A woman caught coronavirus twice in the space of 20 days in what is believed to have been the shortest time between two infections since the start of the pandemic.
First infected with the Delta variant, the 31-year-old healthcare worker from Spain then caught Omicron in less than 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 booster vaccine.
The first positive result was picked up through routine healthcare testing.
She had no symptoms and went into isolation for 10 days.
But just days after returning to work in January she began to show symptoms of Covid – a cough and fever as well as feeling generally unwell – and took a test which yielded a positive result.
Lab tests showed that she was initially infected by the Delta variant of the virus which causes coronavirus, 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."
She continued: "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 infections.
Work by the Office for National Statistics suggests the risk of reinfection is 10 times higher with the Omicron variant compared with the Delta variant.
Omicron is now the dominant variant in most of the world.
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