The risk of hospitalisation or death from Covid-19 is as much as 90% lower at least for 10 months for those who have previously been infected, compared with those who have been uninfected, according to a review published in the Lancet medical journal.

An analysis of five studies reporting on severe disease (hospitalisation and death) found that protection remained high for 10 months – up to 90% for the ancestral coronavirus variants including Alpha and Delta, and 88% for Omicron BA.1.

“For people who have been infected with Covid-19 at least once before, natural immunity against severe disease (hospitalisation and death) was strong and long-lasting for all variants (88% or greater at 10 months post infection),” the report said.

Another 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% at one month, which fell to about 79% at 10 months. Protection from a pre-Omicron variant infection against reinfection from the Omicron BA.1 variant was 74% at one month but declined rapidly to 36% at around 10 months.

The analysis also suggests that the level and duration of protection against reinfection, symptomatic disease and severe illness are 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 variants. The study did not include data on infection from Omicron XBB and its sub-lineages.

The researchers, however, warned that the findings should not discourage vaccination as it was the safest way to acquire immunity.

“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’s School of Medicine, USA.Six studies specifically evaluating protection against Omicron sub-lineages (BA.2 and BA.4/BA.5) suggested significantly reduced protection when the prior infection was from a pre-Omicron variant. But when the past infection was Omicron, protection was maintained at a higher level.