Natural Covid-19 infection elicits a different type of immunity than the vaccine, with the vaccines appearing to be more effective in protecting against reinfection. How can the superiority of the vaccine over “natural” immunity be explained? Does this protection also apply to the new variants?
Does the vaccine provide better immunity than a natural infection? Hard to say. “Some diseases, such as measles, rubella, chickenpox, or hepatitis B, generate a sterilizing immunity, i.e. if you are infected once, you cannot be infected a second time. On the other hand, with viruses in the respiratory tract, such as the coronavirus that causes colds or the respiratory syncytial virus (RSV), we are often reinfected. You can even catch four or five colds in one season.
In the case of Covid-19, immunity does not appear to be sterilizing, as several cases of reinfection have been reported. However, the natural immune response varies greatly from individual to individual. In general, the more severe the form, the stronger the immunity. In asymptomatic patients, on the other hand, antibody production is sometimes very low, with 2-8% of patients producing no antibodies at all. According to another study from the University of Oxford, 26% of symptomatic patients no longer have a significant immune response six months after infection.
After two doses of RNA vaccine, antibody levels are up to 10 times higher than after natural infection.
What happens after vaccination?
Although the immune response is again quite different (for example, the vaccine is less effective in the elderly), the vaccine seems to protect better against reinfection and severe forms. A small study published on the BioRxiv server found that antibody levels were up to 10 times higher after two doses of RNA vaccine than after natural infection.
Why is the immune response different after the vaccine and natural infection?
“The truth is we don’t know,” admits Sabra Klein, an immunologist at the Johns Hopkins Bloomberg School of Public Health in Maryland. “One would think that the natural infection would provide stronger immunity because it relies on all the antigens in the virus, while vaccines focus only on the spike protein. But that’s not the case. The immune system puts on a sort of blindfold and focuses on that part of the virus. However, by spreading antibody production to less useful parts, the natural immune response would be less effective.
It is also possible that this difference can only be explained by the very high doses of high-tech proteins administered in vaccines (for example, up to 100 micrograms of messenger RNA per dose in the case of the Moderna vaccine). On the other hand, because vaccines are administered through the muscles, they do not provide protection at the level of the mucosa. This means that the virus can persist in the nose or throat and the vaccinated person can potentially remain infectious.
Should people who are already infected be vaccinated?
The CDC recommends that people infected with the virus be vaccinated also. The infection generates a humoral and a mucosal response, both of which are reactivated by vaccination. Even when antibody levels are no longer detectable in infected individuals, the vaccine induces memory B cells that again produce ad hoc antibodies.
According to a study published in Science, 95% of individuals with natural immunity to SARS-CoV-2 who had received a dose of the vaccine developed a T-cell response directed against the spike protein, compared with 70% of individuals who had never been infected with SARS-CoV-2 — and had also only received a single dose. In the individuals with immunity that also received a single dose of the vaccine the number of B cells directed against the spike protein was 63 times higher.