The study was based on data from the ONS Covid Infection Survey, the largest regular survey of COVID-19 infections in the general population. To investigate this, the researchers used data from the Office for National Statistics Covid Infection Survey (ONS-CIS), which tested participants approximately monthly. Of these, 381 individuals tested positive with the same viral infection over a period of a month or longer. In some cases, individuals remained infected with viral variants that had gone extinct in the general population. Of the 381 persistent infections, 65 had three or more PCR tests taken over the course of their infection.