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Simplistically, you get 3 main benefits from infection or vaccination: antibodies which circulate in your body, and can respond right away to invaders; memory B cells, which make more antibodies ("secondary response") but take a few days to activate (I found various claims, from 4-7 days to 24-48 hours, so I can't be more precise); and killer T cells, which will purge infected cells, and I've got nothing about their response time except obviously *after* infection.
Ideally, the circulating antibodies stop re-infection cold. But antibody levels drop naturally, and we particularly don't seem to keep high levels in our mucus membranes, so there's a high chance that a respiratory disease is *not* stopped cold. What happens next?
In the case of measles, its generation time (from you being infected to you infecting someone else) is maybe 11-12 days. So even if an 'immune' person is breached, that infection will have to face a fully mobilized immune system, secondary response and killer T and all, so it probably gets clobbered before it can propagate. One breakthrough infection doesn't lead to more.
But in the case of covid, the generation time is just a few days, maybe only 2 days for Omicron. So it's possible that by the time your memory B cells get to work, not only have you infected more people, but those people have infected another wave of people in turn. Your T cells and any new antibodies will probably beat down the rest of the illness and keep it from killing you, but for infection-control purposes it's too late.
Is covid uniquely fast? Probably not: this says "The generation time is the doubling time, or the time required for the number of infections to double in size. Epidemiologic field studies of novel H1N1 flu infections in several states indicate that the generation time for acute respiratory illness (ARI) was 2.0-3.1 days and 2.4-3.1 days for influenza like illness (ILI)."
Flu, of course, is another disease that's been hard to contain via vaccination, for various reasons. But I would now guess that the main hope of a flu shot protecting you is from the initial wave of antibodies; if you were exposed to the exact same virus a year later, you would be counting on memory B cell protection, and likely get (mildly) sick.
Ideally, the circulating antibodies stop re-infection cold. But antibody levels drop naturally, and we particularly don't seem to keep high levels in our mucus membranes, so there's a high chance that a respiratory disease is *not* stopped cold. What happens next?
In the case of measles, its generation time (from you being infected to you infecting someone else) is maybe 11-12 days. So even if an 'immune' person is breached, that infection will have to face a fully mobilized immune system, secondary response and killer T and all, so it probably gets clobbered before it can propagate. One breakthrough infection doesn't lead to more.
But in the case of covid, the generation time is just a few days, maybe only 2 days for Omicron. So it's possible that by the time your memory B cells get to work, not only have you infected more people, but those people have infected another wave of people in turn. Your T cells and any new antibodies will probably beat down the rest of the illness and keep it from killing you, but for infection-control purposes it's too late.
Is covid uniquely fast? Probably not: this says "The generation time is the doubling time, or the time required for the number of infections to double in size. Epidemiologic field studies of novel H1N1 flu infections in several states indicate that the generation time for acute respiratory illness (ARI) was 2.0-3.1 days and 2.4-3.1 days for influenza like illness (ILI)."
Flu, of course, is another disease that's been hard to contain via vaccination, for various reasons. But I would now guess that the main hope of a flu shot protecting you is from the initial wave of antibodies; if you were exposed to the exact same virus a year later, you would be counting on memory B cell protection, and likely get (mildly) sick.