• Shalini Guleria

Understanding Herd Immunity and COVID-19

Since the beginning of Corona virus pandemic there have been many questions from general public in regards to how does a virus work? How does a vaccine work? What is herd immunity? Etc. This article focusses on providing you with a general overview on these questions. So let’s begin!

There is a huge misconception regarding the mode of infection between viruses and bacteria. Usually people think that a viral infection could be treated by antibiotics (similar to a bacterial infection) but that is not true. Viruses operate quite differently to a bacteria, in terms of mode of infection. Unlike bacteria, viruses require a host such as human or animal to multiply and they cause infection by entering and multiplying inside the host’s healthy cells. Due to such a mode of infection, viral infections can only be tackled by the person’s own immune system or a vaccine and not with antibiotics. In many cases viral infections subside in time as the immune system fights the infection. The only thing that can be done with such infections is to manage the symptoms, allowing the body to slowly heal. There are different forms of viral infections that depend on the viral strain, some are as follow: common cold, bronchitis, chickenpox, measles, HIV/AIDS etc.

Depending on the severity of infection, vaccination is suggested. Many viral infections such as influenza are recurrent and require a vaccine every year to help with disease management. However viral infections like chickenpox only require a vaccine to be administered once to control the disease. So how does vaccination work? Well a vaccine can be eluded to a training course for the immune system or a warm up before an intense workout. When a foreign invader such as a virus attacks the body, the immune system is the first line of defence where the immune cells respond by producing antibodies. These antibodies have the ability to fight the foreign invader also known as the antigen and protect against further infections. The immune system is made of variety of cells that have a specific role in fighting disease, some immune cells’ function is to memorise the antigen that once invaded the body to prepare for the next fight if necessary. So vaccine provides the body with a chance to get familiar with the antigen, in its partially active form to be ready if an infection occurs again. Unfortunately, the first time the body faces a particular infection it may take several days for the antibody response to completely activate. This is seen in new infections such as in SAR-CoV-2 or influenza where the infection can spread so rapidly and kill the person before the immune system can fight back.

Virus can transmit between hosts with various degree of efficiency which is indicated by the parameter called basic reproductive number (RO). This parameter provides a quantitative estimate of the transmission efficiency, it is the mean number of secondary infections that one case would produce in a fully susceptible population. In order for an outbreak to occur the RO needs to be higher than 1and in case of SARS-CoV-2 pandemic the average RO value is 3.28. You might be surprised that a virus can cause some serious outbreaks at global scale despite having a low transmission efficiency, but the short generation time of some viruses can cause its spread. It is also found that the viruses with a high RO number are antigenically stable and contain a single or a limited number of serotypes. For such viruses vaccines are highly effective e.g. Rubeola virus (causes measles), due to a low chance of escaping mutants. In contrast, viruses with a low RO such as influenza virus are highly diverse and can have new antigenic variants that are consistently emerging to replace the pre-existing strains. Such viruses show comparably high mutation rates and need to be controlled by altering the vaccine to accommodate for seasonal outbreaks.

So what is herd immunity? Herd immunity is also known as the heard effect and it is where when a large percentage of the population has become immune to an infection, whether it is through previous infection or vaccination. It is a form of indirect protection, which provides a measure of protection for individuals who are not immune. This means that greater the number of immune individuals in the community, the smaller the probability of non-immune individuals getting in contact with infectious individuals. In turn allowing the non-immune or most vulnerable individuals to be protected from the infection. Individuals can gain immunity either by recovering from an earlier infection or through vaccination; once a certain threshold is reached herd immunity gradually eliminates the disease from a population and this can be achieved worldwide. However the concept of herd immunity only applies to contagious diseases that can be transmitted from one person to another. In the case of corona virus, it is suspected that eventually herd immunity will be achieved, especially once a vaccination is made however until then the most vulnerable people must be protected.

Governments around the world are applying various models to counter this new virus but at this stage no one really knows which model would work the best. For several countries, a complete lockdown has worked in terms of lowering the community transmission cases, however some countries are functioning with fewer restriction such as Sweden, where they believe herd immunity could be reached as soon as May. Due to novelty of this virus, at this stage it is hard to say which approach is the best, considering the economic strain the countries will face after all this is over. There are many questions out there, like what will happen once the countries get out of lockdown? Will there be another spike in cases and would everyone need to shut down again? Or if a country does have everything under control after the lockdown will international travel even be possible, as majority of the cases were transmitted via international travel. From a scientific view point there isn’t much data that could predict the future of this outbreak, as mentioned earlier viruses do tend to mutate which could be for the worse or better. So right now all we can do is stay safe and protected, follow the rules and wait out this pandemic.


  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3782273/pdf/WJV-1-131.pdf

  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3171704/pdf/sinf43-683.pdf

  • https://www.livescience.com/32617-how-do-vaccines-work.html

  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3906279/pdf/hvi-9-1763.pdf

  • https://www.aljazeera.com/news/2020/04/sweden-ambassador-stockholm-reach-herd-immunity-200427072044548.html

Views expressed are solely those of the author.

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About the Author

Shalini Guleria is currently holds a Masters in Tissue Engineering where her research focused on developing better treatment and detection techniques for Cancer. She has been associated with Scion Research, New Zealand and holds a Bachelor's Degree in Chemical and Biological Engineering from the University of Waikato, New Zealand. Shalini has won two consecutive national awards at the prestigious Sir Paul Callaghan Eureka Awards for engineers and scientists. Apart from sciences, she is also a highly talented artist.

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