Since I’ve been asked about it a bunch today, here’s my take. Herd immunity is a journey not a destination. Asking “if we’ll get there” is just the wrong question, both technically and operationally 1/
Technically, “herd immunity” describes the incremental reduction or risk to everyone of being in a population where an increasing number of people are immune. That journey starts with the first immune person … it’s just not much of an additional push then 2/
The “herd immunity threshold” is not the endpoint of this journey. The HIT is about preventing outbreaks from starting in the first place and is not practically predictive of elimination (due to randomness, heterogeneity, metapopulation effects) 3/
Operationally, predicting the population immunity necessary for elimination during an epidemic is hard bordering on impossible, because the course of the outbreak is responding to what we’re doing ... and will respond to what we do in the future 4/
My field is littered with errant predictions of when we’ll eliminate malaria, polio, measles (that last one was me … and I sat in that big room at the WHO and was dressed down by luminaries in my field) 5/
As I’ve said before here - the herd immunity threshold is better assessed by a historian than predicted by an epidemiologist because the consequences of getting it wrong can lead folks to take their foot off the brakes too early 6/
Rather than focus on an imprecise theoretical target, we should focus on the benefits of vaccines to hasten the journey 7/
Vaccines MIGHT get us to elimination (only one, smallpox, has done that in human history) but WILL get risk down to low enough levels that we don’t have to structure all of society around this virus and we can get back to addressing other issues 8/
We must humanize the benefits of vaccination. Rather than efficacy and herd immunity, we should be talking about protecting ourselves and protecting our communities. Vaccines does this on all fronts - healthy people, healthy populations, healthy economies /x

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More from @TheFerrariLab

2 May
My MSC (wildlife) advisor wrote a paper asking “is over abundance a conservation concern?”, basically making the case for preparing way in advance for the success of interventions. We’re seeing the failure to do think ahead manifest in misguided COVID policies now 1/
We knew vaccines were coming, we knew lack of documentation would create incentives for cheating. People claiming immunity to avoid masking/testing/distancing. Yet we’re still seeing ham handed messaging & rules that put the cost of compliance on private citizens 2/
Rather than the cost of compliance borne by the state; i.e. the state takes an unpopular stance for the public good rather than making shop owners confront individuals who differ in risk tolerance (or just want to be bullies picking a fight) 3/
Read 10 tweets
19 Mar
Only a fool would wager on the herd immunity threshold for COVID after the year we’ve had … but I would wager this — 10 years from now we will still be debating the HIT for SARS-CoV-2 🧵1/

Five reasons why COVID herd immunity is probably impossible…
The HIT is a magnificent, elegant theoretical result. It is very powerful for planning — it is the reason that we can make plans now to open schools and businesses in later even if we won’t have 100% of the population immune (vaccine or exposure) 2/
But you don’t need to know the exact magnitude of the HIT to plan … and frankly, you’d be a fool to plan around any exact magnitude. The magnitude of the HIT when you start planning is unlikely to be the magnitude of the HIT when you get there - Why? 3/
Read 11 tweets
16 Jan
Because it’s 2021 and we have to say the quiet parts out loud:
Vaccinated individuals must still wear masks, distance, and be treated exactly the same as non-vaccinated individuals. This isn’t just about immunity … 1/
The risks of creating a vaccinated/unvaccinated class system are:
1. It would certainly exacerbate existing inequities in access to services
2. It will create incentives for cheating and forgery of vaccine documents
3. It will put the onus on citizens to police that system 2/
If you thought mask confrontations at Starbucks were hard now, imagine when anti-mask Karen is waving about homemade vaccine paperwork drawn in crayon claiming she is immune 3/
Read 8 tweets
4 Dec 20
IHME has produced some excellent work (I count its staff members among my collaborators and friends), but also a variety of challenging conflicts of interest. The 2018 MOU with WHO is particularly so 1/
The WHO reliance on IHME for burden of disease metrics creates a too-big-to-fail problem that also disincentivizes investment in in-country capacity. LMIC researchers will struggle to convince WHO that their efforts stand up to the IHME juggernaut 2/
There are governance models that could be used to address some of the potential pathologies; e.g. see work by @devisridhar and @marleetichenor…
Read 4 tweets
3 Dec 20
I’ve been thinking about vaccination cards in the US a lot and am struggling to figure out what side to fall on. Cards are a huge part of monitoring and communication in LICs. Without them we’re really in a fog about vaccination coverage 1/
We haven’t got recent experience in the US, so this would be a new strategy and could go awry, leading to stigma and adverse consequences 2/
We also will need to document coverage, and in the absence of a centralized health delivery system in the US (e.g. MCA), a simple, low-tech solution like cards could make follow-up and monitoring of coverage much simpler to implement than trying to work with many providers 3/
Read 4 tweets
2 Dec 20
Hey PSU folks — It’s come to my attention that there is a rumor circulating that I have tested positive for SARS-CoV-2 and am ill. Both are false. It is heartwarming to receive the well-wishes but I am fine 1/
It is also a moment to reflect on the fact that testing positive for a communicable infectious disease is not a condemnation of the individual. If anything, it reflects a failure of the public health infrastructure to prevent that outcome 2/
Now, as we stand at the edge of the COVID-19 vaccine era, this is more even more important to consider. We have always had (blunt) tools (w/ significant off-target consequences) to prevent people from getting sick. We will soon have better tools in the form of a vaccine 3/
Read 7 tweets

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