Thread by @GYamey

And we're off!

Kudos to @malar0ne for organizing this.

First up: a discussion of the "let the virus" rip strategy pushed by the Great Barrington Declaration.

GBD wanted herd immunity from natural infection.
They influenced policy:

"From Oct 2020-Jan 2021, we allowed 270,000 people to die in the US," said Prof Lauren Ancel Meyers

Fascinating discussion of "overshoot," as described by @CT_Bergstrom & @nataliexdean. Prof Meyers says that Britain's strategy of regular testing is valuable in this context.

nytimes.com/2020/05/01/opi…

Prof Meyers shows data on Austin, Texas, showing that communities with higher social vulnerability have a higher attack rate AND are less likely to have vaccination access & high coverage. She says we need to tackle this inequity to reach vaccine herd immunity.

Dr Meyers discussed a study she co-authored. GBD is based in part "on unpublished research that suggests low herd immunity thresholds (HITs) of 10-20%. We re-evaluate these findings & correct a flawed assumption leading to COVID-19 HIT estimates of 60-80%"
medrxiv.org/content/10.110…

Next speaker is Prof Julie Parsonnet, who discusses historical studies on herd immunity, in humans & other animals.

Herd immunity against several infectious diseases in the US was only achieved by high efficacy vaccines, says Dr Parsonnet (smallpox, polio, measles).

"I can't think of any disease where on a large scale we've achieved herd immunity without a vaccine," says Prof Parsonnet.

She now pivots to the global picture.

"We live in a world where there are a LOT of unvaccinated people," says Prof Parsonnet.

We won't end the pandemic unless we address this.

Next up is Prof Marc Lipsitch. He'll discuss "Herd immunity: necessary, sufficient or possible?"

He kicks off by saying that while we often focus on R0 (which could be up to 6 in some settings for COVID-19), we should also be considering vaccine effectiveness against transmission.

Here he answers his questions:

Vaccines work very well in the most at-risk (e.g. elderly), which he says makes reaching herd immunity itself less important. The study in Israel that he co-authored is very encouraging:

nejm.org/doi/full/10.10…

Prof Lipsitch notes that the data so far show that vaccines do reduce transmission, which is very encouraging.

But to reach vaccine herd immunity, based on what we know so far about the magnitude of this reduction in transmission, we *may* need to vaccinate everyone, he says:

Even if we reach the herd immunity threshold in the US and other rich nations, the virus will continue to circulate worldwide, says Prof Lipsitch. So it won't be enough JUST for the US to reach the HIT.

Global inequity in vaccination; vaccine hesitancy; evolution; and immune waning, he says, will be ongoing challenges to reaching global vaccine herd immunity.

Last panel presentation is from Prof Rustom Antia.

He notes that for endemic human coronaviruses (which don't tend to cause severe disease), immunity from natural infection wanes over time.

science.sciencemag.org/content/371/65…

Prof Antia et al's Science paper:

"The transition from epidemic to endemic dynamics is associated with a shift in the age distribution of primary infections to younger age groups, which in turn depends on how fast the virus spreads."

"Longer-lasting sterilizing immunity will slow the transition to endemicity. Depending on the type of immune response it engenders, a vaccine could accelerate establishment of a state of mild disease endemicity."

His key unknowns for the transition to benign endemicity:

PANEL Q&A TIME!

Q1. Is Brazil's push for herd immunity through natural infection (the GBD approach) the right strategy?

Prof Lipsitch: 1 flaw is that we don't know how protective herd immunity from natural infection is; natural immunity is accomplished with a lot of deaths

Q2. On CATS and herd immunity😿

"Cats are sociopaths and want to kill us," says Dr Parsonnet

(hey, I am just the MESSENGER of this panel).

She was joking. She's not worried about cats getting in the way of humans reaching vaccine herd immunity.

Q3. Do superspreader events affect the HIT?

Dr Meyers: they can make it difficult for our interventions to be effective. It's a challenge to mitigation. It depends on the REST of the population (i.e. are these isolated events but on top of little background transmission?)

Q4. Cross-protection against other coronaviruses: could this help us get to herd immunity for COVID-19?

Marc Lipsitch: there's no evidence that past infection with a different coronavirus provides reliable protection against COVID-19 infection

Q5. Do variants of concern mean that the herd immunity threshold is now higher?

Dr Lipsitch: if VOC is more transmissible (e.g. B.1.1.7), yes, the HIT is higher
Dr Meyers: HIT does have modifiers e.g. durability of immunity, evolution of variants, etc.

Dr Meyers: masking is crucial. Keep masking as it will allow us to get to herd immunity faster. Testing can be HUGELY helpful (e.g. see the UK's approach) as it can reduce transmission *if* people isolate & are supported to isolate (@BillHanage has made this point too).

Dr Parsonnet: we need to "press ahead" and vaccinate folks as fast as possible. The mRNA vaccines may be adaptable to variants. We have a window right now to really push to get folks immune. Use masking to reduce susceptibility while also scaling up vaccines. Protect people now!

Q. What's the value of testing?

Dr Meyers: there are lots of really great examples of using testing to stop cases at the school gate! We published a study on testing & isolation: when R0 is over 1, this is VERY cost effective.

Here's her study:

thelancet.com/journals/lanpu…

Q. What's the biggest misconception you've seen about herd immunity?

Dr Lipsitch: when people talk about reaching herd immunity, they think it is 'yes' or 'no,' but every little helps and it is not about being on or off. Lower transmission is better! Herd immunity is continuous

Dr Parsonnet: people think the vaccines will solve all problems & nil else will benefit. This is false. We need a "whole toolbox" of efforts: need to wear masks, avoid superspreader events, etc. The vaccine is also not perfect.

Dr Antia: vaccines can be very effective even if they don't engender vaccine herd immunity, as they reduce transmission.

Dr Meyers: we have 'hot seasons' of COVID-19 in the future, & we may need periods of interventions e.g., masks in winter if cases rise.

Dr Meyers: vaccines are going to transform COVID-19 into an endemic illness, like seasonal flu.

We as scientists need to study: what would indicate to us that the disease has transitioned to low endemicity?

Dr Meyers: we as scientists can help guide and shape policy on things like masking & testing. We know these work, & can guide the community on when they are needed the most.

Marc Lipsitch: there's pretty good evidence that people with past infection make a great antibody response to a single dose of vaccine. That's why Israel is vaccinating those who were previously infected.

Q: what are you optimistic about?

Lipsitch: many nations eliminated COVID-19 without a vaccine. It can be done. It is implausible that the US can achieve this, he says, perhaps with the exception of Hawaii.

He says vaccines will at least 'defang' the pandemic in rich nations

Prof Meyers: I'm hopeful about vaccines. Locally, our hospitalization data suggests that lengths of stay have fallen, so it looks like we're already defanging it. Mortality rate has fallen.

Antia: eradication is not possible, but low endemicity is possible. We'll probably need 2 vaccinate adults repeatedly 4 low endemicity

Dr Parsonnet: the no. of patients in our hospital is way down. I'm hopeful because there's political will to deal with COVID-19, unlike in 2020

OK folks, that's it for me. I typed as fast as I could & did my best to find the papers that the panelists mentioned. Over 750 people listened in.

CONGRATS TO @malar0ne for a world class panel!