Evidence is mounting that having COVID-19 may not protect a person against getting infected again with some of the new variants emerging around the world.
New research also suggests that people can get second infections with earlier versions of the coronavirus if they mounted a weak defense the first time.
How long immunity lasts from natural infection is a big question.
Scientists still think reinfections are fairly rare and usually less serious than initial ones. But recent cases in Brazil and South Africa have caused concern. Health officials say the solution is to get vaccinated as soon as possible and take steps to avoid infection.