Indicators of past COVID-19 infection status: Findings from a large occupational cohort of staff and postgraduate research students from a UK university

BackgroundDefinitive diagnosis of COVID-19 requires resources frequently restricted to the severely ill. Cohort studies must rely on surrogate indicators to define cases of COVID-19 in the community. We describe the prevalence and overlap of potential indicators including self-reported symptoms, suspicion, and routine test results, plus home antibody testing.MethodsAn occupational cohort of 2807 staff and postgraduate students at a large London university. Repeated surveys covering March to June 2020. Antibody test results from ‘lateral flow’ IgG/IgM cassettes in June 2020.Results1882 participants had valid antibody test results, and 124 (7%) were positive. Core symptoms of COVID-19 were common (770 participants positive, 41%), although fewer met criteria on a symptom algorithm (n=297, 16%). Suspicion of COVID-19 (n=509, 27%) was much higher than positive external tests (n=39, 2%). Positive antibody tests were rare in people who had no suspicion (n=4, 1%) or no core symptoms (n=10, 2%). In those who reported external antibody tests, 15% were positive on the study antibody test, compared with 24% on earlier external antibody tests.DiscussionOur results demonstrate the agreement between different COVID indicators. Antibody testing using lateral flow devices at home can detect asymptomatic cases and provide greater certainty to self-report; but due to weak and waning antibody responses to mild infection, may under-ascertain. Multiple indicators used in combination can provide a more complete story than one used alone. Cohort studies need to consider how they deal with different, sometimes conflicting, indicators of COVID-19 illness to understand its long-term outcomes.THUMBNAILWhat is already known on this subject?Research into the effects of COVID-19 in the community is needed to respond to the pandemic, and guidance is needed as to how cohort studies measure COVID-19 infection status retrospectively, particularly given that objective testing for infection was not widely available in the first wave of COVID-19 in many countries. Retrospective testing might be possible using antibodies as a proxy for previous COVID-19 infection.What this study adds?Antibody testing is feasible in community cohorts but sensitivity may be poor. Self-report of suspected infection, recall of symptoms and results of tests received elsewhere add different aspects to the ascertainment of COVID-19 exposure. Combining self-report and objectively measured indicators may enable tailored algorithms for COVID-19 case definition that suits the aims of different research studies.