My understanding about mRNA Covid-19 vaccines and memory cell is that they do induce memory T cell response similar to natural infection, and thus provide long term protection via the memory cell response.
If so, then there is no need to have a third booster shot. However, the recent news report came out from Israel challenges my understanding. It is said that because the protection offered by Pfizer against Covid 19 -- specifically the Delta variant -- is reducing a lot (to 39% currently), the Israeli Health officials are thinking whether to provide a third shot.
I thought that the memory cell will be able to do the job of long term protection for the first few years, but then third dose "is still considered" after only 6 months? Is there anything that I miss?