The more time passes, the more perspective we have on the natural immunity developed by ex-Covid-19 patients. In November, we could say that antibodies against Sars-CoV-2 are effective at least six months. This week, a new study published in Science lengthens the delay: most former patients seem to remain immune for up to eight months after infection, and probably even longer.
A complete vision
California researchers from the La Jolla Institute of Immunology analyzed the immune response of 188 patients who contracted Covid-19, including 43 samples from people infected for more than six months – these provide particularly valuable information on the long-term immunity. And to get a complete picture of immune memory, scientists have analyzed its different facets: not only the presence of antibodies, but also the cells that produce them and other cells that support the battle against the virus.
“We measured antibodies, memory B cells, helper T cells and killer T cells at the same time. I think this is the largest study ever, for a severe infection, that measures all four components of immune memory ”, says Shane Crotty, who co-directed the work.
All these players in the immune response do not appear at the same time from the day of infection. Some come into play quickly to get rid of the virus, then gradually transfer their defense skills to other cells and molecules that take care of guarding for months and months. The level of certain components in the blood therefore drops within a few weeks, while other measures increase and persist over time.
Poor immune memory in a fraction of patients
To start with the bad news, the long-term immune response appears to be heterogeneous in former patients: a small portion of them fail to remember the first infection. Gender and disease severity clearly play a role (men have more antibodies than women, for example), but other criteria are still unknown and remain to be understood. “Maybe the heterogeneity comes from a low viral load or a weak initial inoculation”, the researchers in California assume.
In any case, there are loopholes in some patients. “It is therefore possible that a fraction of the population infected with Sars-Cov-2 with poor immune memory is more likely to be reinfected relatively quickly.”
Protection of at least five to eight months for 95% of people
But for the vast majority of the population, i.e. 95% of the former patients studied, “Data show that at least three components of immune memory are measured five to eight months after onset of symptoms”. These people have therefore, a priori, developed real anti-Covid protection over time.
The memory B lymphocytes, especially, “Are more abundant at six months than at one month after the onset of symptoms.” These cells are the guardians of the memory of the infection. Initially simple, somewhat generic white blood cells patrolling the blood and lymph, they reacted to contact with a pathogen and specialized in knowing how to retain and recognize the characteristics of the enemy (here the coronavirus). They have a long lifespan and a good reactivity: if the same aggressor dares to point the tip of his nose, the memory B lymphocytes quickly produce highly targeted antibodies. This counterattack is more intense than the first infection, to nip the viral invasion in the bud.
Researchers at the La Jolla Institute also measured the level of IgG (type G immunoglobulins), a certain type of antibody that appears two weeks after the initial infection. And much later, they continue to answer the call: the IgG level is “Relatively stable after more than six months”.
Other components gradually disappear from the body over time. The helper T lymphocytes, which help cells on the defense front by secreting a protein, “Decline with a half-life of 3-5 months.” Same verdict for cytotoxic T lymphocytes, which destroy foreign cells by perforating their membranes. This means that helping white blood cells and killer white blood cells remain present, even in reduced amounts, long after infection.
Pending studies on vaccine immunity
The former patients have therefore “A good chance of having protective immunity, at least against a severe form of the disease”, for eight months and “Probably well beyond”, says Shane Crotty. “Our data suggest the immune response is here – and stays”, summarizes the Italian immunologist Alessandro Sette, one of the main authors of the study.
It is now to be hoped that the immunity provided by vaccines will be as long. We can assume, “But we will have to wait for the data to be sure”, says Daniela Weiskopf, co-author of the study. “Studies on the vaccine are at an early stage, and so far show strong protection. We hope they will bring out a similar pattern of immune responses that last over time. ”