Comment: How herd immunity actually works: vaccination is the key to stopping the spread of infectious disease

Epidemiologist Professor Amira Roess of George Mason University’s College of Public Health explains how herd immunity protects entire populations from infectious diseases by preventing viruses and bacteria from finding new hosts and why widespread vaccination remains vital to safeguarding public health

When experts aim to prevent wildfires, they sometimes ignite small, controlled burns to remove excess vegetation and reduce the available fuel to any future uncontrolled blaze. The same kind of principle applies to infectious-disease control. Vaccination limits the number of susceptible individuals and prevents infections from spreading through a population. The effect is that disease spread is stopped before it can take hold – and so burns out.

“Herd immunity happens when most people are immune to a disease, so the infection hits a dead end and transmission stops,” explained Professor Amira Roess.

Herd immunity occurs when a critical proportion of the population – typically more than 95 per cent for many diseases – has developed protection through vaccination or prior infection. Once this threshold is reached, viruses and bacteria struggle to find new hosts, and the chain of transmission is broken.

Critically, the other 5 per cent of the population will include infants who are too young to yet receive any vaccinations and therefore rely on the rest of the ‘herd’ – their families, older siblings, friends and neighbours – to take on the protective weight of responsibility and shield them from risk with their own immunity.

Immunity develops when the immune system learns to recognise and combat a harmful virus or bacterium, either through natural infection or vaccination – especially for more serious and deadly diseases where exposure is too high risk. After exposure, the immune system produces antibodies that can target an invading organism. In case of a later further exposure, the body remembers how to create those antibodies and respond quickly and efficiently to prevent illness. Vaccination provides this immune memory in a safe and controlled way without the risk of developing the disease which is the target of the immunisation.

Before the development of widespread vaccination programmes, diseases such as measles caused hundreds of childhood deaths each year. Routine immunisation has made such fatalities extremely rare among healthy vaccinated individuals. And subsequent decades of careful research have shown that vaccines are safe and effective, with side effects that are minor and short-lived compared with the consequences of infection.

Individuals who can receive vaccines are urged to do so, as viruses and bacteria rely on non-immune hosts to survive. If diseases are allowed to circulate freely mutations tend to emerge that weaken vaccine protection – as was notable during the COVID-19 pandemic.

And achieving herd immunity through allowing infection alone would cause unnecessary illness and death, particularly among the very young and people who are immunocompromised and unable to mount a strong immune response. These individuals depend on widespread vaccination for others to protect them.

Herd immunity through vaccination is one of public health’s greatest achievements and continued defences. It prevents needless suffering, protects the vulnerable and ensures that diseases once thought inevitable remain under control. Many infectious diseases can now be entirely prevented – such as the near complete eradication worldwide of polio – and when safe, effective vaccines exist, the question should not be whether to vaccinate, but why anyone would choose not to?

Amira Roess is a professor of global health and epidemiology at George Mason University's College of Public Health, Department of Global and Community Health. She is an epidemiologist with expertise in infectious diseases epidemiology, multidisciplinary and multi-species field research and evaluating interventions to reduce the transmission and impact of infectious diseases. Roess currently oversees several longitudinal studies to understand emergence and transmission of zoonotic infectious diseases globally, including the emergence and transmission of Campylobacter (with support from The Bill and Melinda Gates Foundation), MERS-CoV (with support from the U.S. National Science Foundation), and the development of the microbiome during the first year of life. She studies links between food animal production and emerging infectious and zoonotic diseases emergence globally, and mHealth (especially apps) technology integration and evaluations to reduce the impact of infectious disease outbreaks, promote health care, and help reduce disparities.

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