Efficacy of previous infection in preventing re-infection with Omicron and other variants of SARS-CoV-2 in Qatar

In a recent study published on the medRxiv* preprint server, researchers estimate the effectiveness of a previous infection in preventing reinfection (PE) with Omicron and other variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) for the population resident of Qatar.

Study: Protection offered by previous infection against reinfection by SARS-CoV-2 with the Omicron variant. Image Credit: creativeneko / Shutterstock

Previous infection with SARS-CoV-2 elicits strong protection against reinfection with variants Alpha (B.1.1.7), Beta (B.1.351) and Delta (B.1.617.2). However, this is not true for the Omicron variant (B.1.1.529) harboring multiple mutations in its Spike protein that mediate immune evasion.

About the study

PE is the proportional reduction in susceptibility to infection with coronavirus disease 2019 (COVID-19) in people who have been previously infected vis à vis those who have not previously been infected with SARS-CoV-2. Researchers in the present study estimated PE with Omicron and other variants of SARS-CoV-2 in Qatar using the negative test case-control study design, a methodology validated for robust estimates of PE to ” study the protection conferred by the previous SARS. CoV-2 infection in the prevention of reinfection with SARS-CoV-2 variants.

The source of the data for the current study was Qatar’s National and Federated SARS-CoV-2 Databases, which included details of all cases of COVID-19 polymerase chain reaction (PCR) tests, vaccinations, clinical infections, hospitalizations, deaths and others. demography. At the time of the study, while almost everyone in Qatar was vaccinated, vaccination records of people arriving from other countries were also recorded in Qatar’s SARS-CoV-2 databases at the airport.

EPs were estimated for Alpha, Beta and Delta variants in subjects identified between March 23, 2021 and November 18, 2021. Both positive cases by PCR with a variant genotyped infection, confirmed by real-time reverse transcription PCR ( RT-qPCR) genotyping of positive clinical specimens and controls were exactly matched in a ratio of 1: 5 by sex, 10-year age group, nationality, and PCR test calendar week to help control for known differences or bias in the risk of exposure to SARS-CoV-2 infection.

Likewise, PEs were estimated for the Omicron variant in subjects identified between December 23, 2021 and January 2, 2022. Omicron-infected COVID-19 cases and controls were matched in a ratio of 1: 3 by sex. , age of 10 years group, nationality and calendar day of the PCR test.

COVID-19 re-infections with negligible symptoms and high Ct the values ​​are not significant from a public health point of view. Therefore, the study focused only on estimating PE against re-infections with an RT-qPCR cycle cutoff (Ct) of ≤30.

The researchers also conducted two sensitivity analyzes to check the robustness of the PE study estimates. The first analysis further adjusted vaccination status and the second analysis excluded individuals with a vaccination record prior to PCR testing.


In the present study, the estimated EPs were one minus the ratio of the odds of previous infection in PCR positive cases to the odds of previous infection in controls. Previous infection referred to infection confirmed by PCR ≥ 90 days before a new positive PCR test.

Study results showed PEs of 90.2% for Alpha, 84.8% for Beta, 92.0% for Delta and 56.0% for Omicron. Of all the cases of COVID-19 re-infections, only one Alpha, two Beta, zero Delta and two Omicron re-infections have progressed to severe COVID-19 while none have progressed to critical or fatal COVID-19. The median time from previous infection to PCR testing in cases and controls of the Alpha, Beta, Delta, and Omicron variants was 279, 284,253, and 314 days, respectively.

Each study participant who tested positive on a PCR test and who was hospitalized was assessed for the severity of the infection every three days until discharge or death. This protocol assessing severity, criticality and mortality has been applied to all Alpha, Beta and Delta cases, but only to a small number of Omicron cases. The results estimated the PEs against hospitalization or death due to reinfection at 69.4% for Alpha, 88.0% for Beta, 100% for Delta and 87.8% for Omicron.


In summary, the study results predicted about 90% protection against Alpha, Beta or Delta variants of SARS-CoV-2 conferred by a previous infection with SARS-CoV-2 +, when it was considerably weaker against Omicron at almost 60%. Regardless of the variant, the previous infection also confers robust protection against hospitalization or death upon reinfection.

Since Qatar has an unusually young and diverse demographics, the study’s findings may not apply to other countries where senior citizens make up a large portion of the population. Qatar has only 9% of residents over the age of 50 and 89% of residents are expatriates from more than 150 countries.

*Important Notice

medRxiv publishes preliminary scientific reports which are not peer reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.

Sara H. Byrd