But it is impossible to know exactly how widespread the phenomenon is. To confirm a case of reinfection, scientists must look for significant differences in the genes of the two coronaviruses responsible for the two diseases.
In the United States, where screening was a scarce resource much of this year, many people were only given it if they were sick enough to be hospitalized. Even then, their samples were generally not kept for genetic analysis, making it impossible to confirm suspected re-infections.
A large majority of people who are re-infected may go undetected. For example, the Hong Kong man showed no symptoms the second time around and his infection was only discovered through routine screening at the airport.
“There are a lot of people who are also going to have been exposed who don’t have symptoms, which we’ll never hear from,” said Marion Pepper, an immunologist at the University of Washington in Seattle.
People with a more serious second infection are more likely to be identified because they return to the hospital. But these are likely to be even rarer, experts said.
“If this was a very common occurrence, we would have seen thousands of cases,” said Dr Iwasaki.
In most people, the immune system works as expected.
Reinfections can occur for a 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 problems. Occasionally, a patient may be exposed to a large amount of the virus that has triggered an infection before the immune response can respond.
This variability is fully expected, the experts said, and has been observed in patients with diseases such as measles and malaria.