Viki Male Profile picture
Sep 18, 2020 23 tweets 6 min read Read on X
#HerdImmunity is a talking point again. So let's talk about it! What conditions do we need to fulfil for a herd immunity strategy to work? And is it possible to do this for #COVID19 #SARSCoV2? 🧵
The idea behind herd immunity is simple. We know infectious diseases spread in a population that is susceptible to them. We also know people can become immune. If enough people are immune, the virus doesn't have enough people to spread to. Then small outbreaks will die out. 2/21
Instinctively, we can feel that the more contagious a disease is, the more people need to be immune to prevent it from spreading. 3/21
You might remember R0 - the number of people each infected person passes a disease on to, if everyone is susceptible. For #COVID19 this number is around 3. For measles, it's more like 13. 4/21
The number of people who need to be immune to achieve herd immunity is given by 1 - 1/R0. So for #COVID19, that's about 66% (2/3) of the population. For measles, it's more like 92% of the population. 5/21
This really tells us all we need to know to decide if a herd immunity strategy will work: can we achieve a situation where that percentage of the population is immune? 6/21
Let's think first about measles, where a herd immunity strategy famously can work. 7/21
There are two ways we can get immunity. One is if the virus induces long-lived immunity in people who have recovered. Measles does this. People who had the virus >60 years ago are still immune! 8/21

tinyurl.com/r3qya2p
But wait! Anyone who is old enough to remember the pre-vaccine days will tell you that wasn't enough. Measles epidemics would still happen every couple of years. 9/21
This is because susceptible individuals were joining the population: babies. Although "herd immunity" from natural infection prevented outbreaks of measles for a bit, after a couple of years, there were enough susceptible people that the proportion immune was < 92%. 10/21
And then another outbreak would occur. 11/21
The way around this is the second way we can induce immunity in a population: vaccination. For measles, this works great. The vaccine works really well and so long as almost all children get it, we shouldn't see measles outbreaks. 12/21
So even for measles, which is famous for inducing long-lasting immunity, a herd immunity strategy was possible only once we had a vaccine. 13/21
Now let's think about #COVID19. We don't have a vaccine yet, but can we achieve herd immunity with natural infections? To do that we have to get to 66% of the population immune. 14/21
As a first pass, we might say that 66% of the population therefore needs to catch the virus. But we're seeing that not everyone makes a good response, so maybe it would have to be more like 80% of people catching it for 66% to be immune. 15/21
In Britain, this would be about 54 million people catching #COVID19, and we might expect something like half a million of them to die. 16/21
Now we *can* have a debate about whether it's worth sacrificing half a million people to achieve this aim. But I think that's the wrong discussion to have, because this strategy assumes that #COVID19, like measles, gives long-lasting immunity. 17/21
And we know that most coronaviruses *don't*. Studies on the coronaviruses that cause colds suggest that immunity tends to wear off after about 18 months (most of us know this instinctively, from our experience with colds!). 18/21
Furthermore, we're now seeing some people who are getting reinfected five or so months after their first infection, so #COVID19 also seems to follow this pattern of not giving long-lasting immunity. 19/21
If immunity wears off, this will make that 66% immunity even harder - I would probably say impossible, though it depends exactly how quickly the immunity wears off - to achieve. 20/21
So to summarise: even for diseases where natural infection induces long-lasting immunity (eg measles) #HerdImmunity strategies rely on vaccination. #COVID19 probably does *not* induce long-lasting immunity, so to for a herd immunity strategy, we definitely need a vaccine. \thread
Afterthought: R0 depends on population mixing factors, as well as viral factors. So one thing I suppose we could do to make a "herd immunity" strategy more viable is to reduce R0 by reducing social interactions, mandatory mask wearing etc...
This would reduce the proportion of people who would have to be immune. BUT my feeling is that people who want to pursue a "herd immunity" strategy want to do so precisely to avoid have to reduce social interactions, mandatory masks etc. So I'm not sure this would help.

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More from @VikiLovesFACS

Jul 2
COVID vaccination causes short-lived increases to menstrual cycle length in some people. But how?

A new paper has come out that examines the possibility that this occurs via a direct effect on the ovaries. But does it show that?

