Why post-recovery Covid-19 immunity doesn’t last

High levels of cytokines — a broad and loose category of small proteins important in cell signalling — associated with coronavirus could prevent long-term response of antibodies, according to a study published in the journal Cell on August 19.

The researchers believe that this is the reason why coronavirus immunity cannot last long after the recovery.

Co-senior author Dr Shiv Pillai, professor at Harvard Medical School, MIT, and a member of the Ragon Institute of Massachusetts General Hospital, said in a statement: “We’ve seen a lot of studies suggesting that immunity to Covid-19 is not durable because the antibodies decline over time. This study provides a mechanism that explains this lower-quality immune response.”

The study was carried out in convalescent Covid-19 patients. It indicated that humoral immunity is often short-lived, and most SARS-CoV-2 antibodies exhibit limited somatic hypermutation — adaptation to new foreign elements.

The scientists noted that the life-long memory of a pathogen allows the body to quickly and effectively identify and attack pathogens in the case of reinfection. This is achieved by the formation of T follicular helper cells found in the secondary lymphoid organs.

However, in the case of SARS-CoV-2, due to the evasion of antiviral aspects of innate immunity, T follicular helper cells get blocked.

The authors suggested that possibly cytokines in Covid-19 lymph nodes block the final step in T follicular helper cell differentiation, which is required for the formation of germinal centres.

Vaccine-induced immunity

“Without the formation of germinal centres, there is unlikely to be long-term memory to this virus developing from natural infections, meaning that while antibodies may protect people for a relatively short time, a single person who recovers from the disease could get infected again, perhaps six months later, or even multiple times with SARS-CoV-2,” added Pillai. “This suggests that developing herd immunity may be difficult,” he speculated.

The study mentioned that developing herd immunity may be difficult in the case of Covid-19 due to impaired infection-induced protective immunity and low durability antibody responses.The authors noted that their findings should not affect vaccine-induced immunity, as vaccines do not induce cytokine storms.