Herd Immunity – what’s in a name?

“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:

  1. Horton, R. (2020) Offline: COVID-19 – a reckoning. Lancet, 395, 935.
  2. https://public.oed.com/updates/new-words-list-march-2020/.
  3. Eichhorn, A. & Potter, G.M. Contagious Abortion of Cattle. In: Farmer’s Bulletin 790, Washington DC: United States Department of Agriculture, 1917.
  4. Winslow, K. & Eichhorn, A. Veterinary Materia Medica and Therapeutics, Eighth Edition. Chicago: American Veterinary Publishing Co, 1919, pp.525-563.
  5. Beechy, L.P. (1920) Abortion disease in cattle. Bulletin of the Ohio State University Agricultural College Extension Service. Vol. XVI, No. 1.
  6. Smith, T., Little Further studies on the etiological role of Vibrio fetus. J Exp Med, 32, 683-689, R.B. &Taylor, M.S. (1920).
  7. Topley, W.W.C. & Wilson, G.S. (1923) The spread of bacterial infection. The problem of herd-immunity. J Hyg, 21, 243-9.
  8. Topley, W.W.C. Wilson, J. & Lewis, E.R. (1925) Immunisation and selection as factors in herd-resistance. J Hyg, 23, 421-436.
  9. Greenwood, M. & Topley, W.W.C. (1925) A further contribution to the experimental study of epidemiology. J Hyg, 24, 45-110.
  10. 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.
  11. Dudley, S.F. The Spread of Droplet Infection in Semi-isolated Communities. Medical Research Council, Special Report Series, No.111, London: HMSO, 1926.
  12. Anon. (1927) The spread of infection in schools and ships. BMJ, 1(3443), 34, 1 Jan.
  13. Dudley, S.F. (1928) Natural and artificial stimuli in the production of human diphtheria antitoxin. Brit J Exp Pathol, 9, 290-298.
  14. Dudley, S.F. (1929) Herds and individuals. J R Army Med Corps, 53, 9-25.
  15. Fine, P., Eames, K. & Heymann, D.L. (2011) “Herd immunity”: a rough guide. Clin Infect Dis, 52, 911-916.
  16. https://www.nhs.uk/conditions/vaccinations/why-vaccination-is-safe-and-important/.
  17. https://vk.ovg.ox.ac.uk/vk/herd-immunity.
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  19. Hakim, H. et al. (2019) Interventions to help people understand community immunity: a systematic review. Vaccine, 37, 235-247.

Can history help us in the COVID-19 epidemic?

1918 flu epidemic: the Oakland Municipal Auditorium in use as a temporary hospital. Photo by Edward A. “Doc” Rogers. From the Joseph R. Knowland collection at the Oakland History Room, Oakland Public Library. Digital copy via http://content.cdlib.org/ark:/13030/kt3q2nc9rt/?&query=

In this time of great uncertainty around the impact that Coronavirus disease 2019 (COVID-19) will have on populations and health systems globally, can we look to history to help us in its management?

Many have already drawn comparisons between COVID-19 and the 1918 influenza pandemic, also known as ‘Spanish Flu’. The 1918 influenza pandemic which spanned a couple of years from 1918-1920 infected 27 per cent of the world’s populations, and killed between 17 and 50 million, making it one of the deadliest pandemics in modern history.

While it may have occurred over a century ago, in many ways the situation with COVID-19 is similar to that facing nations in 1918. With no specific treatment or vaccination available except best supportive care, governments are turning to epidemiologists to help stop the spread and mitigate the damage caused by the disease.

A widely circulated graphic from the paper, ‘Public health interventions and epidemic intensity during the 1918 influenza pandemic’ by Hatchett et al. shows how differing public health responses resulted in different death rates between two American cities: Philadelphia and St Louis.

1918 excess mortality in philadelphia and St Louis

Excess P&I mortality over 1913–1917 baseline in Philadelphia and St. Louis, September 8–December 28, 1918. Source: Hatchett et al. https://doi.org/10.1073/pnas.0610941104

While authorities in Philadelphia became aware of the disease on 17 September 1918, they downplayed its significance and still allowed large social gatherings to take place including, a city-wide parade. They only implemented measures such as school closures and a ban on public gatherings on 3 October.

By contrast St Louis reported its first cases of the disease on 5 October and authorities mobilised containment measures rapidly on 7 October. The difference in the responses between both cities appear to have borne out in the excess pneumonia and influenza death rates seen in both cities.

Philadelphia experienced a peak weekly pneumonia and influenza excess death rate of 257 per 100,000 whereas St Louis experienced a rate of 31 per 100,000.

The above example appears to demonstrate the impact of early interventions such as social distancing to help contain the spread of the disease. If anything is to be learned from history a rapid implementation of such measures may be required to contain the spread of COVID-19.

For further reading on the impact of ‘Spanish Flu’, please refer to this post by Jane Orr.


Words by Flora Malein


Sources used:

Taubenberger JK, Morens DM (2006). “1918 Influenza: the mother of all pandemics”. Emerging Infectious Diseases. 12 (1): 15–22. doi:10.3201/eid1201.050979. PMC 3291398. PMID 16494711.

Spreeuwenberg; et al. (2018). “Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic”. American Journal of Epidemiology. 187 (12): 2561–2567. doi:10.1093/aje/kwy191. PMID 30202996

Richard J. Hatchett et al. (2007) Public health interventions and epidemic intensity during the 1918 influenza pandemic. PNAS May 1, 2007 104 (18) 7582-7587; first published April 6, 2007 https://doi.org/10.1073/pnas.0610941104