5. On average each infected person will then transmit disease to 2 × 0.25 = 0.5 susceptible others. At this rate, the infection quickly fizzles out. That's the idea behind herd immunity. Vaccinate enough people that on average each case generates fewer than one subsequent case.
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16. If we continue control measures once we reach herd immunity, the number of cases will rapid decline and we can soon relax. If we relax as soon as we reach herd immunity, cases will only very slowly decline and many unnecessary infections will occur.
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17. Q: What about escape variants? Won't they prevent us from reaching herd immunity? A: I'm optimistic that we'll be ok on this front. I haven't seen any compelling evidence that the current variants of concern (P1/2, B.1.351, etc) escape the US-authorized vaccines.
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18. While sera from vaccinated individuals do exhibit reduced activity against some of these strains, my guess is that the immune response remains sufficiently strong even against these variants to prevent disease.
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19. Even if escape variants do arise, we can readily incorporate these variants into booster shots. There is no reason why with concerted effort, we shouldn't be able to stay out ahead of any evolving escape variants with a proactive program of vaccine boosters.
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20. Q: Immunity from COVID infection or vaccines may not last all that long? Will waning immunity prevent us from reaching herd immunity? A: Of course we don't know yet, because we haven't had enough time to find out.
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21. That said, my guess is that we'll get at least a couple of years of immunity from natural infection, and at least 3-5 years from vaccination. If so, that generates a modest schedule of booster shots not that different from the tetanus vaccine.
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22. Q: You said "in principle" we can reach 70-80% immune. Why did you qualify that? A: Three reasons. The first two, escape variants and waning immunity, I've already addressed. The third, and I think the big one, is vaccine hesitancy.
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23. Without vaccine hesitancy, we'd be in really good shape. We have Pfizer and Moderna with <90% efficacy and J&J somewhere in the 66-75% range. Any reasonable mixture of these, applied across the entire population, would confer more than 80% immunity and would do the trick.
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24. With vaccine hesitancy, it could be close here in the US. I'm hoping that much of the hesitancy we see is really more like what
@DrMelissaClarke describes as vaccine deliberation. A lot of people feel their communities have not been so well served by US healthcare.pic.twitter.com/PFnCXj68GK
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25. Understandably, they would like more information *that they consider trustworthy* before deciding what to do. As the vaccines prove themselves safe and effective, and as we improve health equity, those deliberating about what to do may decide to take the vaccine.
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26. Q: If we don't reach herd immunity, what would it look like? A: I suspect we'd see some seasonality, much as we do for influenza. Minimal cases in the summer, followed by a winter wave. With vaccination, most people will be protected from serious illness at the very least.
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27. Ongoing boosters or "updates" would remain essential, though. Notice also that even if we reach herd immunity in the US, disease is a global problem. Without aggressive vaccination programs worldwide, we'll face continual reintroductions into the US.
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28. Unmitigated spread worldwide also offers abundant opportunity for escape variants to arise. Ultimately the best way to ensure COVID safety in the US will be to take care of the US — and the same time recognize and address the global nature of the problem.
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29. Q: If we have a safe, readily available vaccine, who cares whether we reach herd immunity or not? If people don't want the vaccine, that's their problem. A: Recall that not everyone can safely take the vaccine, and not everyone has an immune system that will respond to it.
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30. One very important function of herd immunity for diseases such as measles is that by vaccinating the majority, we are able to protect a minority for whom vaccines are not safe or effective. For many diseases this includes *all* young infants.
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31: Q: I asked a question in the replies and you didn't answer. Do you hate me? A: Unlikely. I probably just don't know the answer, and want to defer to those who do.
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End of conversation
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