Many people have heard of Darwin’s Theory of Survival of the fittest; only those who are the best adapted for their environment and able to adapt to change survive and pass on their genes to future generations. Herd immunity is an idea which follows a similar concept. However, in this case, it is a way of limiting the spread of diseases. The concept involves the majority of the population (i.e. the herd) becoming immune to a disease so that the likelihood of transmitting the disease to those who have not been in contact with the disease is reduced significantly.
This is a concept that has been used for many years and dates back to 1923, where Topley and Wilson published a series of reports named “The spread of bacterial infection: the problem of herd immunity”, where they studied laboratory mice who were exposed to infections. Through their investigations they noted that the immunised mice had lower mortality rates, where they started to question how resistance built up among those who were not exposed to the bacteria given that the majority of the population was immune to it. Herd immunity was further developed to prove that it provided indirect protection against diseases and was a natural defence for centuries against epidemics like smallpox in the 1840s.
Herd immunity is nowadays used synonymously with vaccinations. Vaccines are one way to encourage herd immunity; all or the vast majority are exposed to a disease at the same time.
“However, there is a catch with vaccines: they only work if the pathogen does not mutate”
Many of us UK residents may remember the Measles, Mumps Rubella (MMR) vaccine when growing up – where an inactive version of the MMR pathogens are injected into the bloodstream, to trigger white blood cells to act to create antibodies to fight against the evil microbes. Antibodies remain in the bloodstream such that if the same person was infected by the same pathogen again, the body would be able to react quicker and fight it off before any adverse effects occur on the person. Antibodies have the potential to be passed down to future generations during the child carrying stages of each pregnancy. However, there is a catch with vaccines: they only work if the pathogen does not mutate. It is the reason why a vaccine cannot be made for the common cold because it mutates so frequently that by the time it is commercially available the likelihood it has genetically changed is much higher. Cold and flu tablets only help to reduce the symptoms of a cold and provide relief for someone struggling.
“If the healthy and the able can take the vaccine for the greater good, the protection is so much greater that the risk of infection is diluted down to a small minority”
Does Herd Immunity Actually Work?
The short answer is yes. It protects those who do not have the ability to be vaccinated, such as premature babies and immunodeficient people. If the healthy and the able can take the vaccine for the greater good, the protection is so much greater that the risk of infection is diluted down to a small minority. If 80% of the population are immune to a disease the likelihood of transmission is greatly reduce but not eradicated. If one person is infected in a group of 100 people, with 80% being immune, there are only 19 potential people who could become infected – lessening the effects of transmission.
Imagine that you come into contact with someone who is not immune to the disease – they are likely to be a carrier of the disease. There is a hole to transmission of the disease and the disease is likely to replicate within them. Unfortunately, regardless of the many prevention strategies, transmission is still possible, and the disease goes from being eradicated to a surge in cases. This not only means that 20% are at risk of being in contact with the pathogens, but it provides an opportunity for mutation and the whole concept of protection going out the window. This is the current concern with the Covid-19 Virus; there isn’t a safe vaccine for it yet and it is an invisible virus (it has the same early symptoms as the common flu).
Featured image courtesy of chad glenn via Flickr. Image license found here.
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