What is Herd Immunity?

In the wake of the coronavirus pandemic the U.K. has strayed from the pack and into the herd.

On Thursday it was reported that the United Kingdom government has moved out of the containment phase and into the delay phase of their four stage plan to attempt to tackle the pandemic novel coronavirus disease, COVID-19.

It was expected that the U.K. would follow in the footsteps of its continental neighbours and begin to close schools, colleges, and banning mass gatherings to help prevent the spread of the virus. However, this was not the case.

The U.K. Prime Minister Boris Johnson announced Thursday that the country will not be undertaking similar action to that of affected countries such as Italy, France, Germany, Spain and the Republic of Ireland. At least, not for now. Instead, it seems the kingdom is hunkering down for the long haul.

The U.K. government has said that the unexpected move is based purely in science, and that since the infection may not peak for another 14 weeks, it is unlikely the same extraordinary social distancing measures that other countries have adopted could be kept up. In short, it’s just not feasible to lock the country down for three months, both socially and economically.

And then came that term: herd immunity.

Herd Immunity

The U.K. government believes that the novel coronavirus (SARS-CoV-2), which causes the disease COVID-19, isn’t going anywhere for the foreseeable future and, like seasonal flu, could become a yearly menace. This indefinite outbreak has caused the government to switch tactics and opt for a herd immunity approach.

Herd immunity is not a new concept, in fact it was a homegrown Briton by the name of Sir William Heaton Hamer who came up with the theory in 1906, although the actual term wasn’t coined until 1923.

The concept of herd immunity is a relatively simple one. If enough people in a community or population have an immunity to a communicable disease, those with no immunity will be protected by proxy.

So for example, if there is an isolated community of 20 people and 19 of those are inoculated against measles, the one person who isn’t immune won’t be infected with measles due to the disease not being able to spread from the immune 19. This is the very basics of the herd immunity theory.

Of course, communities are not isolated and there are many more factors that can contribute to the spread of a disease. Also, without a vaccine for at least twelve months (and likely longer) immunity comes at a cost: you actually have to get sick and let your own immune system do all the work (see: What Actually is the Novel Coronavirus and How Does is Affect Us?).

Due to the expected long term nature of COVID-19, the U.K. government has opted to accept a widespread infection as all but inevitable now, and have redirected their efforts into controlling the spread of the disease as not to overwhelm the National Health Service (NHS). The idea being that if the majority of the healthy population catch the virus and develop their own immunity to it, the most vulnerable and at risk in the population will be much more protected by proxy, as it will now be much more difficult for the virus to spread. Those who are immune to COVID-19, even if reintroduced to the virus, do not become infectious and therefore the virus cannot spread. Sir Patrick Vallance, the government’s chief scientific adviser, believes that 60 per cent of the population would need to develop an immunity to COVID-19 for herd immunity to take effect.

Herd immunity has been used, successfully, in the past in the complete global eradication of smallpox and the eradication of measles in many countries. It has also been observed that when vaccination uptakes fall low, a disease can resurge in a population. This was documented in the 2012–2013 measles outbreak in Wales, where due to historically low MMR (measles, mumps, and rubella) vaccinations at least 1400 people were reported to have the disease over six months.

There are vulnerable people in communities who today already rely on herd immunity to reduce their chances of infections, which could be potentially deadly. These being immunocompromised individuals, newborn babies, the elderly, and more.

Divided Opinions

The efficacy of the herd immunity strategy and the lack of social distancing measures undertaken by the U.K. government has a somewhat divided opinion within the medical community.

Some, such as Richard Horton, editor-in-chief of the journal The Lancet, have expressed their concerns that without more extreme social distancing measures the U.K. could soon find itself in the same situation as Italy is in now.

Others, however, have come to the defence of this strategy, such as Dr Clare Wenham who told CNN that “Johnson is listening to the current evidence that’s out there” and hasn’t “bowed to pressure” unlike other political leaders. Keith Neal, emeritus professor in the epidemiology of infectious diseases at the University of Nottingham, also said “the plans are sensible” and remarked that calls for more action are easy but lack evidence to make any decisions.

As the world reacts to combat the spread and toll of the novel coronavirus, it seems that the U.K. is taking its own sage advice to keep calm and carry on.

Further Reading

Fine, P., 1993. Herd Immunity: History, Theory, Practice. Epidemiologic Reviews, 15(2), 265–302.

Fine, P., Eames, K. and Heymann, D., 2011. “Herd Immunity”: A Rough Guide. Clinical Infectious Diseases, 52(7), pp.911–916.

Vk.ovg.ox.ac.uk. (2020). Herd immunity (Herd protection) | Vaccine Knowledge. [online] Available at: https://vk.ovg.ox.ac.uk/vk/herd-immunity [Accessed 14 Mar. 2020].

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