What is the Risk of COVID19 Reinfection?
Three recent studies have investigated the risk of COVID19 reinfection. The SARS-CoV-2 Immunity and Reinfection Evaluation (SIREN) study followed more than 20,000 health care workers in the United Kingdom for five months. In the group of 14,000 participants who had not been previously infected, 318 (2.2%) tested positive for SARS-CoV-2. In the group of 6,600 participants who had been previously infected, 44 (0.7%) became reinfected. Only 30% of the people with reinfections reported any symptoms, compared with 78% of participants with first-time infections. Some of the people who became reinfected carried high levels of the virus in their nose and throat, even when asymptomatic. The study concluded that immune responses from past SARS-CoV-2 infection reduce the risk of becoming reinfected by 83% for at least 5 months.
A study by researchers at the Cleveland Clinic published in Clinical Infectious Diseases, found similar levels of protection. The study included test data from more than 150,000 patients tested prior to August 30, 2020, including 8,845 who tested positive. Among the positive patients, 1,278 were retested at least 90 days later, and the researchers identified 63 possible reinfections. Thirty one of the 63 were symptomatic. The researchers estimated an 81.8% protection against reinfection.
Researchers in Denmark found that natural infection confered lasting immunity against SARS-CoV-2, but to a lesser degree in older adults. The study included data from more than 500,000 individuals during Denmark’s first surge, including 11,068 who tested positive. Among those who tested positive, 72 tested positive again at least 90 days later, compared to 16,819 who tested negative. This corresponded to 80.5% protection against reinfection. A secondary cohort analysis of more than 2.4 million individuals found similar overall results (78.8% protection), but lower protection (47.1%) in adults aged 65 years and older.
All three studies determined that the immune response from SARS-CoV-2 infection reduces the risk of becoming reinfected by approximately 80%. The study from Denmark demonstrated significantly lower protection for adults aged 65 years and older. These studies acknowledged that PCR tests can be positive for prolonged periods of time following the resolution of acute infection due to persistent fragments of SARS-CoV-2 RNA, but these studies provided further evidence regarding the degree of protection conferred by natural infection and helped characterize the risk of reinfection.
References
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. medRxiv, posted Jan 15, 2021, https://doi.org/10.1101/2021.01.13.21249642 (2020).
Sheehan MM et al. Reinfection Rates among Patients who Previously Tested Positive for COVID-19: a Retrospective Cohort Study. Mar 15, 2021, Clinical Infectious Diseases, ciab234, https://doi.org/10.1093/cid/ciab234
Hansen CH et al. Assessment of protection against reinfection with SARS-CoV-2 among 4 million PCR-tested individuals in Denmark in 2020: a population-level observational study. March 17, 2021, https://doi.org/10.1016/S0140-6736(21)00575-4
