Natural immunity, that is, that conferred by having been infected by covid, protects us from reinfection or severe covid for 10 months, against most variants of the coronavirus. The same period as vaccination with two doses of RNA vaccines. This has been verified by the largest review carried out to date with 65 studies from 19 countries that is published in “The Lancet”.
According to this systematic review, the risk of hospitalization or death for a person previously infected with Covid-19 is 88% lower for at least 10 months than for those who have not been infected.
The analysis also suggests that the level and duration of protection against reinfection, symptomatic and severe covid is similar to that provided by two doses of the mRNA vaccines (Moderna, Pfizer-BioNtech) for the ancestral, Alpha, Delta and Omicron BA.1. The study did not include data on infection by Omicron XBB and its sublineages.
“Vaccination is the safest way to acquire immunity, whereas the acquisition of natural immunity must be weighed against the risks of severe illness and death associated with the initial infection,” said lead author Stephen Lim of the Institute for Health Metrics. and Evaluation (IHME) at the Washington University School of Medicine (USA).
As co-author Caroline Stein explains, “Vaccines continue to be important to protect high-risk populations, such as those older than 60 years and people with comorbidities. This also includes previously uninfected populations and unvaccinated groups, as well as those who were infected or received their last dose of vaccine more than six months ago. Decision makers need to take into account both natural immunity and vaccination status to get a complete picture of an individual’s immunity profile.”
Since January 2021, several studies and reviews have reported on the efficacy of natural immunity in reducing the risk of reinfection and how immunity wanes over time. But none have comprehensively assessed how long protection will last after natural infection and how long that protection will be against different variants.
This review, which includes 65 studies from 19 countries, evaluated the protection conferred by previous infection against reinfection, symptomatic and severe covid, the variant, and the time elapsed since infection. Studies examining natural immunity in combination with vaccination (ie hybrid immunity) were excluded from the analyses.
How long does immunity last?
Thus, 21 studies reporting time since infection with a pre-Omicron variant estimated that protection against reinfection in this case was approximately 85% at one month, falling to approximately 79% at 12 months. 10 months. Protection against reinfection by the Omicron BA.1 variant was lower (74% at one month) and declined more rapidly to 36% at 10 months.
However, analysis of five studies reporting severe illness (hospitalization and death) revealed that protection remained universally high over 10 months: 90% for ancestral, Alpha, and Delta, and 88% for Omicron BA.1.
Six studies that specifically evaluated protection against the Omicron sublines (BA.2 and BA.4/BA.5) suggested significantly reduced protection when the previous infection was the pre-Omicron variant. But when the previous infection was Omicron, the protection was kept at a higher level.
“The lower cross-immunity with the Omicron variant and its subraces reflects the mutations they carry, which make them escape cumulative immunity more easily than other variants,” says Hasan Nassereldine.