COVID-19 survivors develop natural immunity against severe illnesses: Lancet

Previous exposure to pre-Omicron variants significantly weakened natural immune defences against Omicron BA.1 reinfection. Sixty-five studies from 19 countries were included in the meta-analysis and review.

The research offers a thorough analysis of studies that have calculated the protection against prior COVID-19 infection by variant and length of time since infection. The results for ancestral, alpha, and delta variants for all significant outcomes demonstrates strong levels of protection against re-infection.

Also, the research reveals that the amount and duration of protection against re-infection, symptomatic disease and severe illness is at least comparable to that offered by two doses of the mRNA vaccines (Moderna, Pfizer-BioNtech) for ancestral, Alpha, Delta, and Omicron BA.

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However, researchers caution that their findings should not discourage vaccination because it is the most secure method of obtaining protection. They suggested people should recognise the natural immunity in individuals who have recently been infected with COVID-19.

“Vaccines continue to be important for everyone in order to protect high-risk populations such as those who are over 60 years of age and those with comorbidities,” the Institute for Health Metrics and Evaluation (IHME) co-author Dr. Caroline Stein said.

The relevance of a study or report to infection immunity from COVID-19 was evaluated based on the title and abstract. In that case, the main text and any supporting materials were evaluated by two separate reviewers to see if they satisfied the inclusion requirements.

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The protection from re-infection against symptomatic disease was 82% or greater for ancestral, alpha, beta, and delta variants, and was again substantially reduced for the omicron BA.1 variant (pooled estimate of 44%.

By contrast, although based on data from 12 studies, protection against severe disease (hospitalisation or death) was universally high, with mean protection of 78% or greater for ancestral, alpha, beta, delta, and omicron BA.1.

Risk measures of SARS-CoV-2 infection in individuals with previous infection compared with those who were infection naive were extracted from each study. The research used adjusted effect sizes where available, otherwise it used unadjusted effect sizes.