There’s been a lot of talk about Covid-19 becoming “endemic”. But what does it actually mean?

First published in July 2021.

There’s been a lot of talk about Covid-19 becoming “endemic”.

Which means it circulates normally.

It doesn’t mean “trivial” or unimportant.

Polio was endemic in many countries in the mid-twentieth century.1 So was smallpox, but for a much longer period.2 Both caused death and disability.

Populations which had been exposed to them had lower mortality rates; but that didn’t mean the disease was trivial.

Sometimes the lower mortality rates related to the age at which you get infected.

The “childhood diseases” that everybody got used to include (among others) measles, mumps, rubella, varicella (chickenpox), and pertussis (whooping cough).3-7

If you get measles, chickenpox or mumps after puberty it is generally much more serious. But getting it as a child of, say, 4-9 years of age, it is much less so. (Not trivial. Mumps caused a lot of deafness, for example.8)

And rubella – german measles… (Aside – the rash is similar to a measles rash; “germane to”, meaning similar, mutated into “german”. Possibly the name stuck because people thought it meant “German” – we do have a dreadful habit of blaming nasty diseases on our neighbours, and naming them accordingly. But the name has nothing to do with Germany.)

Rubella is a mild disease. OK, a few people with rubella get idiopathic thrombocytopaenic purpura – a frightening name for a generally mild, self-limiting (gets better by itself) condition; and some get transient joint pains; but overall it’s pretty insignificant. Except...

While the fetus’ organs are forming, rubella infection can cause devastating harm known as “congenital rubella syndrome” (CRS), if the mother catches the virus while pregnant.9 (Ironically, given the suggested-but-debunked association between the MMR vaccine and autism, one of the few known causes of autism is CRS!10)

Zika may be similar. Remember all those babies in Brazil with microcephaly?11-13 The disease has been endemic in parts of Africa for decades. There, nearly everybody gets infected before they’re old enough to get pregnant. And it’s only when you are infected for the first time during pregnancy that it causes fetal anomalies.

Perhaps, as the infection becomes endemic in Brazil, the same will happen.

Another infection that is endemic in most of the world is pertussis – whooping cough.7 14 Some time, if you’re looking for an interesting rabbit hole to go down, look up the common names for this disease worldwide. Some names (eg “100-day cough”) accurately describe the typical duration of the symptoms. Others – including “whooping” describe the sound of the coughing.

“How can pertussis be endemic?”, you ask. “We vaccinate against it?”

Unlike vaccination against, say, measles, which provides excellent, long-lasting (probably usually life-long) immunity, pertussis immunity doesn’t last as long. With the less reactogenic acellular vaccines we switched to in 2004, immunity lasts about a decade, two if you’re lucky.

(As an aside… If you test people who have been fully vaccinated some years ago, but who have been exposed to a measles case – eg a staff member on a paediatric ward – you sometimes – quite rarely – find the virus’ DNA in their nasopharynx, and sometimes they have extremely mild symptoms, perhaps a slight rash. They have an attenuated form of measles. Very rarely indeed they can be infectious to others, although usually only to very close/intimate contacts.15 16)

So, if you’re vaccinated against pertussis as an infant (in the first year of life), there’s a good chance you’ll catch the infection in your teens or later. The “epidemic cycle” continues, with peaks of infections every few years.

People who have prior immunity tend to get much less ill. They have “attenuated disease”. But they can still be infectious.

We don’t worry too much about the fact that they can be infected: many will have a persistent, annoying cough that can last for about 100 days. It can be very unpleasant, but they won’t usually get seriously ill.

What we do worry about is if they infect small babies.

In the first 3 months of life, pertussis can be very serious. A small baby’s tiny airways can easily be blocked by mucus caused by the infection. As the airways (and the baby) get larger, the airways are less likely to be blocked. But in small babies, it can block off their oxygen supply, so they die or suffer brain damage.

That’s why we now vaccinate pregnant women. They’re usually already immune – their immune system has been “primed” by previous vaccination, so when they encounter the vaccine their immune system rapidly produces a large amount of antibodies (as it would if they encountered the disease).

These antibodies pass through the placenta and into the fetus’ bloodstream; and they can persist for months after birth, protecting the baby until after it has been vaccinated and can produce its own defences.

So... Some diseases are endemic, but vaccination allows us to live with them relatively safely. Others have been effectively eliminated through vaccination.

What are the prospects for Covid-19?

Since it first spread widely in human populations, in late 2019 and early 2020, the virus has changed. More infectious (“transmissible”) variants have – because they’re more transmissible – largely replaced the original variants.

Ideally, we’d achieve herd immunity.17 But the more infectious the disease, the higher the proportion of the population that has to be sufficiently immune to not be infected, or at least, to not pass the disease on to others if they are infected.

Recent estimates suggest – well, here’s Dr Diego Bassani’s take on a (preprint) paper:18 19

“The transmissibility of Delta means that vaccination of 87.5% of the population with a highly efficacious vaccine against infection (95% efficacy) is necessary.”

