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Outbreaks of infectious diseases have been recorded throughout history. Currently, SARS-CoV-2, a contagious virus that causes the disease called COVID-19, is impacting individuals worldwide. Many countries have set their sight on an effective vaccination program to end their infectious disease outbreaks. The term “herd immunity” has been mentioned in association with ending the COVID-19 outbreaks. So, how do you reach herd immunity and how does herd immunity work?

How does herd immunity work?

Herd immunity, or community immunity, refers to the indirect protection experienced by susceptible individuals (i.e., those who are not immunized) when enough individuals in a population have become immune to a disease, reducing the spread of the infectious disease in that community.

Reaching herd immunity works to lower the risk of susceptible individuals being infected and developing a potentially life-threatening disease since the infectious disease is less prevalent in the population. This is especially beneficial for individuals at higher risk for serious complications from infectious disease.

The number of people that it would take to reach herd immunity – the herd immunity threshold – depends on a variety of factors at work. Pathogens vary in their infectiousness which impacts the proportion of the population needing to be immune before the protective effect of herd immunity is realised.

Measles, a highly contagious disease, needed at least 95% population immunity before the spread of the disease was considered to be effectively eliminated in countries such as the USA and Canada.

Other factors that are important to consider in relation to the efficacy of herd immunity are the strength and the duration of the immunity. For some infectious diseases, such as measles, acquired immunity lasts a lifetime, whereas the immunity for some other infectious diseases will gradually diminish.

For infectious diseases with weaker or shorter durations of immunity, it is possible for outbreaks to occur occasionally. In this case, an immunity boost may be required every so often to ensure that a large enough fraction of the population is immune to reach the herd immunity threshold.

How do you reach herd immunity

There are two ways for herd immunity to be achieved: through infection and recovery or through vaccination. Both of these methods generate immunity in individuals, allowing herd immunity to be established when there are too few susceptible individuals for the infectious disease to successfully spread in a population.

Whether an individual has become immune through infection and recovery or through vaccination, their immune system has become trained for fighting off the virus to protect against the infectious disease. This means that an individual has developed an adequate immune response to the virus if they should subsequently be infected.

Relying on individuals to become infected and recover at a population level reaching the herd immunity threshold is generally regarded as infeasible and costly for society. For example, in order to reach the herd immunity threshold for the novel SARS-CoV-2 virus, some estimates claim 67% or more of the population need to become immune.

Without a vaccine, reaching the estimated herd immunity threshold would likely burden healthcare systems and result in a higher COVID-19 death rate. It is also likely that it would take a long time for a sufficiently high proportion of the population to become immune from natural infection.

These high costs associated with the infection and recovery method of achieving herd immunity makes it undesirable in many scenarios.  Further, due to the novelty of SARS-CoV-2, it is not yet clear the duration of the naturally acquired immune response in individuals who were infected and recovered.

If the immunity to SARS-CoV-2 is short-lived, then it would become more difficult to reach the herd immunity threshold and it is possible that an individual would need to be re-infected and recover several times in order to boost their immunity as it diminishes over time.

This strategy of achieving herd immunity also risks an “immunity gap” if only some regions within a country reach the level required to obtain the protective effect of herd immunity but others do not. In this case, outbreaks could occur if there is not uniform herd immunity across a population. A similar effect could occur if herd immunity is not reached in enough countries across the world with this method of infection and recovery.

The second method of reaching herd immunity is through vaccination, in which an individual is able to prepare their body’s immune response for a future infection without experiencing the effects of the disease.

When enough people are immunized from the infectious disease through vaccination, the herd immunity effect occurs, protecting those who are susceptible to the infectious disease (i.e., those who cannot be vaccinated).

In some cases, such as smallpox, vaccination programs have even led to the eradication of the infectious disease. If the immunity for an infectious disease wanes over time, it is possible for vaccine boosters to be distributed to maintain the herd immunity. 

SARS-CoV-2

Clinical trials for numerous potential COVID-19 vaccines are underway; some of which have delivered sufficient safety and efficacy results for usage approval in certain countries. In some countries, vaccination programs have already begun. Initially, the production and delivery of vaccines for COVID-19 may be limited, with vulnerable people and essential workers likely being vaccinated first.

Until a high enough proportion of the population is immune from COVID-19 from vaccination, social distancing measures, mask wearing, and proper hand hygiene are recommended by health professionals to slow the spread of COVID-19. Further, it is important for adequate vaccination to occur globally to ensure the spread of COVID-19 is controlled and herd immunity is achieved worldwide.

References:
Desai, A. N., & Majumder, M. S. (2020). What is herd immunity? Journal of the American Medical Association, 324(20), 2113. doi:10.1001/jama.2020.20895

Fontanet, A., & Cauchemez, S. (2020). COVID-19 herd immunity: Where are we? Nature Reviews Immunology20(10), 583–584. https://doi.org/10.1038/s41577-020-00451-5

Kim, T. H., Johnstone, J., & Loeb, M. (2011). Vaccine herd effect. Scandinavian Journal of Infectious Diseases43(9), 683–689. https://doi.org/10.3109/00365548.2011.582247

Papachristodoulou, E., Kakoullis, L., Parperis, K., & Panos, G. (2020). Long-term and herd immunity against SARS-CoV-2: Implications from current and past knowledge. Pathogens and Disease78(3). https://doi.org/10.1093/femspd/ftaa025

Randolph, H. E., & Barreiro, L. B. (2020). Herd Immunity: Understanding COVID-19. Immunity52(5), 737–741. https://doi.org/10.1016/j.immuni.2020.04.012

World Health Organization. (2020). Coronavirus disease (COVID-19): Herd immunity, lockdowns and COVID-19. Retrieved from https://www.who.int/news-room/q-a-detail/herd-immunity-lockdowns-and-covid-19#:~:text=Research%20is%20still%20ongoing%20into,develop%20an%20immune%20response.

Xia, Y., Zhong, L., Tan, J., Zhang, Z., Lyu, J., Chen, Y., Zhao, A., Huang, L., Long, Z., Liu, N. N., Wang, H., & Li, S. (2020). How to Understand “Herd Immunity” in COVID-19 Pandemic. Frontiers in cell and developmental biology8, 547314. https://doi.org/10.3389/fcell.2020.547314

Image by PIRO4D from Pixabay 

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