9 ms·

The herd immunity threshold changed with Delta, which I've heard has an R0 of 8-10 (compare 3.5-4.5 for the UK Alpha variant, and about 2.5 for the original str

The herd immunity threshold changed with Delta, which I've heard has an R0 of 8-10 (compare 3.5-4.5 for the UK Alpha variant, and about 2.5 for the original strain). Instead of 60% vaccination, you'd need 90%, and will likely overshoot that. It wouldn't surprise me if the UK's cases are now dropping because they reached that threshold, but I doubt the US is there now. Probably Delta will blow through literally all the remaining unvaccinated people and that'll be the last surge, at least unless we get a vaccine-resistant strain.

deleted 5y ago
[deleted]
mensetmanusman 5y ago
“ Instead of 60% vaccination, you'd need 90%, and will likely overshoot that.” 60% (Vaccinated + Previously Infected)
- crowbahr 5y ago
  Delta variant can reinfect.
  - arwhatever 5y ago
    It's not a binary consideration, but you have expressed it as such.
samizdat311 5y ago
These are very good points. I'd add that herd immunity isn't a clearly defined threshold, either. It's the point at which the effective R in a population is < 1 -- AKA exponential decay. With a circulating virus, depending on how much below 1 you get, exponential decay might take a few months or even a few years to get down to elimination. The current dip in the UK is trending toward the former. But it's further complicated by the fact that levels of immunity in the population will continue to oscillate up and down. (Nevermind that once schools open in September, 20% of the population with relatively sparse vaccination will be mixing in large cohorts). Another factor in herd immunity is vaccine efficacy - which is somewhere between 86% and 95% for the mRNA vaccines (and will continue to change). A herd immunity threshold of 90% presupposes a vaccine with 100% efficacy - when your vaccines are not that effective (not to mention, losing effectiveness over time) it bumps the threshold even higher. Right now, much of the population has some level of "sterilising immunity" from the first round of vaccines and previous infection (i.e. having enough circulating antibodies to potentially block symptomatic infection and transmission). But everything we know about existing respiratory viruses (and indications so far this year) suggest antibodies will wane. Of course, indications are most people will come away with robust cell-based immunity. (Those plucky T-cells and B-cells). But eradication for the UK looks *very* close to impossible given the near-total-abandonment of public health measures recently.
- nicoburns 5y ago
  > eradication for the UK looks very close to impossible given the near-total-abandonment of public health measures recently. I don't support the recent abandonment of restrictions, but could that policy, followed up by stricter restrictions in a few months time actually be helpful for eradication? Eradication is likely a hopeless prospect in the UK anyway, because there is no political will for closing the borders. But it's interesting to conjecture.
  - XorNot 5y ago
    The ship has sailed on eradication. We're ALL going to get Delta at some point now, it's just a question under what circumstances (and whether the antivaxxers will have sufficient numbers to denial of service the healthcare system as they die).
    - dragonwriter 5y ago
      > We're ALL going to get Delta at some point now, That's by no means certain; even ignoring the possibility of effective countermeasures, a more transmissible variant could displace it (if we’re lucky, it could also be one producing less severe disease, the way it was when Epsilon variant for a while displaced most others in California, before Delta, which is more transmissible than Epsilon, took hold.)
      - manwe150 5y ago
        Tangential question: what is the mechanism by which we observe ‘displacement’ of earlier strains? Is it just the exponentially higher transmission factor of the variant that is making it relatively disappear? Or are they somehow also get out-competed directly?
        XorNot 5y ago
        The former: once you have COVID you're immune to it. Unless a strain totally evades the antibody response of recovered individuals (very unlikely), then they're no longer capable of spreading it. So if there's overall less replicants of one strain then the other, then the population will effectively select for the more infectious one. Actual geography of course is also key here though: if a carrier flies over to an unexposed area with any strain, it's going to take over if the other one doesn't make it there.
adventured 5y ago
