A healthcare worker was infected with Covid-19 twice in the space of 20 days in what is believed to have been the shortest time between two infections since the pandemic began.
The 31-year-old woman from Spain became infected with the Delta variant followed by the Omicron variant of the virus in under three weeks.
Researchers said the case shows that even vaccinated people who have had Covid-19 "cannot assume they are protected against reinfection".
The woman, who has been kept anonymous, became infected in December 2021, 12 days after she received her Covid booster vaccine.
The first positive result was picked up through routine healthcare testing.
She had no symptoms and went into isolation for ten days.
But just days after returning to work in January she began to show symptoms of Covid-19 - a cough and fever, as well as feeling generally unwell.
She took a test and that yielded a positive result.
Lab tests showed that she was initially infected by the Delta variant of the virus that causes Covid-19, and then by the Omicron variant.
Her case, which is being presented to the European Congress of Clinical Microbiology and Infectious Diseases in Portugal, is believed to be the shortest known gap between infections.
Dr Gemma Recio, of the Institut Catala de la Salut in Spain, one of the study's authors, said: "This case highlights the potential of the Omicron variant to evade the previous immunity acquired either from a natural infection with other variants or from vaccines.
"In other words, people who have had Covid-19 cannot assume they are protected against reinfection, even if they have been fully vaccinated.
"Nevertheless, both previous infection with other variants and vaccination do seem to partially protect against severe disease and hospitalisation in those with Omicron.
"This case also underscores the need to carry out genomic surveillance of viruses in infections in those who are fully vaccinated and in reinfections.
"Such monitoring will help detect variants with the ability to partially evade the immune response."
Reinfections are the second or subsequent Covid-19 infections.
Work by the Office for National Statistics in the UK suggests the risk of reinfection is ten times higher with the Omicron variant compared with the Delta variant.
Omicron is now the dominant variant in most of the world.