The research group that previously found anti-spike plasma cells in the bone marrow of people recovered from covid has now taken samples of cells from the lymph nodes of people who have been vaccinated (with Pfizer). They found B-cells in the lymph nodes undergoing a process called somatic hypermutation. This fine-tunes the antibodies made by the B-cells to better bind to the spike protein. They found that this process is continuing for at least 15 weeks after the first shot, which means that the immune system is taking the spike protein very seriously, and trying to make the best antibodies that it can against it.
This is not a surprise, since we have already seen the resulting antibodies becoming better at neutralizing virus over time. We are also seeing people with antibodies remaining almost a year after vaccination, and that's not going to happen if you didn't get a full-blown immune response.
However, the leader of this research group is telling the media that this result means that booster shots will probably not be needed. I don't think the data are telling us that at all.
Given this type of robust immune response, if you are fully vaccinated and responded to the vaccine, you will probably have memory B and T cells for decades, maybe the rest of your life, even if your antibodies wear off completely. If you catch covid in the future, they would then be able to produce antibodies and T-cells much faster than someone who had never been vaccinated (days as opposed to weeks). But memory B and T cells can't prevent an infection from happening. They can only respond to clear the infection.
If we want to end the pandemic, we need a critical mass of people to have enough antibodies to prevent infection and break the chain of transmission. Vaccinated people with breakthrough infections are about half as likely to give the virus to someone else, at least for the alpha variant. But the delta variant is more transmissible, and even for the alpha variant we could see an uptick in cases once antibodies start to wear off.
On an individual level, breakthrough infections are less severe, but for some people, they are still pretty unpleasant. And we still don't have any data at all on long covid in breakthrough infections.
So don't be confused by the media reports on this study :
https://www.nature.com/articles/s41586-021-03738-2_reference.pdf