Effectiveness of previous infection in preventing reinfection with SARS-CoV-2 variants |
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Effectiveness of previous infection in preventing reinfection with SARS-CoV-2 variants
Dr Surya Kant, Professor and Head, Dept. of Respiratory Medicine, KGMU, UP, Lucknow. National Vice Chairman IMA-AMS,  11 February 2022
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A previous Covid infection protects against symptomatic reinfection with SARS-CoV-2 variants, according to a study from Qatar published February 9, 2022 in the New England Journal of Medicine.1

This study conducted in the resident population of Qatar evaluated the effectiveness of previous Covid-19 infection in preventing reinfection with the different variants. Data was sourced from the national databases of test results, disease course and vaccinations. Cases detected between March 23, 2021 and November 18, 2021 were examined. A test-negative, case-control study design was used for this purpose. Only those cases where the cycle threshold (Ct) value was ≤30 and persons suspected to have symptoms compatible with Covid-19 were included. Vaccinated persons were excluded from the study. The median age of the participants ranged from 31 to 35 years. Previous infection was defined as positive RT PCR at least 90 days before another positive test.

Analysis of data showed 90.2% effectiveness of previous infection in preventing reinfection with alpha variant; the effectiveness against the beta variant was 85.7%, against delta variant the effectiveness was 92.0%, while the effectiveness against the omicron variant was found to be 56.0%.

Among the patients with reinfection, severe disease occurred in only one patient with the alpha variant, two patients each with the beta variant and the omicron variant, while no patient with delta variant progressed to severe disease. There were no critical cases or deaths in patients with reinfection. The effectiveness against severe, critical or fatal disease was estimated to be 69.4% against alpha, 88.0% against beta, 100% against delta and 87.8% against Omicron.

This study has demonstrated that previous infection provides robust protection against reinfection with the alpha, beta and delta variants of SARS-CoV-2. Although comparatively the degree of protection against reinfection with the Omicron variant was lower, it was still significant at roughly 60%.

India’s total vaccine coverage has crossed 170 crores. More than one crore adolescents aged 15-18-years are now fully vaccinated. Around 16 lakh precaution doses have been administered till date. However, consistent use of well-fitting three-layer masks (double masking) and strict adherence to covid-appropriate behavior are as important now as they were during the earlier waves for everyone, including the fully vaccinated persons.


  1. Altarawneh HN, et al. Protection against the Omicron variant from previous SARS-CoV-2 infection. N Engl J Med. 2022 Feb 9. doi: 10.1056/NEJMc2200133.
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