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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?

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    This is an excellent question. The bottom line is that the efficacy of some or all of the COVID-19 vaccines seems to decline over time, especially with older people and likely for people who are immunocompromised or immunosuppressed. This question asks "why", which I view as a truly excellent and worthwhile question. If no quality answers are posted here, it may be worth posting on Biology SE. Biology SE has many amazing biology experts who may be able to help provide quality science-based answers.
    – Amazon Dies In Darkness
    Commented Jul 28, 2021 at 11:17
  • IIRC, the efficacy of at least some COVID-19 vaccines seems to increase over time. We're lacking info, and I think it's important to keep clear the difference between protection declining in a given person against the same virus and protection declining because the population of circulating virus now has changed.
    – Armand
    Commented Jul 28, 2021 at 16:17
  • @Armand wrote the efficacy of at least some COVID-19 vaccines seems to increase over time. I hope that is true, but everything I have read is to the contrary. Of course, we have to define "over time". If we arbitrarily assign "over time" to mean "over 3 months", have you been able to find any research supporting the hypothesis that the efficacy of some SARS-CoV2 vaccines improve beyond that window?
    – Amazon Dies In Darkness
    Commented Jul 29, 2021 at 12:33
  • @RockPaperLz-MaskitorCasket This may have been the work I was thinking of: bmj.com/content/373/bmj.n1088 The AstraZeneca vaccine kept getting better out past their last bin of 35+ days after second dose, unlike the Pfizer analyzed at the same time.
    – Armand
    Commented Jul 29, 2021 at 12:54
  • @Armand Thank you for the link. Much appreciated. I'll add it to the top of my reading list.
    – Amazon Dies In Darkness
    Commented Jul 29, 2021 at 23:57

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Re:

If so, then there is no need to have a third booster shot.

Why do you think this? Consider chicken pox/shingles. Clearly, infection with chicken pox once leads to immunity lasting for many many decades. However, that immunity can wane, allowing shingles to develop. Immunity is not an all or nothing status.

Also, consider which specific variant is used to make the vaccine, and which might be circulating in the population.

Finally, consider that "it is said" is not actual science. Maybe that is correct, maybe it isn't, maybe it's incomplete or mistaken.

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  • 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? That's the crux of my question
    – Graviton
    Commented Jul 28, 2021 at 1:53
  • @Graviton I think this is the key part: "consider which specific variant is used to make the vaccine, and which [variant] might be circulating in the population." Are they the same or different?
    – Armand
    Commented Jul 28, 2021 at 2:19

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