It’s not quite as simple as “87.5% of the population”, of course. People in their teens and twenties have (on average) a lot more contact with other people than people aged 0-10 or >50 years.

So it’s particularly important that this age group is well-vaccinated if you are to have any chance of stopping the spread of the virus through vaccination.

We do not, at present, routinely offer Covid vaccination to people under the age of 18 in the UK, and until we do, there’s no chance of herd immunity.

Even if we reach herd immunity thresholds of immunity in the UK, this could be undermined by even more transmissible variants. (It could also be undermined by vaccine escape variants – but I expect that these can and will, in due course, be prevented by tweaking the vaccines.)

And we will continue to see the disease spreading widely in other parts of the world where the disease is not under control – generating new variants, and importing cases into the UK.

It will be some years before transmission of the virus is fully controlled in the UK through vaccination. It will therefore become “endemic”.

But will it remain a serious infection?

Of course, that depends on how you define “serious”.

Once most people have been vaccinated, or had the disease and acquired natural infection, some will still become infected (sometimes referred to as breakthrough infection, or “vaccine failure”). But it is likely that relatively few of them will require hospital infection, critical care (ICU), or will die.

What we don’t know yet is what proportion will have “long Covid”; or how serious or long-lasting these will be.

Burden of disease

In public health, we talk about “burden of disease”. This includes hospitalisation, critical care, and deaths. But it also includes all the adverse consequences of the disease.

It includes GP consultations. It includes economic consequences: time off work to look after a sick child; consequences for the patient.

We know that a fairly high proportion of patients admitted to hospital have significant organ damage.20 21

Some suffer cognitive deficits (can’t think as well).22

Long Covid, by definition, continues for months after the initial infection and can impair people’s lives for months, maybe years.23-30

The long-term consequences of Covid-19 may limit people’s ability to contribute to society through working, earning, paying taxes, volunteering...

All of these consequences contribute to the “burden of disease”.

And these consequences might persist for years – possibly for the rest of the person’s life. The “cost” of such persistent sequelae can add up to become very significant.

The highly infectious delta variant is overwhelmingly the predominant strain in the UK at present (and likely soon, if not already, in the world).

With highly infectious variants like the delta variant, it will be much harder for people who are not immune (not vaccinated or it didn’t work) to avoid infection, and all the short- and long-term consequences of the disease.

As Covid-19 becomes an endemic disease, there will continue to be cases, some of whom will have serious acute (short-term) illnesses, and some will have long Covid.

Exactly what the burden of disease will be when it becomes endemic is hard to predict. It is likely to remain high enough to justify attempts to reach herd immunity through vaccination – which will require vaccination of children.

Future of Covid vaccination

It may be that Covid-19 vaccines will become part of the routine childhood schedule, if prior immunity means that any subsequent infections cause (mostly) only mild disease.

It is too soon to know if we will need boosters, either occasionally (as with eg tetanus and polio);¹ ³¹ or if we will need regular revaccination with new vaccines that have been tweaked to address new variants that can escape protection by previous vaccines (as with influenza).  


Dr Peter English, Public Health Doctor. Particular interests in vaccination and health intelligence.


Going Further:

Note that I have included some Twitter threads amongst the references – something that might enrage purists. But where I have done so, they not only make a point or develop an argument, but they mostly provide links to proper published papers or pre-prints. So I make no apology for this!

