Sunetra_herd_immunity.md 5.3 KB
This is an example of the Pfizer vaccine actually showing major reductions in our T-cells after you get vaccinated. Study published in Nature in 2020 using the Pfizer vaccine, and you can see here the count of the lymphocytes, the T-cells, actually goes down following the initial injection with a Pfizer vaccine. Now it doesn't go down as much later on, but it certainly doesn't go up. This is a Rhesus monkey study using the Moderna vaccine, and what this study clearly showed, it was published in NEJM when they looked at the memory CD-4 cells, the T-helper 2 response, there is no T-helper 2 cell response using the Moderna vaccine. And so there is no complete innate response, and these cells are the ones that are critical for antibody production.
Herd immunity probably takes the prize for being the most misunderstood term of 2020, it is also widely misused to mean the willful culling of vulnerable people in a population during an epidemic.
The way to understand herd immunity is by recognizing that your risk of infection depends very much on how many people around you are already immune. When a virus survives in a population, it starts to spread very quickly, because people are not immune, so everyone is available for infection.
But as it spreads, it limits its own possibilities, because people stop being immune, putting up a barrier to its spread. Eventually it settles to a level that we call an endemic equilibrium.
So it moves from being epidemic, to being endemic, that is to say that it persists at a stable level, fluctuating around on such factors as seasonality. That is exactly how we live with the other coronaviruses.
So there'll always be an increase in cases around winter, and then they drop off again in the summer. This is also true of influenza, pneumoccocal pneumonia, a whole range of other respiratory infectious diseases.
So there are 4 other coronaviruses that we currentlly live with, that are all circulating in th epopulation. And the reason you don't hear about them is because there is herd immunity to each one of them, which keeps the risks of being infected very low.
Immunity to these current viruses is established upon infection. but it doesn't last forever. So after a while you become infected again, you are re-infected.
Our relationship with these coronaviruses is one of repeated re-infected. But re-infection doesn't carry with it a significant threat of disease or death.
So this is a slightly different situation to measles, where you become infected and remain immune for life. As you get older you suffer what we call immune sunaissance, which is that your immune system doesn't work as well as it used to. And this is why the elderly are particularly vulnerable to disease and death.
Every year a number of elderly people do die of these other coronaviruses, but the risk of coronavirus infection among the elderly is kept low by herd immunity.
Well there's no reason to believe that this novel Coronavirus is more virulent, intrisically, than the other seasonal coronaviruses, as we call them. The difference is that when this new coronavirus arrived, it encountered a population where there was no immunity to this new vrus, other than that which accumulates from exposure to the other coronaviruses already in circulation.
Given how the other coronaviruses behave in human populations, it is reasonable to assume that this novel coronavirus will also settle down to that endemic equilibrium with a risk of infection in the vulnerable population is of the same level that we accept for all the other coronaviruses.
So people have questioned the idea whether herd immunity can be established to the new virus, for a number of reasons. First of all there was a question surrounding whether we wuold develop immunity at all to this virus. Not surprisingly we have learned that we do develop strong protective immune responses just as we do to all the other coronaviruses.
So we know that immunity is established upon infection, what we don't know is how long it lasts. If we go by the other coronaviruses, it is not going to be life-long. But this does not mean that you cannot established herd immunity.
Indeed, we know that we have herd immunity to all these other viruses. How is that possible?
The rate of loss of immunity doesn't really affect the level of immunity that's maintained in the population.
In an endemic state, the loss, the rate at which people lose immunity is matched by the rate at which people gain immunity. It's a bit like water flowing into a system. To start with, when you have an empty system, the water gushes in, but as the system fills up, the rate at which water pours in slows down. So the build-up of immunity in the population is like the filling up of the system.
Now, once the system fills up, it will remain at that level, even though some water might leak away from it, because it will be replnished by water coming into it. It could leak away very slowly, as it does for measles, because immunity is life-long, and replenished very slowly by people being born and getting measles. Or it could leak away very quickly as it does for the coronaviruses, but then it is replnished also very quickly by reinfection, so you end up maintaining this level of water in the system, or population-level immunity, or herd immunity, whether or not the leakage from the system is slow or fast.