> It wouldn't surprise me if the UK's cases are now dropping because they reached that threshold, but I doubt the US is there now. Given the enormous US Covid outbreak, the US is likely not far away. We're at 70% adult vaccination and had a minimum of 100 million prior Covid infections. Also throw in tens of millions of people that likely have natural immunity to Covid. The biggest question with Delta is going to be the matter of repeat infections and how long the vaccines, the vaccines + prior infections, and prior infections (without vaccination) all hold up against it. To say nothing of what variations will follow next after Delta. Is there actually herd immunity to be found with Delta, or will people just keep getting variations of Covid from here forward. I don't believe there will be herd immunity for any nations, there will just be far lower mortality rates due to prior virus exposure and the vaccines. Some non-trivial % of the vaccinated population will continue to get Covid. If there was a chance to squash Covid with the early strain via herd immunity, those days are very long gone. The great lie still being persisted for the benefit of the agitated public, is that Covid is going to go away courtesy of vaccinations. The authorities have pitched that to the public from the early days of the vaccines to try to max out the vaccination rates. Do this and Covid goes away. The vaccines are never going to stop Covid, they're going to continue to dramatically reduce the mortality rates (which is critical, however the public still doesn't widely grasp the reality of the forever Covid future yet).
- sokoloff 5y ago
  If the forever-COVID reality for 95+% of the population is "you really, really ought to get that annual booster shot" (instead of "get the flu shot iff it's convenient"), that's not actually so terrible. If it becomes century-long endemic in the population, kids are exposed to it, likely creating a productive exposure-related immune reaction until they're old enough to get an mRNA vaccine.
  - adventured 5y ago
    Definitely. I don't think it's terrible at all given the alternative is tens or hundreds of thousands of additional people dying from Covid per year in the US. I guess we'll soon see the extent to which the public can be convinced to get booster shots every year. The governments of the world will have their jobs cut out for them, there will be a lot of resistance to forever Covid vaccinations as time goes on (and very lucrative for big pharma; Pfizer, BioNTech and Moderna are going to make a stupid amount of money over the next few years). What will be interesting to find out in the near future, is if high vaccination rates + prior infections reduce Covid deaths below, say, 50k people in the US every year, or whether we're forever stuck with a sizable population killer no matter what (short of implementing elaborate forever cultural systems of lockdown; as in, those counter measures will have to become ingrained deeply into the culture, accepted and automatic). The next decade is going to be very weird and interesting on that front.
eloff 5y ago
> Probably Delta will blow through literally all the remaining unvaccinated people and that'll be the last surge, at least unless we get a vaccine-resistant strain. I think you're right, but that will still happen over years in multiple waves. Barring some sudden uptick in people changing their mind about vaccination.
dTal 5y ago
>at least unless we get a vaccine-resistant strain. ...which will happen promptly. The selection pressures are perfect. You already have a variant with vaccine breakthrough, circulating in a partially vaccinated population.
- robbiep 5y ago
  I don’t know if you can make that bet - evolution of the virus needs to conserve the current mode of cellular entry and attachment and fitness of spike appears to be the main driver at present - and then spike mutation has driven drift from the original vaccine target leading to a degree of immunity ‘escape’. So the question is, is the virus now in a local minima where it is highly adapted for spread (ie big increase in R0 from ‘original’ virus) and are those selective pressures in a situation where the virus can’t jump into a full immunity escape mode without climbing down off mount improbable and taking another face? My bet would be that the selective pressures may continue to drive fitness for reproduction, but won’t lead to immunity escape because spike is too conserved
  - D_Alex 5y ago
    I agree with everything you wrote, however - when we bet, we need to consider the stakes and the payoff. So the important question is: should "we" take the bet on that immunity escape? Or should we not take that chance? My opinion is that we should act decisively to avoid the bet.... but the UK government clearly feels lucky.
  - bhickey 5y ago
    I'll echo that this is a reasonable assessment. In evolutionary hill climbing, the fitter a protein the fewer paths there are to climb higher [0]. Art Poon has some interesting work that effectively maps viral phenotype space by looking at compensatory mutations [1]. He induces deleterious mutations in page phage, grows them until they recover fitness and then assesses if they regained fitness by reversion or compensation. The worse a deleterious mutation, the more likely a compensatory mutation is to arise. The more optimized the covid spike becomes, the harder it is to get better. What frightens me are polymerase mutations. [0] Darwinian Evolution Can Follow Only Very Few Mutational Paths to Fitter Proteins https://science.sciencemag.org/content/312/5770/111 https://science.sciencemag.org/content/312/5770/111 [1] The rate of compensatory mutation in the DNA bacteriophage phiX174https://pubmed.ncbi.nlm.nih.gov/15911582/ https://pubmed.ncbi.nlm.nih.gov/15911582/
