Severity of SARS-CoV-2 Reinfections as Compared with Primary Infections

Qatar had a first wave of infections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from March through June 2020, after which approximately 40% of the population had detectable antibodies against SARS-CoV-2. The country subsequently had two back-to-back waves from January through May 2021, triggered by the introduction of the B.1.1.7 (or alpha) and B.1.351 (or beta) variants.¹ This created an epidemiologic opportunity to assess reinfections.

Using national, federated databases that have captured all SARS-CoV-2–related data since the onset of the pandemic (Section S1 in the Supplementary Appendix, available with the full text of this letter at NEJM.org), we investigated the risk of severe disease (leading to acute care hospitalization), critical disease (leading to hospitalization in an intensive care unit [ICU]), and fatal disease caused by reinfections as compared with primary infections in the national cohort of 353,326 persons with polymerase-chain-reaction (PCR)–confirmed infection between February 28, 2020, and April 28, 2021, after exclusion of 87,547 persons with a vaccination record. Primary infection was defined as the first PCR-positive swab. Reinfection was defined as the first PCR-positive swab obtained at least 90 days after the primary infection. Persons with reinfection were matched to those with primary infection in a 1:5 ratio according to sex, 5-year age group, nationality, and calendar week of the PCR test date (Fig. S1 and Table S1 in the Supplementary Appendix). Classification of severe, critical, and fatal Covid-19 followed World Health Organization guidelines, and assessments were made by trained medical personnel through individual chart reviews.

Table 1.