should be achieved by protecting the community through vaccination, not by exposing them to the virus that causes the disease.

Controlling SARS-CoV-2 and its new variants presents a much more formidable challenge. Like influenza, SARS-CoV-2 mutated continuously into its modified new variants that can escape the immunity which was derived from infections and vaccines. It can also be transmitted asymptomatically and without pathognomonic signs which would be impeding public health control.

SARS-CoV-2 appears not to substantially engage the systemic immune system, as do viruses such as smallpox, measles, and rubella that consistently have a pronounced viremic phase. Moreover, no vaccination or infection appears to promote a prolonged protection against SARS-CoV-2 in majority of the people. Substantially, the public health community has found substantial resistance to take measures to control the pandemic of SARS-CoV-2 by wearing mask, vaccination, maintaining distance and other interventions.

MECHANISM

Individuals who are immune or who are immunologically strong serves as a barrier in the transmission of disease, reducing or preventing the transmission of disease to others. A person’s immunity could be adaptive (through natural infection) or artificial (such as vaccination).

When the majority of the population becomes immunized it is termed as herd immunity threshold (HIT) or herd immunity level (HIL). The disease can no longer make any existence in the population, ceasing the transmission and becoming endemic.

Herd immunity works on the principle that within a population not each individual needs to be immune to cease disease transmission. In a community where critical part of population are immune, those who are not immune are indirectly saved or protected. Here's how it works:

• Infected individual will dominantly come into contact with immune person.

• This disrupts path of transmission of disease

• Hence the disease has very less chances to transmit to non-immune beings or immunologically weak person.

The effectiveness of herd immunity majorly depending on the disease having ability to spread - vaccine efficacy Virulence of disease - it is population dynamics and density.

For an example measles outbreak occurred in a population or community. If 95% of the population is protected against measles (it is a highly transmissible disease) the measles spread will be significantly reduced. Because the 5% people who aren't protected or vaccinated or immunized will survive against the disease indirectly because the disease pathway had been disrupted for transmissibility.

IMPORTANCE OF HERD IMMUNITY IN MICROBIOLOGY

Under the view of lens of microbiology, the herd immunity majorly reduces the impact of survival and multiplying of the disease causing microorganisms, as hosts to infect are decreased. The spread of such microorganisms are reduced proportionally and thus ceasing the transmission of disease in a community.

The study of community immunity plays an important role to microbiology. It gives insights into the dynamics of contagious disease transmission which also influences the strategies of vaccine efficacy and development. Vaccines which make immunity without illness to a patient plays a major role in attain the herd immunity.

Microbiology and its lab research is important in producing vaccines against disease causing strains of microorganisms. Its microbiological surveillance and genetics permitting the identification of strains of circulating pathogens are necessary for the production of vaccines as in the cases recorded of Influenza vaccine.

So, elaborated herd immunity is important in the field of microbiology as this

• Draws light on disease and its dynamics and influences the strategically development of vaccine

• This even allows the identification of disease causing strains.