sandworm101 5y ago
Delta's R numbers are not uniform, but certainly are not 0. Some easy-to-grab stats from Canada's worst covid province which is dealing with a Delta wave: Alberta provincewide: 1.48 (1.38-1.59) Edmonton Zone: 1.37 (1.15-1.61) Calgary Zone: 1.50 (1.37-1.64) Rest of Alberta: 1.48 (1.26-1.73) https://www.alberta.ca/covid-19-alberta-data.aspx https://www.alberta.ca/covid-19-alberta-data.aspx (Fyi, Alberta is Canada's version of West Virginia. It is the national heart of COVID resistance/denial.)
- manwe150 5y ago
  I don’t know any specifics about this, so this as an attempt at asking questions of what to possibly expect from those numbers. Alberta seems to be 63% vaccinated, which seems to put them well above average for the US? And it looks like those numbers are currently computed for the day before the latest surge in cases took off? Edit: later learned the parent is talking about R0 and you are talking about Rt, so those numbers are not directly comparable anyways.
  - sandworm101 5y ago
    Oh. Whenever i see "R number" i think Rt. Thats what the news and government spokespeople use.
stadium 5y ago
Add to that, the more it multiplies and spreads, the more chance of mutations leading to more variants. Needing updated vaccines, booster shots of existing, and/or higher rates needed to reach herd immunity. Didn't the UK have another lockdown for this wave too? Herd-masking seemed like a no-brainer from the beginning. It will be really hard to go back to that in the US after the CDC giving the all clear to go maskless.
- nradov 5y ago
  You can't expect people to wear masks forever.
  - stadium 5y ago
    True, but maybe just until the pandemic is actually under control. Which would happen faster if people kept their masks on.
kkvcdyh 5y ago
Where did you "hear" it has an R0 of 8-10? It's not time to be imprecise. If you're referencing the CDC's case study from Provincetown, I think it's important to point out that the R value cited is the R value during Bear Week, not on average.
- amirhirsch 5y ago
  When people present R0 as a property of the pathogen, you can take it as hot panicky air. From wiki: "R0 is not a biological constant for a pathogen as it is also affected by other factors such as environmental conditions and the behaviour of the infected population."
  - manwe150 5y ago
    Wiki also lists this number as the current R0 (though without a source, as far as I can see). Found this study too in attempting to answer your question: https://www.fil.ion.ucl.ac.uk/spm/covid-19/forecasting/ https://www.fil.ion.ucl.ac.uk/spm/covid-19/forecasting/ And learned the difference with Rt today (which some others are quoting, eg for Alberta), is that Rt is computed, while R0 is a guess based on a model, to try to normalize Rt so that it is more comparable over time and place. So I thought that was cool!
- mdp2021 5y ago
  > It's not time to be imprecise :) Many of us have felt something along those lines since the early times. But e.g. the viral load for said variant was reported as _1000x_ Wuhan weeks ago, value changed to _3x_ days ago. I read the emergence of Tinnitus after COVID-19 weeks ago at _67%_, today at _0.shmbrmbsomething_. We are flooded in bad numbers. (And of course, that R0 is also circumstantial, vs the examples above...)
fshbbdssbbgdd 5y ago
The UK Office for National Statistics actually puts out estimates for the prevalence of covid antibodies (from both vaccines and infections combined), and you are right on the money! They estimated that as of June 14, 89.8% of the population would test positive for antibodies. https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/conditionsanddiseases/bulletins/coronaviruscovid19infectionsurveyantibodyandvaccinationdatafortheuk/7july2021 https://www.ons.gov.uk/peoplepopulationandcommunity/healthan... Indeed, even with Delta’s terrifying R-value, this seems like enough to suppress the virus.
mdp2021 5y ago
I also kept in mind that amount of R0 for the "Indian variant", until last week I saw it halved to 4..5×. Then you realize that a transmission rate among the timid in ShyFleetersLand and the compulsive in LetsHugLand cannot be the same. Then you realize that that very average number needs explanations of the underlying model to be meaningful - it comes from societal behaviour not just from intrinsic viral properties.
mdp2021 5y ago
Look, just fresh in came an article from Shabir A. Madhi, Dean Faculty of Health Sciences and Professor of Vaccinology at University of the Witwatersrand; and Director of the SAMRC Vaccines and Infectious Diseases Analytics Research Unit, University of the Witwatersrand COVID-19 herd immunity? It's not going to happen, so what next? https://theconversation.com/covid-19-herd-immunity-its-not-going-to-happen-so-what-next-165471 https://theconversation.com/covid-19-herd-immunity-its-not-g... > we would need to get close to 84% of the global population developing protection against infection (in the absence of non-pharmacological interventions) in as brief a period of time as possible (...also, there, you find «the reproductive rate [...] is probably closer to six rather than three»)

