Hi Kim,
For the PREV-7 measure, it is auto-populated if it was determined that the patient received a flu shot. Additional details can be found in the Frequently Asked Questions (FAQ) section of the Web Interface. I'm including an excerpt from the FAQ below:
PREV-7 (flu shot) is the only instance where numerator-specific data are prepopulated. Note that influenza immunization data are not prepopulated for all beneficiaries ranked in PREV-7, but only those for whom an immunization could be identified in the claims data. If influenza immunization data has been prepopulated for a patient, the ACO or MIPS group does not need to validate that data. If the ACO or MIPS group is selected for an audit, the ACO or MIPS group will not have to provide medical record documentation for prepopulated influenza immunization data. However, if influenza immunization data are not prepopulated, the ACO or MIPS group should refer to the patient’s medical record to determine if an influenza immunization was administered in accordance with the measure specifications, and should document their findings in the CMS Web Interface. Should your organization be selected for an audit, the influenza immunization data that is obtained from the medical record (i.e., not prepopulated from claims data) is subject to provision of supporting documentation.