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If vaccinated people can still spread the virus why would a "critical mass" of vaccinations change anything? Herd immunity relies on the fact that either havin

If vaccinated people can still spread the virus why would a "critical mass" of vaccinations change anything? Herd immunity relies on the fact that either having been vaccinated or recovered from the disease prevents you from spreading the disease to those that have not yet been vaccinated or can't get vaccinated.

arrosenberg 6y ago
A virus spreading to a pandemic level is a stochastic process. If you have enough people vaccinated, then there aren't enough carriers to push r over 1 even if unvaccinated people are catching and actively transmitting the disease. Doesn't matter at an individual level, but the restrictions are a societal issue due to lack of capacity for care and treatment.
- SigmundA 6y ago
  Right so if you are vaccinated you are no longer a carrier and cannot infect those who are not vaccinated. If enough people get vaccinated they act as a barrier between the carriers and the unvaccinated, hence herd immunity.
  - arrosenberg 6y ago
    Ah, I misunderstood your original question a bit. If the vaccinated were still acting as carriers, the critical mass would still be important (though I'd imagine the math would be different) because the window of transmissibility would be much shorter and drive R way down. Vaccine would mean less viral load, less viral load means less transmission overall. This seems less likely to me than what is actually happening (vaccine inhibits transmission), but I could picture mechanisms by which that would happen.
  - refurb 6y ago
    It’s not black and white “vaccinated = no longer a carrier”, but rather a gradient where vaccinated people have antibodies which reduce the amount of viral shedding to such low levels (some will be zero) that transmission is potentially an order of magnitude less.
maxerickson 6y ago
Vaccines tend not to be 100% effective. The Covid-19 vaccines appear to be very good in real world use, but the less effective a given vaccine is, the larger percentage of the population you need vaccinated to stop the virus spreading. That can get misscommunicated/misunderstood, and in addition lots of public health folks have been very conservative about saying anything that wasn't demonstrated in the trials, which didn't measure overall infection rates (they tracked symptomatic infections). Data is finally coming out about overall infection rates in vaccinated people and they appear to be very low.
- fossuser 6y ago
  It's wild to me that in the trials they didn't test all participants (including asymptomatic people) when asymptomatic transmission is the reason we're in a pandemic.
  - SigmundA 6y ago
    So have there ever been vaccines that prevent you from getting getting the disease while still allowing you to spread it? This is a genuine question.
    - fossuser 6y ago
      I don't know - I'm also not the person that was arguing that. I have pretty high confidence that if you have the vaccine you won't transmit the disease - I suspect the scientists are just being overly cautious because they don't have the data yet (though it is now starting to prove this out). They don't have the data in part, because the trials did not test asymptomatic people. This means it's possible that some of the vaccinated people in the trial caught the virus, but at a reduced severity that was asymptomatic (but still transmissible). Pretend that the vaccine reduced severity, but most people still caught it and were just asymptomatic. In that world, the vaccine wouldn't be enough (and the trials would not have revealed this). Since we know that some people who get the vaccine do catch the virus, but just have a mild case - it seems like a plausible outcome that others catch it without symptoms, but can still transmit it. That said, it's looking like that's not the world we're in - and the vaccines do just stop you from contracting the disease in addition to reducing severity in the rare case you catch it anyway. mRNA vaccines are new and given the asymptomatic transmission of the virus is why we have a pandemic, it's a reasonable thing to be cautious about (which is why I was surprised the trials only tested symptomatic patients).

arrosenberg 6y ago

A virus spreading to a pandemic level is a stochastic process. If you have enough people vaccinated, then there aren't enough carriers to push r over 1 even if unvaccinated people are catching and actively transmitting the disease. Doesn't matter at an individual level, but the restrictions are a societal issue due to lack of capacity for care and treatment.

SigmundA 6y ago
Right so if you are vaccinated you are no longer a carrier and cannot infect those who are not vaccinated. If enough people get vaccinated they act as a barrier between the carriers and the unvaccinated, hence herd immunity.
- arrosenberg 6y ago
  Ah, I misunderstood your original question a bit. If the vaccinated were still acting as carriers, the critical mass would still be important (though I'd imagine the math would be different) because the window of transmissibility would be much shorter and drive R way down. Vaccine would mean less viral load, less viral load means less transmission overall. This seems less likely to me than what is actually happening (vaccine inhibits transmission), but I could picture mechanisms by which that would happen.
- refurb 6y ago
  It’s not black and white “vaccinated = no longer a carrier”, but rather a gradient where vaccinated people have antibodies which reduce the amount of viral shedding to such low levels (some will be zero) that transmission is potentially an order of magnitude less.

maxerickson 6y ago

Vaccines tend not to be 100% effective. The Covid-19 vaccines appear to be very good in real world use, but the less effective a given vaccine is, the larger percentage of the population you need vaccinated to stop the virus spreading. That can get misscommunicated/misunderstood, and in addition lots of public health folks have been very conservative about saying anything that wasn't demonstrated in the trials, which didn't measure overall infection rates (they tracked symptomatic infections). Data is finally coming out about overall infection rates in vaccinated people and they appear to be very low.

fossuser 6y ago
It's wild to me that in the trials they didn't test all participants (including asymptomatic people) when asymptomatic transmission is the reason we're in a pandemic.
- SigmundA 6y ago
  So have there ever been vaccines that prevent you from getting getting the disease while still allowing you to spread it? This is a genuine question.
  - fossuser 6y ago
    I don't know - I'm also not the person that was arguing that. I have pretty high confidence that if you have the vaccine you won't transmit the disease - I suspect the scientists are just being overly cautious because they don't have the data yet (though it is now starting to prove this out). They don't have the data in part, because the trials did not test asymptomatic people. This means it's possible that some of the vaccinated people in the trial caught the virus, but at a reduced severity that was asymptomatic (but still transmissible). Pretend that the vaccine reduced severity, but most people still caught it and were just asymptomatic. In that world, the vaccine wouldn't be enough (and the trials would not have revealed this). Since we know that some people who get the vaccine do catch the virus, but just have a mild case - it seems like a plausible outcome that others catch it without symptoms, but can still transmit it. That said, it's looking like that's not the world we're in - and the vaccines do just stop you from contracting the disease in addition to reducing severity in the rare case you catch it anyway. mRNA vaccines are new and given the asymptomatic transmission of the virus is why we have a pandemic, it's a reasonable thing to be cautious about (which is why I was surprised the trials only tested symptomatic patients).