Whether re-infection in individuals who have recovered from COVID-19 is possible and thus pose a risk to others is currently unclear.
The interim results from a large UK-based study of healthcare staff have revealed that prior infection protects the majority of individuals against re-infection for at least 5 months.
The SARS-CoV-2 Immunity and Reinfection Evaluation (SIREN) study is a large, national and multi-centre, prospective study of hospital healthcare workers across the UK, designed to determine whether the presence of antibodies to COVID-19 is associated with reduced risk of subsequent symptomatic or asymptomatic reinfection over the next 12 months. In the study, participants were assigned to one of two cohorts: a positive and negative group. Allocation to the positive group included individuals who were: antibody positive at enrolment or from a prior test, with or without a prior positive PCR test; antibody negative on enrolment, but with a prior antibody positive test and a prior positive PCR result and finally individuals who were antibody negative on enrolment but with a prior positive PCR test. In contrast, those assigned to the negative cohort had a negative antibody test when enrolled and also no evidence of a prior PCR positive test. For the purposes of the study, a possible re-infection was defined as a participant with two positive PCR test samples, 90 days apart or an antibody positive individual with a new positive PCR test at least 4 weeks after the first antibody positive test. A probable reinfection required supportive quantitative serological data and/or viral genomic data confirmatory samples. In addition, the authors also documented whether or not reinfected participants were symptomatic.
The study included 20,787 individuals with a mean age of 46 years and approximately 83% female and who were predominately of White (84.8%) ethnicity. By November 2020 there were 318 new PCR positive infections in the negative cohort, 78% of which were symptomatic, although only 62% of cases displayed typical COVID-19 symptoms and 12% (40) were asymptomatic. In the positive cohort there were a total of 44 re-infections, of which 34% (15) were symptomatic although only 9.1% (4) presented with typical COVID-19 symptoms and 47.7% (21) were asymptomatic. Using these data, the authors calculated that previous infection reduced the risk of a subsequent reinfection by as much as 83% and that this protective effect lasts for at least 5 months.
They concluded by noting that detailed studies on the longevity of this protective antibody response and reassessment of re-infection rates and the challenges posed by the new variant are ongoing.
Hall V et al. Do antibody positive healthcare workers have lower SARS-CoV-2 infection rates than antibody negative healthcare workers? Large, multi-centre prospective cohort study (the SIREN study), England: June to November 2020