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Immunity after COVID-19 infection: How long it lasts? Severe vs. mild cases. (10)
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What is the current view on the ability to form immunity after having clinical COVID-19 disease? Let me start with that question by making a comment on how information is being disseminated right now. We, as scientists, need to look primarily at the center of the bell curve of any effect, to understand it fast. Whereas, if you want to draw attention, you look at the tails of the bell curve. So for example, if a small number of people do not make good antibody response, and a large number of people do, the real story is that a large number of people make strong, durable antibody responses after they get the infection. But the news is that a small number of people do not make those antibody responses. And that gets turned into a concern, which it is. But it needs to be put in perspective and on a scale. And that’s similarly with these individuals who are being apparently re-infected with COVID-19 coronavirus, although there’s multiple explanations for that. And so the overwhelming trend, most of the individuals who are infected, certainly with a moderate or serious case of the coronavirus, is that they make antibodies that should last for at least a year or two. We don’t know how long antibody protection will last. But every experience with SARS-1 and with other viral infections suggests that [recovered COVID-19 patients] will have antibodies that will protect them for some time. And there are two implications of that. One is those antibodies can be useful for other individuals if they’d like to donate their blood and help somebody who’s quite sick. And the second implication is they can, for the most part, not worry about getting infected again, [at least] for a year or two, certainly. And so I think that needs to be reiterated. However, there are always the ends, the tails of the bell curve. And there will always be individuals who could not mount an effective antibody response [against COVID-19 coronavirus]. We need to keep an eye on those individuals. And we need to be alert to that possibility. But a simple serological test can not only tell you whether you have had the infection, but it will give you a very rough quantitative sense of how intense your antibody response is. So if you have a weak immune response, you may want to be more careful and also consider getting a vaccine [against COVID-19 coronavirus] in addition. So that’s very interesting. But the bell curve could have a different shape. So if those tails are thvery small, then you really need to focus on the top of it, on the middle [part of the bell curve]. But those bell curve tails could be long. Because, for example, up to 40% of people have an asymptomatic COVID-19 coronavirus infection or very mild symptoms. And the question is, could that affect the size of those tails of the bell curve for the antibody formation? What do we know about people who are asymptomatic or have very mild symptoms of the coronavirus infection? As far as immunity is concerned. That’s a great question. We don’t know enough yet about those individuals [with COVID-19] who would test serologically positive. They may not have known they had an infection. Or they tested positive [for COVID-19 coronavirus antibody], but they didn’t experience any symptoms. Those individuals indeed may not have generated a robust antibody response. And should they perhaps be exposed to a larger dose [of COVID-19 coronavirus], they may indeed be vulnerable. But I should point out that there is more than one mechanism for protection against viruses. The immune system has multiple arms, and we are only looking at its most important arm, which is antibodies. But there are, indeed, other arms [of the immune system], what we call the adaptive and the innate immune systems that could be playing roles as well. So I think those individuals who have had COVID-19 disease are probably in a better position, regardless of the intensity of the disease than those who have not. Although, again, there’s going to be a wide range of human variation. And you’re absolutely right, that the nature of [antobody response bell curve] tails is not well described at this point. And it is important. Well, it’s certainly true that at least if they recovered COVID-19 patients some antibody response, then if they get re-infected [with COVID-19 coronavirus], that hopefully, the infection might be milder compared with what it was otherwise. Even if they have a very modest antibody response [after first COVID-19 infection], the rate at which the antibody response will return on new exposure to the coronavirus will be far faster than it was originally. So, I think we can probably, for the most part, include those individuals with mild cases [of COVID-19] in our understanding of when herd immunity will emerge. I think there was a paper investigating the 1918 flu pandemic that said that allowing the 1918 flu virus to circulate for some time allowed for more antibody response. So the second peak was either non-existing or milder, which brought the cumulative death numbers down. Well, that was so. Just to remind you a little bit in more detail. The second wave of the 1918 influenza pandemic was more virulent than the first. So the protection that people obtained from the first [wave of 1918 flu] was very good against the more virulent second wave. We’re not clear [of COVID-19 waves]. COVID-19 coronavirus is pretty virulent as it is. And we’re not clear that the second wave of COVID-19 will be more virulent. There’s every reason to believe it might be less virulent. We don’t know; it could go either way, actually.