deleted 5y ago

[deleted]

mensetmanusman 5y ago

“ Instead of 60% vaccination, you'd need 90%, and will likely overshoot that.” 60% (Vaccinated + Previously Infected)

crowbahr 5y ago
Delta variant can reinfect.
- arwhatever 5y ago
  It's not a binary consideration, but you have expressed it as such.

samizdat311 5y ago

These are very good points. I'd add that herd immunity isn't a clearly defined threshold, either. It's the point at which the effective R in a population is < 1 -- AKA exponential decay. With a circulating virus, depending on how much below 1 you get, exponential decay might take a few months or even a few years to get down to elimination. The current dip in the UK is trending toward the former. But it's further complicated by the fact that levels of immunity in the population will continue to oscillate up and down. (Nevermind that once schools open in September, 20% of the population with relatively sparse vaccination will be mixing in large cohorts). Another factor in herd immunity is vaccine efficacy - which is somewhere between 86% and 95% for the mRNA vaccines (and will continue to change). A herd immunity threshold of 90% presupposes a vaccine with 100% efficacy - when your vaccines are not that effective (not to mention, losing effectiveness over time) it bumps the threshold even higher. Right now, much of the population has some level of "sterilising immunity" from the first round of vaccines and previous infection (i.e. having enough circulating antibodies to potentially block symptomatic infection and transmission). But everything we know about existing respiratory viruses (and indications so far this year) suggest antibodies will wane. Of course, indications are most people will come away with robust cell-based immunity. (Those plucky T-cells and B-cells). But eradication for the UK looks *very* close to impossible given the near-total-abandonment of public health measures recently.

nicoburns 5y ago
> eradication for the UK looks very close to impossible given the near-total-abandonment of public health measures recently. I don't support the recent abandonment of restrictions, but could that policy, followed up by stricter restrictions in a few months time actually be helpful for eradication? Eradication is likely a hopeless prospect in the UK anyway, because there is no political will for closing the borders. But it's interesting to conjecture.
- XorNot 5y ago
  The ship has sailed on eradication. We're ALL going to get Delta at some point now, it's just a question under what circumstances (and whether the antivaxxers will have sufficient numbers to denial of service the healthcare system as they die).
  - dragonwriter 5y ago
    > We're ALL going to get Delta at some point now, That's by no means certain; even ignoring the possibility of effective countermeasures, a more transmissible variant could displace it (if we’re lucky, it could also be one producing less severe disease, the way it was when Epsilon variant for a while displaced most others in California, before Delta, which is more transmissible than Epsilon, took hold.)
    - manwe150 5y ago
      Tangential question: what is the mechanism by which we observe ‘displacement’ of earlier strains? Is it just the exponentially higher transmission factor of the variant that is making it relatively disappear? Or are they somehow also get out-competed directly?
      - XorNot 5y ago
        The former: once you have COVID you're immune to it. Unless a strain totally evades the antibody response of recovered individuals (very unlikely), then they're no longer capable of spreading it. So if there's overall less replicants of one strain then the other, then the population will effectively select for the more infectious one. Actual geography of course is also key here though: if a carrier flies over to an unexposed area with any strain, it's going to take over if the other one doesn't make it there.

adventured 5y ago

> It wouldn't surprise me if the UK's cases are now dropping because they reached that threshold, but I doubt the US is there now. Given the enormous US Covid outbreak, the US is likely not far away. We're at 70% adult vaccination and had a minimum of 100 million prior Covid infections. Also throw in tens of millions of people that likely have natural immunity to Covid. The biggest question with Delta is going to be the matter of repeat infections and how long the vaccines, the vaccines + prior infections, and prior infections (without vaccination) all hold up against it. To say nothing of what variations will follow next after Delta. Is there actually herd immunity to be found with Delta, or will people just keep getting variations of Covid from here forward. I don't believe there will be herd immunity for any nations, there will just be far lower mortality rates due to prior virus exposure and the vaccines. Some non-trivial % of the vaccinated population will continue to get Covid. If there was a chance to squash Covid with the early strain via herd immunity, those days are very long gone. The great lie still being persisted for the benefit of the agitated public, is that Covid is going to go away courtesy of vaccinations. The authorities have pitched that to the public from the early days of the vaccines to try to max out the vaccination rates. Do this and Covid goes away. The vaccines are never going to stop Covid, they're going to continue to dramatically reduce the mortality rates (which is critical, however the public still doesn't widely grasp the reality of the forever Covid future yet).

