From the Document: "Vacancies in the office of U.S. Representative occur when an incumbent Member dies, resigns, or leaves office for any reason before the end of their term. Multiple special elections to fill vacancies in the House of Representatives typically take place in each Congress. The same procedures apply if a Representative-elect dies between the election and the day on which the new Congress to which they were elected convenes."
CRS In Focus, IF11722
Congressional Research Service: https://crsreports.congress.gov/