Thread by @petermbenglish

1/ There's been a lot of talk about Covid-19 becoming "endemic".

Which means it circulates normally.

It doesn't mean "trivial" or unimportant.

(Long thread.)

2/ (A version of this thread is available at my blog: peterenglish.blogspot.com/2021/07/will-c… .)

3/ Polio was endemic in many countries in the mid-twentieth century. Smallpox ditto, for a much longer period. 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.

4/ Sometimes the lower mortality rates relate to the age at which you get infected.

The "childhood diseases" that everybody got used to include (among others) measles, mumps, rubella, chickenpox, and pertussis (whooping cough).

5/ 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.)

6/ And rubella - german measles… (Aside - the rash is similar to a measles rash; "germane to", meaning similar, mutated into "german".

7/ 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.

8/ OK, a few people with rubella get idiopathic thrombocytopaenic purpura - a frightening name for a mild, self-limiting (gets better by itself) condition; and some get transient joint pains; but overall it's pretty insignficant. Except…

9/ Except if you get rubella when you're pregnant, while the fetus' organs are forming, when it can cause devastating harm known as "congenital rubella syndrome" (CRS).

10/ Zika may be similar. Remember all those babies in Brazil with microcephaly? The disease has been endemic in parts of Africa for decades. There, nearly everybody gets infected before they're old enough to get pregnant.

11/ 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.

12/ Another infection that it endemic in most of the world is pertussis - whooping cough. Some time, if you're looking for an interesting rabbit hole to go down, look up the common names for this disease worldwide.

13/ Some names (eg "100 day cough") accurately describe the typical duration of the symptoms. Others - including "whooping" describe the sound of the couging.

14/ "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.

15/ With the less reactogenic acellular vaccines we switched to in 2004, immunity lasts about a decade, two if you're lucky.

16/ (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 an paediatric ward - you sometimes - quite rarely - find the virus' DNA in their nasopharynx, and sometimes they have…

17/ …extremely mild symptoms, perphaps a slight rash. They have an attenuated form of measles. Very rarely indeed they can be infectious to others, although usually only very close/intimate contacts.)

18/ 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.

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

20/ 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, but they won't get seriously ill.

21/ 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 babies tiny airways can easily be blocked by mucus caused by the infection.

22/ 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.

23/ That's why we now vaccinate pregnant women.

24/ 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 large amount of antibodies (as it would if they encountered the disease).

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

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

27/ 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.

28/ Ideally, we'd achieve "herd immunity". 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.

peterenglish.blogspot.com/2021/02/select…

29/ Recent estimates suggest - well, here's @DGBassani's take on a (preprint) paper:

"The transmissibility of Delta means that vaccination of 87.5% of the population with a highly efficacious vaccine against infection (95% efficacy) is necessary."
threadreaderapp.com/thread/1418656…

30/ (The paper: medrxiv.org/content/10.110…)

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.

31/ 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.

32/ We do not, at present, routinely offer 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.

33/ (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.)

34/ 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.

35/ 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 how you define "serious".

36/ 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").

37/ 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.

38/ 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.

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

40/ We know that a fairly high proportion of patients admitted to hospital have significant organ damage.
bbc.co.uk/news/health-57…

Some suffer "cognitive deficits" (can't think as well): thelancet.com/journals/eclin…

41/ Long covid, by definition, continues for months after the initial infection, and can impair people's lives for months, maybe years.

42/ The long-term consequences of Covid-19 may limit people's ability to contribute to society through work, to earn, pay taxes, volunteering…

All of these consequences contribute to the "burden of disease".

43/ 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.

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

45/ 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.

46/ 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.

47/ It is likely to remain high enough to justify attempts to reach herd immunity through vaccination - which will require vaccination of children.

48/ Indeed, 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.

49/ It is too soon to know if we will need regular boosters (as with eg tetanus and polio); 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.

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@threadreaderapp NB - the blog version of this thread at peterenglish.blogspot.com/2021/07/will-c… has been tidied up, clarified in places, and had some headings and citations added.