Limitations of the pilot and how they will be addressed in the main study.

(DOCX)</p

Topic guide.

(DOCX)</p

COREQ checklist.

(PDF)</p

Age-stratified LASSO stability selection on testing PCR positive.

(DOCX)</p

Univariate effect of each symptom reported in the week prior to testing on risk of PCR positivity in rounds 2–7 and round 8.

(DOCX)</p

Key characteristics of the REACT-1 study population for rounds 2–7 and round 8.

Results are presented for the full study population. (DOCX)</p

Descriptive statistics for symptoms and SARS-CoV-2 PCR test results among REACT-1 participants, rounds 2–7 and round 8.

(DOCX)</p

Proportion of B.1.1.7 variant cases among all cases in England, October 2020–January 2021.

(DOCX)</p

Characteristics of the round 8 PCR-positive participants in whom lineage data were available.

Numbers are presented for cases with wild-type (N = 181) and B.1.1.7 (N = 898) infections. The statistical significance of differences between these 2 groups is evaluated using a chi-squared test for categorical variables. We report the p-value for the null hypothesis of no difference. (DOCX)</p

Selected symptoms predictive of COVID-19.

Results of LASSO stability selection using 1,000 models (with 50% subsamples of training data from rounds 2–7). Mean (penalized) log odds ratios (log ORs) across all models are shown in the top panel. Positive regression coefficients are presented in teal, and negative in red. Only symptoms selected at least once are displayed. The selection proportions (selection prop.; proportion of 1,000 models that included each symptom) are shown in the middle panel; the horizontal dashed line shows the selection threshold of 50%. Symptoms are ordered according to their selection proportions, and selected symptoms are in black. The bottom panel shows the area under the curve (AUC) of models adding each variable in order of selection proportion (from left to right) in both holdout data from rounds 2–7 (grey) and data from round 8 (red).</p