March 25, 2023


Feb 24, 2023

Past SARS-CoV-2 infection protection against re-infection: a systematic review and meta-analysis

Group 1 Past SARS-CoV-2 infection

Group 2 No past SARS-CoV-2 infection

Effectiveness of past infection by outcome


Symptomatic disease

Severe disease


High levels of protection from infection caused by Alpha, beta, and delta variants

Lower levels of protection from infection caused by Omicron BA.1 variant

Effectiveness against re-infection with the omicron BA.1 variant

Protection against reinfection, 45·3%

Protection against omicron BA.1 symptomatic reinfection, 44%

Protection against severe disease if reinfected with BA.1 is 88.9%

Protection from re-infection with ancestral strains

Alpha and delta variants

Declined over time

78·6% at 40 weeks

Protection against re-infection with omicron BA.1

Declined more rapidly 36·1% at 40 weeks

Protection against severe disease at 40 weeks if reinfected

Remained high for all variants

90·2% for alpha and delta variants

88·9% for omicron BA.1

Data suggests that the level of protection afforded by previous infection is at least as high, if not higher than that provided by two-dose vaccination using high-quality mRNA vaccines

As of June 1, 2022

COVID-19 pandemic had caused an estimated 17·2 million total deaths

6·88 million reported deaths

7·63 billion total infections and re-infections.

Between 15th November 2021 and 1st June 2022 3·8 billion people

46% of the global population, have been infected by omicron and sublineages.

Understanding needed for Predicting future potential disease burden Designing policies, travel, access to venues Informing choices, vaccines Estimate protection from past infection Systematically synthesize studies 65 studies from 19 countries By variant By time since infection Up to Sept 31, 2022

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