A new study shows how well a prior COVID-19 infection protects the next time around. 

Someone previously infected with COVID-19 has at least an 88 per cent lower risk of death and hospitalisation if they get it again 10 months later, according to international researchers. 

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

“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,” says lead author Dr Stephen Lim from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington’s School of Medicine, USA. 

Since January 2021, several studies and reviews have reported the effectiveness of past COVID-19 infection in reducing the risk of reinfection and how immunity wanes over time. But none has comprehensively assessed how long the protection after natural infection will last and how durable that protection will be against different variants.

To provide more evidence, the researchers conducted a review and meta-analysis of all previous studies that compared the reduction in risk of COVID-19 among unvaccinated individuals against a SARS-CoV-2 reinfection to unvaccinated individuals without a previous infection up to September 2022. 

The review included 65 studies from 19 countries.

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, falling to about 79 per cent at 10 months. 

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. 

Nevertheless, 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 pre-Omicron variant. But when the past infection was Omicron, protection was maintained at a higher level.

The full study is accessible here.