A possible explanation on why breakthrough cases happen and why vaccines still offer protection from severe infection (rarer breakthrough hospitalizations tend to be older people with health issues):
Vaccines that are injected into arm muscles aren’t likely to be able to protect our nasal passages from marauding SARS-CoV-2 viruses for very long, even if they are doing a terrific job protecting lungs from the virus.
Vaccines that are injected into the arm have done a spectacular job at preventing severe disease and death. But they do not generate the kind of protection in the nasal passages that would be needed to block all infection. That’s called “sterilizing immunity.”
Florian Krammer, a vaccinologist at the Icahn School of Medicine at Mount Sinai Hospital, thinks existing vaccines likely induce high enough levels of circulating antibodies early after vaccination that some of them end up in the mucus membranes of the nose and throat. But as antibody levels start to drop in the months after vaccination, that early nasal protection seems to wane with it — especially in the face of the onslaught of the Delta variant, he suggested.
https://www.statnews.com/2021/08/10/...asal-vaccines/
Back when the Covid vaccines first began widespread distribution most health experts were skeptical of full sterilizing immunity. The fact that intramuscular injection protects lungs, but not always the upper airways is representative of how other vaccines work.
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