In the United States, where testing was a rare resource much of this year, many people were not tested unless they were sick enough to be hospitalized. Even then, their samples were usually not preserved for genetic analysis, making it impossible to confirm suspected reinfections. In most people, the immune system works as expected. Reinfections can occur for any number of reasons: because the initial infection was too mild to produce an immune response, for example, or because the immune system was compromised by other health conditions. On occasion, a patient may be exposed to a large amount of virus that seeded an infection before the immune response could respond.

Source: New York Times October 13, 2020 22:25 UTC