A systemic review suggests that the protection afforded by a prior COVID-19 infection is at least as high as that from two doses of a vaccine
In a systematic review and meta-analysis, researchers from Washington university in the US found that the protection afforded by a prior COVID-19 infection was high against re-infection from most pre-omicron variants and remained high against severe disease for all variants and was comparable to the protection from a two-dose vaccination with mRNA vaccines.
To date, several studies have suggested that a previous infection with COVID-19 offers some degree of protection against re-infection. Nevertheless, the available studies have included different time periods as well as COVID-19 variants yet there are no analyses that have provided an overview of how the level of protection against re-infection varies over time and in relation to the different variants.
In the current analysis, the US researchers extracted data from studies through to September 2022 that examined the reduction in risk of developing COVID-19 in those with a prior COVID-19 infection compared to those without a previous infection. The data were then analysed to show the effectiveness of a prior infection against several outcomes including the risk of re-infection, symptomatic disease and severe disease based on the variant and time since infection.
Prior COVID-19 infection and re-infection outcomes
A total of 65 studies were included in the analysis from 19 different countries.
The pooled protection against re-infection varied depending on the variant ranging from 82% against delta to 90% against alpha. In contrast, the pooled protection against re-infection with omicron BA.1 was 45.3%. The protection against symptomatic disease was broadly similar, e.g. 85% for delta and 87.2% against alpha and only 44% against omicron BA.1.
The protection against severe infection (i.e., hospitalisation or death) was high for delta (97.2%), slightly lower against alpha (79.6%) but actually also high against omicron BA.1 (81.9%).
The pooled protection against ancestral, alpha and delta variants was initially high at 85.2% after 4 weeks but reduced to 78.6% at 40 weeks. In contrast, protection against re-infection from omicron BA.1 rapidly declined to 36.1% at 40 weeks.
The authors concluded that protection against re-infection was high against most variants pre omicron BA.1 adding that this level of protection was at least equivalent, if not greater than that provided by two-dose mRNA vaccines, which has also been observed in a previous study.
COVID-19 Forecasting team. Past SARS-CoV-2 infection protection against re-infection: a systematic review and meta-analysis. Lancet 2023