For people previously infected with COVID-19, their risk of hospitalisation or death is 88 per cent lower for at least 10 months compared to those who had not been previously infected, according to a study published in The Lancet journal.

The analysis also suggests that the level and duration of protection against reinfection, symptomatic disease and severe illness is at least on a par with that provided by two doses of the mRNA vaccines (Moderna, Pfizer-BioNtech) for ancestral, Alpha, Delta and Omicron BA.1 variant.

The study did not include data on infection from Omicron XBB and its sub-lineages. "Vaccination is the safest way to acquire immunity, whereas acquiring natural immunity must be weighed against the risks of severe illness and death associated with the initial infection,” said lead author Stephen Lim from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington, US.

Study co-author Caroline Stein from IHME explained that 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.

"This also includes populations that have not previously been infected and unvaccinated groups, as well as those who were infected or received their last vaccine dose more than six months ago," Stein said.

"Decision makers should take both natural immunity and vaccination status into consideration to obtain a full picture of an individual's immunity profile," the researcher added.

The team conducted a review and meta-analysis of all previous studies that compared the reduction in risk of COVID-19 among non-vaccinated individuals against a SARS-CoV-2 reinfection to non-vaccinated individuals without a previous infection up to September 2022.

It included 65 studies from 19 countries and evaluates the effectiveness of past infection by outcome (infection, symptomatic disease, and severe disease), variant, and time since infection.

Studies examining natural immunity in combination with vaccination i.e., hybrid immunity were excluded from the analyses.

Analysis of data from 21 studies reporting on time since infection from a pre-Omicron variant estimated that protection against reinfection from a pre-Omicron variant was about 85 per cent at one month -- and this fell to about 79 per cent at 10 months, the researchers said.

Protection from a pre-Omicron variant infection against reinfection from the Omicron BA.1 variant was lower (74 per cent at one month) and declined more rapidly to 36 per cent at around 10 months, they said.

Analysis of five studies reporting on severe disease (hospitalisation and death) found that protection remained universally high for 10 months: 90 per cent for ancestral, Alpha, and Delta, and 88 per cent for Omicron BA.1.

Six studies evaluating protection against Omicron sub-lineages specifically (BA.2 and BA.4/BA.5) suggested significantly reduced protection when the prior infection was a pre-Omicron variant. However, when the past infection was Omicron, protection was maintained at a higher level, according to the researchers.

"The weaker cross-variant immunity with the Omicron variant and its sub-lineages reflects the mutations they have that make them escape built-up immunity more easily than other variants," said IHME co-author Hasan Nassereldine.

"The limited data we have on natural immunity protection from the Omicron variant and its sub-lineages underscores the importance of continuous assessment, particularly since they are estimated to have infected 46 per cent of the global population between November 2021 and June 2022," Nassereldine added.