[Estimating the number of COVID-19 cases using a web-based tool: Results from the first week of the 'Covid-19 Trends' project in the Basque Country].

OBJECTIVE:In the Basque Country, two cases of COVID-19 were diagnosed on February 28 2020. On March 14, the Spanish Government established a state of alarm. Only cases confirmed by molecular biology (reverse-transcriptase polymerase chain reaction [RT-PCR]) were known. We launched a web-based surveillance tool to estimate the number of symptomatic cases of COVID-19 to contribute to Public Health decision-making. MATERIAL AND METHODS:We implemented an anonymous web questionnaire and disseminated it through online social media social. We collected epidemiological information about «time» (date of onset of symptoms), «place» (zip code), and «person» (gender, age). We compared cases detected by RT-PCR with the estimated cases, according to the case definition of the Ministry of Health. We calculated the questionnaire response rate and the cumulative incidence at 14days. RESULTS:Between March 19 and 26, 128,009 people answered the questionnaire (5.5% of the Basque population). Of these, 26,375 met the case definition (symptom prevalence of 21.4%). The estimated cases were almost six times more than COVID-19 positive RT-PCR. The estimated 14-day cumulative incidence was 578.3 per 100,000 population compared to RT-PCR positive cases, which was 139.6 per 100,000 population. CONCLUSIONS:This tool was useful in estimating the minimum number of symptomatic cases in the Basque Country, which could support Public Health actions.