Does he have a compromised immune system? Any autoimmune diseases? Levels of antibodies drop naturally over time for all vaccines - what's important is memory B cells in bone marrow and lymph nodes - two studies published in the last few months showed vaccinated patients had them, they did biopsies to confirm. Previous similar studies on patients infected with SARS 17 yrs ago showed they still had memory B cells in their bone marrow, which the researchers said showed long term protection post-infection and recovery.
Sounds like your BIL was in the 5% group for which the vaccines don't provide protection. Hope he gets better soon. I also know MRNA has measure antibodies in patients at the 6 month post vaccination stage and they didn't see a huge drop in antibodies - again points at your BIL being a non-responder. I have a a feeling there is going to be a group of people who will need annual boosters - the problem is figuring out who they are before they get infected.
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