  1. Public Health England. Chapter 26: Polio. Immunisation against infectious disease. Web page updated 19 Apr 2013; pdf updated 18 Jan 2013 ed. London: HMSO, 2013:313-28 (
  2. Stewart AJ, Devlin PM. The history of the smallpox vaccine. J Infect 2006;52(5):329-334. (
  3. Salisbury D, Ramsay M, Noakes K. Chapter 21: Measles. Immunisation against infectious disease. updated 1 July 2013, v2_0 ed. London: HMSO, 2013:209-234 (
  4. Salisbury D, Ramsay M, Noakes K. Chapter 23: Mumps. Immunisation against infectious disease. updated 28 January 2013 ed. London: HMSO, 2013:255-76 (
  5. Salisbury D, Ramsay M, Noakes K. Chapter 28: Rubella. Immunisation against infectious disease. updated 28 January 2013 ed. London: HMSO, 2013:343-65 (
  6. Public Health England. Chapter 34: Varicella. Immunisation against infectious disease. 26 Jun 2019 ed. London: HMSO, 2019(
  7. Public Health England. Chapter 24: Pertussis. Immunisation against infectious disease. London: HMSO, 2016:1-26 (
  8. Mizushima N, Murakami Y. Deafness following mumps: the possible pathogenesis and incidence of deafness. Auris Nasus Larynx 1986;13 Suppl 1:S55-7  PMID: 3767776. (
  9. Yazigi A, De Pecoulas AE, Vauloup-Fellous C, Grangeot-Keros L, Ayoubi JM, Picone O. Fetal and neonatal abnormalities due to congenital rubella syndrome: a review of literature. The journal of maternal-fetal & neonatal medicine : the official journal of the European Association of Perinatal Medicine, the Federation of Asia and Oceania Perinatal Societies, the International Society of Perinatal Obstet 2016:1-5  PMID: 27002428
  10. Hutton J. Does Rubella Cause Autism: A 2015 Reappraisal? Frontiers in human neuroscience 2016;10:25  PMID: 26869906.
  11. The Lancet. Zika virus: a new global threat for 2016. Lancet 2016;387(10014):96. (
  12. Soares de Araújo JS, Regis CT, Gomes RGS, Tavares TR, Santos CRd, Assunção PM, et al. Microcephaly in northeastern Brazil: a review of 16 208 births between 2012 and 2015. Bull World Health Organ 2016. (
  13. English PMB. Microcephaly in Brazil - not as clear cut as you might think. Peter English's random musings. London, 2016(
  14. English PMB. Pertussis vaccination - it's complicated. Peter English's random musings. Epsom, Surrey:, 2012(
  15. Jones J, Klein R, Popescu S, Rose K, Kretschmer M, Carrigan A, et al. Lack of Measles Transmission to Susceptible Contacts from a Health Care Worker with Probable Secondary Vaccine Failure — Maricopa County, Arizona, 2015. MMWR Morb Mortal Wkly Rep2015;64(30):832-833. (
  16. Hahne SJ, Nic Lochlainn LM, van Burgel ND, Kerkhof J, Sane J, Yap KB, et al. Measles Outbreak Among Previously Immunized Healthcare Workers, the Netherlands, 2014. J Infect Dis 2016;214(12):1980-1986  PMID: 27923955. (
  17. English PMB. How many people have to be immunised to provide herd immunity? Peter English's random musings (blog)2021; Updated 31 May 2021; Accessed: 2021 (31 May): (
  18. Bassani D. @DGBassani: New preprint (not peer reviewed) from China (data-driven model) finds that extending the vaccination program to children 3-17 years is the only way of reaching vaccine-induced herd immunity to original and currently circulating SARS-CoV-2 variants. Twitter thread 2021; Updated 23 Jul 2021; Accessed: 2021 (25 Jul): (
  19. Liu H, Zhang J, Cai J, Deng X, Peng C, Chen X, et al. Herd immunity induced by COVID-19 vaccination programs to suppress epidemics caused by SARS-CoV-2 wild type and variants in China. medRxiv2021:2021.07.23.21261013. (
  20. Pym H. Covid: Younger adults still at risk of serious organ damage - study. BBC News 2021; Updated 16 Jul 2021; Accessed: 2021 (16 Jul): (
  21. Ayoubkhani D, Khunti K, Nafilyan V, Maddox T, Humberstone B, Diamond I, et al. Post-covid syndrome in individuals admitted to hospital with covid-19: retrospective cohort study. BMJ2021;372:n693. (
  22. Hampshire A, Trender W, Chamberlain SR, Jolly AE, Grant JE, Patrick F, et al. Cognitive deficits in people who have recovered from COVID-19. EClinicalMedicine 2021 PMID: 101044. (
  23. Prescott HC. Outcomes for Patients Following Hospitalization for COVID-19. JAMA 2021;325(15):1511-1512. (
  24. Blomberg B, Mohn KG-I, Brokstad KA, Zhou F, Linchausen DW, Hansen B-A, et al. Long COVID in a prospective cohort of home-isolated patients. Nature Medicine 2021. (
  25. O’Dowd A. Covid-19: Third of people infected have long term symptoms. BMJ 2021;373:n1626. (
  26. Osmanov IM, Spiridonova E, Bobkova P, Gamirova A, Shikhaleva A, Andreeva M, et al. Risk factors for long covid in previously hospitalised children using the ISARIC Global follow-up protocol: A prospective cohort study. European Respiratory Journal2021:2101341. (
  27. McConway K, Tang J, Strain D. Expert reaction to preprint from REACT-2 looking at persistent symptoms following SARS-CoV-2 infection in a random community sample of 508,707 people. Science Media Centre 2021; Updated 24 Jun 2012; Accessed: 2021 (24 Jun): (
  28. Ayoubkhani D, Pawelek P. Prevalence of ongoing symptoms following coronavirus (COVID-19) infection in the UK: 4 June 2021. Office for National Statistics (ONS) 2021; Updated 01 Jul 2021; Accessed: 2021 (05 Jul): ( via
  29. Whitaker M, Elliott J, Chadeau-Hyam M, Riley S, Darzi A, Cooke G, et al. Persistent symptoms following SARS-CoV-2 infection in a random community sample of 508,707 people. medRxiv2021:2021.06.28.21259452. (
  30. Crook H, Raza S, Nowell J, Young M, Edison P. Long covid—mechanisms, risk factors, and management. BMJ 2021;374:n1648. (
  31. Public Health England. Chapter 30: Tetanus. Immunisation against infectious disease. 22 Jan 2020 ed. London: HMSO, 2020:1-15 (

[This piece was originally published in Peter English’s Random Musings blog, based on a Twitter thread, and re-published in PMP Magazine on 28 July 2021, with the author’s consent. | The author writes in a personal capacity.]

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