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“Herd immunity” recently made a controversial appearance in the context of the current COVID-19 pandemic. What does the phrase mean, where did it come from, and how helpful is it today?

As of March 2020, the OED defines it as, “resistance to the spread of a contagious disease within a population that results if a sufficiently high proportion of individuals are immune to the disease, typically as a result of having been vaccinated against it”.

The earliest use of the phrase can be traced to a 1917 report from the US Bureau of Animal Industry that dealt with a cattle infection causing death of unborn calves. A cow that had aborted was likely to become immune, and calves born and raised in such an affected herd were tolerant to the disease. The authors concluded that “a herd immunity seems to have developed as the result of both keeping the aborting cows and raising the calves”.

However, the senior author, Dr Adolph Eichhorn, Chief of the Pathological Division, made no reference to herd immunity in a monograph to which he contributed a major section on biological therapeutics just two years later. His biologically apt coinage does seem to have been picked up in US agricultural circles, but it was not universally adopted, with “immunity of the herd” being used instead.

The concept of herd immunity next appeared in British bacteriologist William Topley’s epidemiological studies of bacterial infection, which examined the resistance of a population of mice after immunising animals with suspensions of bacteria. He used “herd-resistance” to describe the natural resistance of individuals within a population. And he discussed the implications of his work with the “mouse herd” for the “human herd”.

The human herd entered this experimental realm at about the same time. In 1922, Surgeon-Commander Sheldon Dudley studied a diphtheria epidemic at Greenwich Hospital School. He found that the longer boys had been resident the greater the proportion who were immune, and that increases in immunity correlated with each outbreak. He extended such studies to other infectious diseases and used herd immunity to explain his findings.

In 1928, all boys in the school were actively immunised against diphtheria. The most senior became immune (Schick-test negative) twice as quickly as the most junior, suggesting prior exposure to the disease (see Figure). These results paralleled earlier work in animals, except for the fact that “a herd of human boys were used in lieu of the guinea-pigs”.

Dudley was unapologetic for using the prefix herd to denote the properties of a community, pointing out that psychologists had earlier popularized the phrase “herd instinct”. Besides, on evolutionary grounds, there was “little fundamental difference between a herd of deer, a herd of swine, and a herd of Homo sapiens”.

Notions of herd immunity have become more sophisticated in recent decades owing to the increased importance of vaccination. Today’s NHS website defines the benefits thus: “If enough people are vaccinated, it’s harder for the disease to spread to those people who cannot have vaccines. For example, people who are ill or have a weakened immune system”.

The reader is also directed to more information and an animation on the website of the Oxford Vaccine Group’s Vaccine Knowledge Project . This site suggests that a better name for herd immunity is “herd protection” because it helps to protect those especially vulnerable to infectious diseases. “Community immunity” appears as an alternative.

Conveying the value of herd protection or community immunity to the public will be critical in successful vaccination against COVID-19. One must worry that the lazy use of a century-old phraseology rooted in the farm, mouse lab and human guinea-pigs, as well as a contemporary profusion of alternative terms, may prove more of a hindrance than a help.

Words by Edward Wawrzynczak

Sources used:

Horton, R. (2020) Offline: COVID-19 – a reckoning. Lancet, 395, 935.
https://public.oed.com/updates/new-words-list-march-2020/.
Eichhorn, A. & Potter, G.M. Contagious Abortion of Cattle. In: Farmer’s Bulletin 790, Washington DC: United States Department of Agriculture, 1917.
Winslow, K. & Eichhorn, A. Veterinary Materia Medica and Therapeutics, Eighth Edition. Chicago: American Veterinary Publishing Co, 1919, pp.525-563.
Beechy, L.P. (1920) Abortion disease in cattle. Bulletin of the Ohio State University Agricultural College Extension Service. Vol. XVI, No. 1.
Smith, T., Little Further studies on the etiological role of Vibrio fetus. J Exp Med, 32, 683-689, R.B. &Taylor, M.S. (1920).
Topley, W.W.C. & Wilson, G.S. (1923) The spread of bacterial infection. The problem of herd-immunity. J Hyg, 21, 243-9.
Topley, W.W.C. Wilson, J. & Lewis, E.R. (1925) Immunisation and selection as factors in herd-resistance. J Hyg, 23, 421-436.
Greenwood, M. & Topley, W.W.C. (1925) A further contribution to the experimental study of epidemiology. J Hyg, 24, 45-110.
Dudley, S.F. (1922) The relation of natural diphtheria antitoxin in the blood of man to previous infection with diphtheria bacilli. Brit J Exp Pathol, 3, 204-209.
Dudley, S.F. The Spread of Droplet Infection in Semi-isolated Communities. Medical Research Council, Special Report Series, No.111, London: HMSO, 1926.
Anon. (1927) The spread of infection in schools and ships. BMJ, 1(3443), 34, 1 Jan.
Dudley, S.F. (1928) Natural and artificial stimuli in the production of human diphtheria antitoxin. Brit J Exp Pathol, 9, 290-298.
Dudley, S.F. (1929) Herds and individuals. J R Army Med Corps, 53, 9-25.
Fine, P., Eames, K. & Heymann, D.L. (2011) “Herd immunity”: a rough guide. Clin Infect Dis, 52, 911-916.
https://www.nhs.uk/conditions/vaccinations/why-vaccination-is-safe-and-important/.
https://vk.ovg.ox.ac.uk/vk/herd-immunity.
Betsch, C. et al. (2017) On the benefits of explaining herd immunity in vaccine advocacy. Nat Hum Behav, 1, 0056.
Hakim, H. et al. (2019) Interventions to help people understand community immunity: a systematic review. Vaccine, 37, 235-247.

