There are two recent studies that suggest that immunity lasts for at least a year and it is possible that it may last for decades, at least for some people. These studies have been made on bone marrow of the persons who have been infected by Corona virus Sars-CoV-2.
The first study on Persistent Cellular Immunity to SARS-CoV-2 Infection is as under:
SARS-CoV-2 is responsible for an ongoing pandemic that affected millions of individuals around the globe. To gain further understanding of the immune response in recovered individuals we measured T cell responses in paired samples obtained an average of 1.3 and 6.1 months after infection from 41 individuals. The data indicate that recovered individuals show persistent polyfunctional SARS-CoV-2 antigen specific memory that could contribute to rapid recall responses. In addition, recovered individuals show enduring immune alterations in relative numbers of CD4+ and CD8+ T cells, expression of activation/exhaustion markers, and cell division.
Summary We show that SARS-CoV-2 infection elicits broadly reactive and highly functional memory T cell responses that persist 6 months after infection. In addition, recovered individuals show enduring immune alterations in CD4+ and CD8+ T cells compartments.from Biorxiv.com
The above report is a pre-print and has not been peer reviewed. A similar such report has been submitted to Nature.Com by another group of scientists which opines as under:
Long-lived bone marrow plasma cells (BMPCs) are a persistent and essential source of protective antibodies1–7. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) convalescent individuals have a significantly lower risk of reinfection8–10. Nonetheless, it has been reported that anti-SARS-CoV-2 serum antibodies experience rapid decay in the first few months after infection, raising concerns that long-lived BMPCs may not be generated and humoral immunity against this virus may be short-lived11–13. Here we demonstrate that in patients who experienced mild infections (n=77), serum anti-SARS-CoV-2 spike (S) antibodies decline rapidly in the first 4 months after infection and then more gradually over the following 7 months, remaining detectable at least 11 months after infection. Anti-S antibody titers correlated with the frequency of S-specific BMPCs obtained from bone marrow aspirates of 18 SARS-CoV-2 convalescent patients 7 to 8 months after infection. S-specific BMPCs were not detected in aspirates from 11 healthy subjects with no history of SARS-CoV-2 infection. We demonstrate that S-binding BMPCs are quiescent, indicating that they are part of a long-lived compartment. Consistently, circulating resting memory B cells directed against the S protein were detected in the convalescent individuals. Overall, we show that SARS-CoV-2 infection induces a robust antigen-specific, long-lived humoral immune response in humans.from nature.com
In plain English it is a good news for those who were unlucky to be infected but were lucky enough to escape with mild symptoms. Human bone marrow has immune cells that continue to evolve even after 11 months of infection and the antibodies they produced became better at fighting infection. Even after the active infection was over, these cells kept learning because the immune system retained a piece of the virus. Over time, the immune cells continued to improve and became broader in their repertoire, so they could work against a much broader range of variants.
Remember earlier we were told that immunity lasts till six months but this new study shows that it may last longer. Future researches may further confirm that immunity may last forever. From these reports it was not clear if the vaccination has similar effect but perhaps we have to wait till such report comes out. After all vaccine is now a trillion dollar industry and any reporting may affect the commercial value of a vaccine.