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.

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."
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.

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.
@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.

• • •

Missing some Tweet in this thread? You can try to force a refresh

Keep Current with Peter English #FBPE

Peter English #FBPE Profile picture

Stay in touch and get notified when new unrolls are available from this author!

Read all threads

This Thread may be Removed Anytime!


Twitter may remove this content at anytime! Save it as PDF for later use!

Try unrolling a thread yourself!

how to unroll video
  1. Follow @ThreadReaderApp to mention us!

  2. From a Twitter thread mention us with a keyword "unroll"
@threadreaderapp unroll

Practice here first or read more on our help page!

More from @petermbenglish

28 Jul
I was just told "my son went to the Latitude festival, and he and all the people he went with have Covid".

This begs so many questions!
Transmission is predominantly airborne, and mainly occurs when aerosolised respiratory droplets can accumulate, and you spend enough time in the space to breathe in an infectious dose.

That's what most of us believe.
An anecdote like this suggests so many hypotheses that could be tested by investigating transmission patterns at events like Latitude.

Who are the people the son went with? What did they do together? Can you identify clusters or chains of transmission?
Read 10 tweets
27 Jul
Chirr App is a powerful Twitter thread maker. It splits text automatically using an intelligent algorithm that makes your threads easy to read.

You can also control where to split your text using the special character sequence below 👇
Thousands of people came to write Twitter threads but stayed for amazing features like:

📅 Scheduler that allows posting great content consistently.

⚡ Blitz that allows you to reply to your audience distraction-free.

📈 Analytics that helps you to know what works the best.
The Editor is free to use, but if you want more - for just $4/mo you can unlock tons of mighty features.

Edit tweets, reply with a thread, add tweets to a thread a much more!

Give it a try: getchirrapp.com
Read 4 tweets
21 Jul
They dismiss PIMS-TS/MIS-C as uncommon and not proven to be caused by Covid-19. (I am comfortable with this.)
They then, similarly, dismiss "Long Covid" in children. Given (the clue's in the name) that it will take time for sequelae, and their duration, to become apparent; and the increasing evidence base on this, this seems an extremely brave decision (as Sir Humphrey would say).
They then briefly mention "indirect health benefits" (not infecting your parents and becoming an orphan etc) and dismiss them as not significant.

Another very brave decision. Where is the modelling?
Read 19 tweets
21 Jul
That JCVI statement in more detail.


It's clear that their top priority was "reductions in hospitalisations and deaths in the population".
1/ Image
They move on to considering vaccine safety - but say, in effect, there's not enough data.

But many other countries have been vaccinated children without significant safety signals. Image
They explain why they recommend vaccination of 12-17yos with certain conditions increasing their risk (and suggest there will be more detail to follow).

And recommend vaccinating those who are household contacts of immunosuppressed.
3/ ImageImage
Read 6 tweets
18 Jul
It has been announced that JCVI has recommended, or likely to recommend, that only at-risk adolescents be offered the vaccine. I suspect that they considered in their modelling only the direct costs to the health service of acute disease.
I would be particularly interested to see their workings on:

* The benefits to children of ending the pandemic and inevitable restrictions sooner;

* The harms from the secondary cases from the children, especially if teachers, parents or carers are infected;
* The effect of Covid-19 (directly or through self-isolation etc) on loss of school;

* And , crucially, given the growing evidence, the long term consequences on the children and adolescents of “Long Covid”.
Read 4 tweets
7 Jul
Doing trace and isolate properly is hugely resource intensive. Ideally you work out where they caught it and identify others potentially infected by the same source, and identify all the contacts that could become secondary cases and try to persuade them to isolate…
It took all of our resources in Surrey and Sussex for over a week to cope with one case in Feb 2020 There are few shortcuts, despite all we've learned (although there are a few more people to do the work).
It might be manageable if the number of new cases per day (in England) were in the very low hundreds - ideally less than 100.
Read 7 tweets

Did Thread Reader help you today?

Support us! We are indie developers!

This site is made by just two indie developers on a laptop doing marketing, support and development! Read more about the story.

Become a Premium Member ($3/month or $30/year) and get exclusive features!

Become Premium

Too expensive? Make a small donation by buying us coffee ($5) or help with server cost ($10)

Donate via Paypal Become our Patreon

Thank you for your support!

Follow Us on Twitter!