Past infection protects against severe disease

2 minute read

Protection against reinfection was high against all variants until Omicron, unfortunately.

Prior covid-19 infection provides lasting protection against reinfection with all pre-Omicron variants, and protects against severe illness from all variants, a global systematic review has found.

Examining 65 studies from 19 countries, researchers found that individuals previously infected had 90% higher protection against Alpha covid, 86% against Beta and 82% against Delta.

“[Our findings] support the idea that those with a documented infection should be treated similarly to those who have been fully vaccinated with high-quality vaccines,” the authors of the review wrote.

Past infection was also more than 78% effective in protecting against severe disease in individuals reinfected with any covid variant, including Omicron BA.1.

Although this protection declined over time, at 40 weeks it remained at 79% for the ancestral Alpha and Delta variants.

In comparison, prior infection was only 45% effective against reinfection by the Omicron BA.1 variant, and it was only 44% effective against BA.1 symptomatic disease.

Protection against reinfection also declined more rapidly over time for those infected with Omicron BA.1, decreasing to 36% after 40 weeks. 

The authors say the findings should inform clinical guidelines about timing covid-19 vaccines, including boosters, and policies regarding vaccine mandates for workers.

“The immunity conferred by past infection should be weighed alongside protection from vaccination when assessing future disease burden from covid-19, providing guidance on when individuals should be vaccinated, and designing policies that mandate vaccination for workers or restrict access, on the basis of immune status, to settings where the risk of transmission is high, such as travel and high-occupancy indoor settings,” they conclude.

The Lancet 2023, online 16 February

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