As a person of limited medical science knowledge I am puzzled by what I read about the different vaccines and their efficacy.
What I read about the covid vaccines suggests to me, that they do not stop the vaccinated from being infected, but stop or lessen the severity of the effects of the disease. This is contrary to my previous understanding of how vaccines work.
I was always given to understand, that if vaccinated and exposed to a virus, your antibodies would slaughter the nasty germs, and you would be rid of them. If the effect is just to lessen the severity of the virus effect, then do you still have those germs in you, so that you can pass them on to some other dude?
The above seems contrary to the notion that we can achieve herd immunity and the effectiveness of vaccination to achieve this.
It is complicated. The immune system is complicated and vaccines differ.
On a simplistic level the immune system polices what happens inside the body, so it cannot have an effect until the germ (virus) is inside of the body - you are infected. Most vaccines do not stop infection but they enable the body to rapidly terminate the infection before you become ill and before you enter into a state when you can infect others. Good examples of this would be smallpox vaccine or injected polio vaccine, or measles vaccine.
However, there is something called mucosal immunity. This is immunity at the level of the layer between you and the outside of the body in the gut and airways. A specific type of immunity occurs here, mediated by immunoglobulin A. Immunity at this point can stop infection getting into your body so preventing infection. A good example of this is oral polio vaccine. A nasal flu vaccine exists which produces mucosal immunity but it really only works reliably in children. By enhancing specific mucosal immunity the virus can be stopped at the barrier between inside and outside of the body so technically no infection occurs.
Rabies vaccine is unusual in that you can give it post infection, and because the infection spreads relatively slowly up nerves from the site of infection to the brain, you can produce an effective immunity before the virus gets to the brain and kills you. (Because the virus hides inside nerves it does not provoke a natural immune response).
A bad vaccine is BCG vaccine for TB. It enhances the immune response to TB this is good in preventing disseminated TB in the body. It is not good in preventing TB in the lungs, infectious TB. So BCG is best at preventing TB meningitis in children (highly fatal) but poor at preventing infectious pulmonary TB, it may even increase the risk, it appears to limit TB to the lungs, so may increase the risk of developing infectious TB. It is very good at preventing leprosy!
So covid vaccines are disease modifying. They probably do not provoke good mucosal immunity so do not prevent infection. They do reduce the severity of the illness, reduce the virus production in your body so reduce the likelihood of you infecting anyone else. The vaccines are near 100% effective at preventing death, but only reduce infections by 60%, and probably reduce transmissibility if infected by half. Nearly but not quite enough to prevent the virus from continuing to circulate in a fully vaccinated community.
trials of nasal vaccines which should provoke mucosal immunity and be more effective at reducing transmissibility are under way, but generating good mucosal immunity is difficult and best done with live attenuated virus. The adenovirus vector vaccines may be able to do this, if given orally / nasally.
