key: cord-0987134-psrgp9hs authors: Breathnach, Dr. Aodhán Seán; Riley, Peter Andrew; Cotter, Meaghan Patricia; Houston, Angela Cara; Habibi, Maximillian Shahin; Planche, Timothy David title: Prior COVID-19 significantly reduces the risk of subsequent infection, but reinfections are seen after eight months date: 2021-01-13 journal: J Infect DOI: 10.1016/j.jinf.2021.01.005 sha: 2aba5eeb2707838eced82549268dc0e62ca290a7 doc_id: 987134 cord_uid: psrgp9hs [Figure: see text] As the second wave of the SARS-CoV-2 Pandemic takes hold, there is still much that is not certain, in particular the degree and duration of protective immunity following infection in the first wave. Hanrath and colleagues 1 have shown that there were no reinfections identified in a cohort of 1,038 previously infected healthcare workers followed for approximately seven months after the first wave of the Pandemic. A study of 12,000 healthcare workers in the UK similarly showed evidence for protective immunity for up to 31 weeks after infection, although both studies ended before December 2020, when the number of cases in the UK increased significantly 2 . Experience with other human coronaviruses suggests that reinfections might be expected even with the same strain within a few months, but the other hand, evidence from SARS suggests that immunity can persist for much longer 3 . There is good evidence that cellular and humoral markers of immunity persist for several months following SARS-CoV-2 infection 4 , but there have been a limited number of concerning reports of SARS-CoV-2 reinfection 5 . This is an important question, with significant implications for COVID-19 control. The lack of evidence for protective immunity means that public health authorities have been cautious about relaxing quarantine guidelines for patients and healthcare workers who have previously had COVID-19. For example, UK currently does not differentiate between recovered patients and those who have never had the infection 6 , with legally enforceable quarantine of all exposed contacts, irrespective of their immune status. US guidance is more pragmatic and exempts recovered patients from quarantine if subsequently exposed, but only if this is within three months of the first illness 7 . Furthermore, the emergence of new genetic variants of SARS-CoV-2 has raised concerns that immunity may be strainspecific, although there is as yet no evidence for this 8 . To address this question, we examined results of SARS-CoV-2 PCR and antibody tests in our southwest London laboratory, which serves four hospitals and a population of 1.3 million. We determined who had evidence of COVID-19 in the first wave of infections in the UK (February to July 2020, with a peak in early April), as shown either by a positive SARS-CoV-2 PCR or a positive antibody test, and determined their risk of having a positive SARS-CoV-2 PCR assay in the first five months of the second wave (August to December 2020), compared with patients who had a previous negative PCR or antibody test. Cases where the second positive result was