For context this is a question I answered on Quora
None of these modRNA transfections establish long lasting immunity by any stretch of that word. For that you need antigen specific long lived antibody secreting plasma cells. Unlike traditional inactivated and attenuated live antigen immunizations such as the tetanus toxoid or annual flu shot, modRNA transfections do not sufficiently produce these and thus have an abysmally short antibody half life (87 days) and are categorically different from what we have typically called vaccines prior to 2020. A serological study of vaxxed adults (for both COVID and Flu) undergoing bone aspiration (n = 19) 2.5–33 months after their last dose found no correlation between anti-SARS2 IgG levels in the blood and SARS-2 specific long lived plasma cells in bone marrow. This was in stark contrast for the strong correlation between IgG levels in the blood and antigen specific long lived plasma cells for tetanus toxoid and the modest correlation for the annual flu shot. They found no SARS-2 specific long lived plasma cells except at one time point (23 months) where it was 0.3% of antigen specific plasma cells and only 35% of participants had detectable SARS2 specific long lived plasma cells which were found at extremely low levels despite receiving multiple boosters.
Infection risk reduction is not only gone after a few months but the modRNA also makes apparent changes in IgG antibody subclass allocation that changes the immune response from an inflammatory response to immune tolerance response that impairs the FC effector function. I have already summarized the extensive serological and observational research documenting immune tolerance to the spike protein following repeated modRNA transfections in three previous answers.
Answer: Why Does it Seem Like the COVID Vaccines Only Partially Work?
Answer: What Makes the modRNA COVID-19 Vaccines So Effective
As I noted in a third answer this appears to be a feature of the modRNA platform itself as evidence by similar results found in trials for HIV and malaria and a lack of similar findings following repeated immunization with other platforms.
Observations of modRNA shots creating immune tolerance to specific antigens and possible enhancement of disease are not new and have previously been observed in clinical trials for HIV [113] and Malaria vaccines [115] which directly linked increased risk of infection with IgG4 selection.
In an HIV vaccine trial statistically significant protection was associated with G1 and G3 immunoglobulin production while negligible VE was associated with G4 immunoglobulin production. The HIV vaccine that offered negligible protection involved a higher number of shots that impaired the Fc effort function of antibodies.
Similarly, in a malaria vaccine trial, doubling the production of G1 and G3 immunoglobulin G in the second year after immunization reduced the relative risk of contracting malaria by 50% and 60% respectively. However, doubling the amount of G4 immunoglobulin G increased the risk of contracting Malaria 3x. Overall, G1 and G3 resulted in a 51% and 56% lower relative risk of infection for vaccinees at 2 years of age while G4 proliferation resulted in an increased risk of infection at that age. A separate Malaria vaccine trial found the same results.