Abstract
A difficulty that arises when studying symptoms that develop or persist after SARS-CoV-2 infection is determining whether the symptoms are being caused by the recent infection. In many studies, investigators reporting frequencies of clinical conditions after COVID-19 do not report corresponding frequencies in individuals with no history of SARS-CoV-2 infection,1 which is necessary for effect estimation. In other words, because some of the clinical manifestations linked to long-term COVID-19 (“long COVID”) might be related to the general impact of the pandemic or to coexisting medical conditions, recruitment of a comparator group is necessary.