sokoloff 5y ago
If the forever-COVID reality for 95+% of the population is "you really, really ought to get that annual booster shot" (instead of "get the flu shot iff it's convenient"), that's not actually so terrible. If it becomes century-long endemic in the population, kids are exposed to it, likely creating a productive exposure-related immune reaction until they're old enough to get an mRNA vaccine.
- adventured 5y ago
  Definitely. I don't think it's terrible at all given the alternative is tens or hundreds of thousands of additional people dying from Covid per year in the US. I guess we'll soon see the extent to which the public can be convinced to get booster shots every year. The governments of the world will have their jobs cut out for them, there will be a lot of resistance to forever Covid vaccinations as time goes on (and very lucrative for big pharma; Pfizer, BioNTech and Moderna are going to make a stupid amount of money over the next few years). What will be interesting to find out in the near future, is if high vaccination rates + prior infections reduce Covid deaths below, say, 50k people in the US every year, or whether we're forever stuck with a sizable population killer no matter what (short of implementing elaborate forever cultural systems of lockdown; as in, those counter measures will have to become ingrained deeply into the culture, accepted and automatic). The next decade is going to be very weird and interesting on that front.

eloff 5y ago

> Probably Delta will blow through literally all the remaining unvaccinated people and that'll be the last surge, at least unless we get a vaccine-resistant strain. I think you're right, but that will still happen over years in multiple waves. Barring some sudden uptick in people changing their mind about vaccination.

dTal 5y ago

>at least unless we get a vaccine-resistant strain. ...which will happen promptly. The selection pressures are perfect. You already have a variant with vaccine breakthrough, circulating in a partially vaccinated population.

robbiep 5y ago
I don’t know if you can make that bet - evolution of the virus needs to conserve the current mode of cellular entry and attachment and fitness of spike appears to be the main driver at present - and then spike mutation has driven drift from the original vaccine target leading to a degree of immunity ‘escape’. So the question is, is the virus now in a local minima where it is highly adapted for spread (ie big increase in R0 from ‘original’ virus) and are those selective pressures in a situation where the virus can’t jump into a full immunity escape mode without climbing down off mount improbable and taking another face? My bet would be that the selective pressures may continue to drive fitness for reproduction, but won’t lead to immunity escape because spike is too conserved
- D_Alex 5y ago
  I agree with everything you wrote, however - when we bet, we need to consider the stakes and the payoff. So the important question is: should "we" take the bet on that immunity escape? Or should we not take that chance? My opinion is that we should act decisively to avoid the bet.... but the UK government clearly feels lucky.
- bhickey 5y ago
  I'll echo that this is a reasonable assessment. In evolutionary hill climbing, the fitter a protein the fewer paths there are to climb higher [0]. Art Poon has some interesting work that effectively maps viral phenotype space by looking at compensatory mutations [1]. He induces deleterious mutations in page phage, grows them until they recover fitness and then assesses if they regained fitness by reversion or compensation. The worse a deleterious mutation, the more likely a compensatory mutation is to arise. The more optimized the covid spike becomes, the harder it is to get better. What frightens me are polymerase mutations. [0] Darwinian Evolution Can Follow Only Very Few Mutational Paths to Fitter Proteins https://science.sciencemag.org/content/312/5770/111 https://science.sciencemag.org/content/312/5770/111 [1] The rate of compensatory mutation in the DNA bacteriophage phiX174https://pubmed.ncbi.nlm.nih.gov/15911582/ https://pubmed.ncbi.nlm.nih.gov/15911582/

sandworm101 5y ago

Delta's R numbers are not uniform, but certainly are not 0. Some easy-to-grab stats from Canada's worst covid province which is dealing with a Delta wave: Alberta provincewide: 1.48 (1.38-1.59) Edmonton Zone: 1.37 (1.15-1.61) Calgary Zone: 1.50 (1.37-1.64) Rest of Alberta: 1.48 (1.26-1.73) https://www.alberta.ca/covid-19-alberta-data.aspx https://www.alberta.ca/covid-19-alberta-data.aspx (Fyi, Alberta is Canada's version of West Virginia. It is the national heart of COVID resistance/denial.)

