ZIP Codes represent mail delivery routes, not areas. To represent the area that the route is in, the US Census Bureau developed ZIP Code Tabulation Areas (ZCTAs). In ZCTAs, some ZIP Codes are bundled together into a single ZCTA.
Modified ZCTA (modzcta) is a geography used to analyze health data. ZCTAs with small populations are combined, which allows for more stable rate calculations by increasing the underlying population of these areas.
If your ZIP Code isn't in the COVID-19 data, use this map or table to see what modzcta it is in.