(TLDR: Not really...) 🧵

nature.com/articles/s4154…
I'll start by saying that this isn't the first mechanism I would investigate myself. As the authors acknowledge, other vaccines - notably COVID vaccines that do not leave the site of injection - have the same effect. This suggests an effect mediated by the immune response. 2/
But there's something to be said for looking at a direct effect, just in case. So the authors add vaccine directly to cultures of ovarian cells.

Props to them for making an effort to use a plausible concentration of vaccine that might get to the ovaries "end organ dose". 3/
Read 13 tweets
May 22
67 studies including 723,988 ppl who received a #CovidVaccine during #pregnancy!

😷 61% less likely to be infected

🏥 94% less likely to be hospitalised

🤰🏻 12% less likely to have a hypertensive disorder and 9% less likely to have a C section

1/

gh.bmj.com/content/9/4/e0…
👶🏻 Babies 8% less likely to be admitted to NICU

👍🏾 No increased risk of any adverse outcomes in pregnancy or to babies.

2/
Great to see an updated systematic review on this!

And I particularly like this one because the authors have used only the highest quality studies in their primary analysis, and report on lower quality studies separately. This gives us both reliability and transparency. 3/
Read 5 tweets
Feb 8
Studies looking for evidence that mRNA COVID vaccines cross the placenta have not found it (192 babies).

A case report finds degraded vaccine mRNA in umbilical cord blood from one baby, born 2d after their mother was vaccinated.

What does this mean? 🧵

ajog.org/article/S0002-…
mRNA degrades quickly, so it’s possible we haven't detected vaccine mRNA in the placenta because we didn’t look quickly enough.

To address this, the authors look at placentas for two babies born two and ten days after their mothers received a dose of COVID vaccine. 2/
The authors used a much more sensitive form of PCR than has been used in previous studies, called ddPCR. This technique also tells us whether the mRNA is intact. 3/
Read 11 tweets
Oct 23, 2023
🇨🇦 85,650 babies born after #CovidVaccination in #pregnancy vs 56,336 babies from unvaccinated pregnancies

👉🏻 Risk of death and disease lower in newborns from vaccinated pregnancies

👉🏻 No difference in hospital admission up to six months old

1/

jamanetwork.com/journals/jamap…
Image
This was designed as a safety study and the results are reassuring.

This is unsurprising, given the very extensive evidence we already have on the safety of mRNA COVID vaccines in pregnancy. 2/

docs.google.com/document/d/19F…
Image
But the finding that outcomes at birth are actually better for babies from vaccinated pregnancies is interesting!

Vaccinated families do tend to have better healthcare, but even when the study team take account of this in their analysis, the finding remains. 🤔

3/
Read 7 tweets
Sep 12, 2023
👩🏿‍🔬 RCT of Pfizer #CovidVaccine given at 24-34 weeks of #pregnancy, with babies followed to six months old.

🤰🏽 173 vaccinated vs 173 saline control

🏥 No difference in rate of severe AEs

👶🏼 No difference in babies' health at birth or up to 6 months

🧵

classic.clinicaltrials.gov/ct2/history/NC…
This came out while I was away last month, so I didn't post about it at the time. But I talk about it on @SkepticJonGuy's video released last night, which prompted me to share a few (belated!) thoughts... 2/

x.com/SkepticJonGuy/…
Some points to note! This trial is not designed to look at effects of mRNA COVID vaccination in early pregnancy.

We have some data on that from the main trials, showing no difference in pregnancy outcomes between vaccinated and control groups... 3/

fda.gov/media/152256/d…
Image
Read 15 tweets
Aug 23, 2023
🇺🇸 Yesterday, the FDA approved an RSV vaccine for use in pregnancy.

There was a non-significant difference in preterm birth in the trial, which patients will be informed about.

Let's look at the minutes of the meeting to understand these decisions... 🧵

fda.gov/news-events/pr…
Before we start, here are links to the trial report...



And the meeting minutes... 2/

nejm.org/doi/10.1056/NE…
fda.gov/media/169361/d…
RSV is the leading cause of death in babies <6 mo, and is implicated in 50% of hospitalisations for respiratory illness, so development of a vaccine is a priority.

I should declare an interest here... my baby son was very sick with RSV 😢 and I would have loved to avoid that. 3/
Read 15 tweets

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