Covid-19 Vaccine Intranasal Study (COV008), UK

https://news.ycombinator.com/item?id=28014448 :

That said, current vaccines don't provoke the exact same response as an actual infection, particularly in that they don't fully stimulate an immune response in the mucous membranes of the upper repository tract. Here's an Scientific American round-up article (from back in March): https://www.scientificamerican.com/article/to-beat-covid-we-...

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The current vaccines are excellent at helping the body stop the virus as it progresses into the lungs and further (thus the massive reduction in severe illness and deaths). But it still means that there's a period of a few days after initial infection when a vaccinated person has a relatively naive immune response (while the virus is mostly just in the nose/throat). This would then result in the person still being pretty contagious.