manwe150 5y ago
I don’t know any specifics about this, so this as an attempt at asking questions of what to possibly expect from those numbers. Alberta seems to be 63% vaccinated, which seems to put them well above average for the US? And it looks like those numbers are currently computed for the day before the latest surge in cases took off? Edit: later learned the parent is talking about R0 and you are talking about Rt, so those numbers are not directly comparable anyways.
- sandworm101 5y ago
  Oh. Whenever i see "R number" i think Rt. Thats what the news and government spokespeople use.

stadium 5y ago

Add to that, the more it multiplies and spreads, the more chance of mutations leading to more variants. Needing updated vaccines, booster shots of existing, and/or higher rates needed to reach herd immunity. Didn't the UK have another lockdown for this wave too? Herd-masking seemed like a no-brainer from the beginning. It will be really hard to go back to that in the US after the CDC giving the all clear to go maskless.

nradov 5y ago
You can't expect people to wear masks forever.
- stadium 5y ago
  True, but maybe just until the pandemic is actually under control. Which would happen faster if people kept their masks on.

kkvcdyh 5y ago

Where did you "hear" it has an R0 of 8-10? It's not time to be imprecise. If you're referencing the CDC's case study from Provincetown, I think it's important to point out that the R value cited is the R value during Bear Week, not on average.

amirhirsch 5y ago
When people present R0 as a property of the pathogen, you can take it as hot panicky air. From wiki: "R0 is not a biological constant for a pathogen as it is also affected by other factors such as environmental conditions and the behaviour of the infected population."
- manwe150 5y ago
  Wiki also lists this number as the current R0 (though without a source, as far as I can see). Found this study too in attempting to answer your question: https://www.fil.ion.ucl.ac.uk/spm/covid-19/forecasting/ https://www.fil.ion.ucl.ac.uk/spm/covid-19/forecasting/ And learned the difference with Rt today (which some others are quoting, eg for Alberta), is that Rt is computed, while R0 is a guess based on a model, to try to normalize Rt so that it is more comparable over time and place. So I thought that was cool!
mdp2021 5y ago
> It's not time to be imprecise :) Many of us have felt something along those lines since the early times. But e.g. the viral load for said variant was reported as _1000x_ Wuhan weeks ago, value changed to _3x_ days ago. I read the emergence of Tinnitus after COVID-19 weeks ago at _67%_, today at _0.shmbrmbsomething_. We are flooded in bad numbers. (And of course, that R0 is also circumstantial, vs the examples above...)

fshbbdssbbgdd 5y ago

The UK Office for National Statistics actually puts out estimates for the prevalence of covid antibodies (from both vaccines and infections combined), and you are right on the money! They estimated that as of June 14, 89.8% of the population would test positive for antibodies. https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/conditionsanddiseases/bulletins/coronaviruscovid19infectionsurveyantibodyandvaccinationdatafortheuk/7july2021 https://www.ons.gov.uk/peoplepopulationandcommunity/healthan... Indeed, even with Delta’s terrifying R-value, this seems like enough to suppress the virus.

mdp2021 5y ago

I also kept in mind that amount of R0 for the "Indian variant", until last week I saw it halved to 4..5×. Then you realize that a transmission rate among the timid in ShyFleetersLand and the compulsive in LetsHugLand cannot be the same. Then you realize that that very average number needs explanations of the underlying model to be meaningful - it comes from societal behaviour not just from intrinsic viral properties.

mdp2021 5y ago

Look, just fresh in came an article from Shabir A. Madhi, Dean Faculty of Health Sciences and Professor of Vaccinology at University of the Witwatersrand; and Director of the SAMRC Vaccines and Infectious Diseases Analytics Research Unit, University of the Witwatersrand COVID-19 herd immunity? It's not going to happen, so what next? https://theconversation.com/covid-19-herd-immunity-its-not-going-to-happen-so-what-next-165471 https://theconversation.com/covid-19-herd-immunity-its-not-g... > we would need to get close to 84% of the global population developing protection against infection (in the absence of non-pharmacological interventions) in as brief a period of time as possible (...also, there, you find «the reproductive rate [...] is probably closer to six rather than three»)