Illustration of several SARS-CoV-2 virus particles in blue on a red background
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Previous coronavirus infection lowers the risk of hospitalization or death by nearly 90 percent for at least 10 months compared with no prior infection, according to the most comprehensive analysis of studies to date.

The protection conferred by past infection against reinfection with pre-omicron variants was very high and remained so even after 40 weeks.

However, this was substantially lower for the omicron BA.1 variant, with protection decreasing more rapidly over time. Nonetheless protection from severe COVID-19 disease was high for all variants.

The study did not include data on infection from Omicron XBB and its sublineages.

Prior SARS-CoV-2 infection offered at least the equivalent protection against future infection to two doses of high-quality messenger (m)RNA vaccines such those by Moderna and Pfizer-BioNTech, researchers report in The Lancet.

They say this finding has important implications for guidance on the timing of vaccine doses, including boosters, and policies that restrict access to travel or venues or require worker vaccination.

“It supports the idea that those with a documented infection should be treated similarly to those who have been fully vaccinated with high-quality vaccines,” the team maintains.

The COVID-19 Forecasting Team, led by Caroline Stein from Institute for Health Metrics and Evaluation in Seattle, Washington, conducted their systematic review and meta-analysis using 65 studies from 19 different countries.

Stein and colleagues found a high level of protection through prior infection against reinfection and symptomatic disease for ancestral, alpha, beta, and delta variants, with a mean pooled estimate greater than 82 percent.

But protection was substantially lower, at approximately 45 per cent, for the omicron BA.1 variant.  Protection against severe disease was high for all variants studied, at more than 85% at 40 weeks.

The protection against reinfection with alpha, beta, and delta variants waned over time but remained greater than 75 percent at 40 weeks.

However, protection against omicron BA.1 infection declined more rapidly, dropping to 36 per cent at 40 weeks.

“Our results show that high levels of protection—on average greater than 85%—are present for ancestral, alpha, delta, and beta variants across all three outcomes (infection, any symptomatic disease, and severe disease),” the researchers summarize.

“The analysis shows the substantially reduced level of protection against reinfection or any symptomatic disease to less than 55% for the omicron variant, but that protection against severe disease from the omicron variant appears to be maintained at a high level.”

South-African researchers Cheryl Cohen, from the National Institute for Communicable Diseases in Johannesburg, and Juliet Pulliam, from Stellenbosch University, examine the findings in a Comment article accompanying the study.

They note that, by September 2021, global SARS-CoV-2 seroprevalence was estimated at 59 per cent globally.

The Commentators suggest: “As SARS-CoV-2 epidemiology shifts to more stable circulation patterns in the context of high levels of immunity, studies of the burden and cost of SARS-CoV-2 infection and risk groups for severe disease are needed to guide rational vaccination policy and decisions around prioritisation in relation to other vaccine-preventable diseases.”

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