Why can and do vaccinated people spread COVID-19?

Viruses spread if they can replicate in large enough numbers with not enough of a response to fight them. Therefore, vaccinated people can spread COVID-19

Generally, if your immune system is doing a good job, it will eradicate the virus to such an extent that the few who do manage to “escape” will be so statistically irrelevant that the chances of finding a new host are basically zero. The UV from the sun would sterilize them long before they found their way to another person. Freak incidents are possible but not likely.

However, it seems that for whatever reason, the strength of the Covid-19 vaccine is not up to par. I don’t think it is the antibodies themselves, because they keep sticking with the same formulation. That said, it seems that for some people with poor immune systems, once the initial response wanes, there is little in the way of protective memory cells left. You need to have a certain number of these to be effective.

To use a WWII reference, if you’re Germany, you might have King Tigers and ME 262s, but if you don’t have enough of them, you’re not going to stop the invasion. Those SARS-CoV-2 viruses are replicating like Russian T34s and American Flying Fortresses and they can’t be stopped.

Don’t get me wrong, it makes absolute sense that the vaccine would prevent a hospital visit. Immunity IS a system, with interconnecting parts. If “acquired” immune cells are responding to an issue, regulatory aspects of your immune system will not employ as many of the “symptom inducing” non-specific immune cells. In other words, issues like that “cytokine storm” you’ve heard about won’t happen, and neither will the associated collateral damage.

That said, if the virus is replicating in large amounts, it can get a foothold even with some acquired immunity, and it just needs to build up in enough numbers that SOME of the droplets you cough up carry enough SARS-CoV-2 to make it to the next person intact. Indeed, many of the variants are even more efficient at transmission and replication than previous. While they are individually just as vulnerable to the vaccine, they play a numbers game where they need less of themselves to infect, and when they do infect, they make more of themselves. Then, vaccinated people can spread COVID-19

So what ends up happening, is that your actual immune response against the virus with the vaccine can be potentially anemic. For starters, the vaccines are dosed to a level that is safe and tolerable, as well as dosed to the point that they can get the maximum benefit before it plateaus. Repeated exposures to an antigen do expand your immunity, which is likely why the shot lead with two doses (at least for the mRNA vaccines). However, its important to remember that the vaccines do not induce nearly as much immune response as an actual covid infection. While this is the obvious point of a vaccine (to avoid sickness), it also means that it runs the risk of not inducing strong immunity.

Research has shown that in asymptomatic non-vaccinated individuals, that their acquire immunity against Covid-19 is also low, with the implication that a lesser presence of the virus correlates to weaker acquired immunity. In theory, the vaccine may well suffer from this limitation.

Even if that is not the case, the other issue can be in patients themselves. Your own immune system’s ability to take in the antigens and develop memory cells (be they B or T cells) can play a large role. Immediately after infection, your immune system will scale up and you will be very protected, but most of the cells mounting this defense on the antibody side are “Plasma cells” and other helpers, that die off after 3–6 months. Hence, your total immunity wanes. That said, ideally you’d have enough memory cells present so that when the infection shows up again, it can scale up faster and stronger. This is why older people are more at risk, their cells are hitting the end of the line and are not replicating the way they used to. Your memory cells may exponentially expand, in response to an infection, but you’re going to be better off starting with hypothetically, 10,000 memory cells rather than 100. The vaccine could be 100% flawless and you’d still struggle if you’re not generating those memory cells.

Unfortunately, there is no good way to know how immune you are. Many of these issues are known, but on the scale of the whole US population, it would be daunting to monitor and identify these people, even if we were to overlook the questionable infringement of people’s rights to do so.

As a result, any number of factors can combine to allow SARS-CoV-2 to transmit from an infected person, be it more efficient variants, anemic vaccines, or variable immunocompetency.

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