Clinical trials

A study is conducted to determine the long term effects of a novel vaccine against the SARS-CoV-2 virus, which causes coronavirus disease 19 (COVID-19). To do so, researchers followed individuals who had received the vaccine for one year. They discovered the vaccine is associated with an increased incidence of fever of unknown origin and new-onset heart failure with reduced ejection fraction in 25 patients out of the 1 million total patients who were followed. Based on these results, an additional warning about these adverse events is added to the investigator brochure for the vaccine. Which of the following clinical phases does this study most likely belong to?