Filter Questions in Symptom Assessment Affect the Prevalence of (A)Symptomatic COVID-19 Cases

Background: It has been reported that a substantial number of COVID-19 infections are asymptomatic, with both symptomatic and asymptomatic infections contributing to transmission dynamics. Yet, the share of asymptomatic cases varies greatly across studies. One reason for this could be the measurement of symptoms in medical studies and surveys. Design: In 2 experimental survey studies (total N > 3,000) with participants from Germany and the United Kingdom, respectively, we varied the inclusion of a filter question on whether participants who tested positive for COVID-19 had experienced symptoms prior to presenting a checklist of symptoms. We measured the reporting of asymptomatic (versus symptomatic) COVID-19 infections. Results: The inclusion of a filter question increased the reporting of asymptomatic (versus symptomatic) COVID-19 infections. Particularly mild symptoms were underreported when using a filter question. Conclusions and implications: Filter questions affect the reporting of (a)symptomatic COVID-19 cases. To account for such differences in the estimation of population infection rates, future studies should transparently report the applied question format. Both symptomatic and asymptomatic infections are important for COVID-19 transmission dynamics. In previous research, symptoms have been assessed either with or without a filter question prior to presenting a symptom list. We show that filter questions reduce the reporting of asymptomatic infections. Particularly mild symptoms are underreported when using a filter question.