Paris: Comité National de Défense contre la Tuberculose; Quimper: Comité d’Hygiène Sociale du Finistère, [between 1930 and 1939?]. Image Credit: Wellcome Collection, CC BY.

Christmas has long been recognised as a time of charity and fundraising. Perhaps familiar to some is the ‘Christmas seal’ – a label placed on post over the Christmas period – to raise awareness and funds for charitable organisations.

Christmas seals fall into a category known as ‘cinderella stamps’: items that resemble stamps but cannot be used in payment for postage

While the Christmas seal concept has been adopted to raise funds for many different types of charitable organisations, they have most closely been allied to raising awareness of tuberculosis. According to a New York Times article, the concept originated in Denmark in 1904, when a Danish postal worker called Einar Holbøll came up with the idea to sell the penny seals to raise money for children with tuberculosis.

Other countries quickly followed suit with fundraising seals being produced to support tuberculosis causes in Sweden, Iceland and Argentina.

At the time, tuberculosis was still one of the major causes of death globally. And despite the discovery of the M. tuberculosis bacillus by Robert Koch in 1882, no effective treatment had been found. Organised national efforts to combat the disease were looking for means to raise awareness and financial support for the disease. These campaigns can be seen as some of the very first public health initiatives, in which the financial contributions of ordinary people were used to fund treatment of a disease.

Picture credit: W. Dibb Private Collection.

The first American Christmas seals were produced in 1907 by a Red Cross volunteer called Emily Bissell, who adopted the idea to help raise money for a tuberculosis sanatorium in Delaware that was under the threat of closure if a sum of $300 was not raised.

Although she could not get permission to have the U.S. national postal service to print and distribute the seals, as they had in Denmark, she was allowed to privately sell the seals in the post office lobbies. The seals were such a success they raised over ten times the amount needed and became an annual tradition that raised major funds for The National Tuberculosis Association (now known as the ‘American Lung Association’). The American Lung Association continues to sell their Christmas Seals™. Selling tuberculosis Christmas seals has also funded major disease prevention programmes in countries, such as Canada, through chest X-ray screening or tuberculin tests.

The Cross of Lorraine

National Tuberculosis Association, 1940. Image Credit: Wellcome Collection, CC BY.

Many examples of the Christmas seals created to raise funds for tuberculosis charities contain the symbol of the double-barred cross. Symbolically known as the ‘Cross of Lorraine’, it was adopted as the symbol of the fight against tuberculosis at the International Conference on tuberculosis held in Berlin, 1902.

A French doctor – Gilbert Sersiron – proposed the emblem which had been the banner of Godfrey of Bouillon, one of the leaders of the first Crusades who successfully laid siege to Jerusalem and became it first ruler. In adopting this Crusader symbol, the cross became a symbol of the new, organised anti-tuberculosis ‘crusade’.

Flora Malein

You can read more about postal items and infectious disease in a previous post, here.

Sources used:

The New York Times: https://www.nytimes.com/1989/12/24/style/pastimes-stamps.html

The American Lung Association: https://www.lung.org/get-involved/ways-to-give/christmas-seals/history.html

The Canadian Lung Association: https://web.archive.org/web/20111115234620/http://lung.ca/involved-impliquez/christmas-noel/history-histoire/index_e.php

TB Alert: https://web.archive.org/web/20090302074617/http://www.tbalert.org/about/cross.php

John, Simon (2017). Godfrey of Bouillon: Duke of Lower Lotharingia, Ruler of Latin Jerusalem, c.1060-1100. Taylor & Francis. ISBN 978-1-317-126300.

https://wellcomecollection.org/works/msbjucxs

https://wellcomecollection.org/works/hdh9a7xw