The 31-year-old woman first tested positive on Dec. 20 last year, on a routine PCR test taken during a staff screening. She was fully vaccinated and had received a booster shot 12 days earlier.

The patient didn’t develop any symptoms, but still self-isolated for 10 days before returning to work. Then on Jan. 10, 2022, just 20 days after first testing positive, she developed a cough and fever and took another PCR test, which also returned positive.

The case, which is being reported at this year’s European Congress of Clinical Microbiology & Infectious Diseases in Lisbon, is backed by genome sequencing, which showed the patient was infected by two different strains of SARS-CoV-2. The December 2021 infection was with the Delta variant and the second, in January, was with the Omicron variant.

Gemma Recio, one of the study’s authors and a doctor at Institut Català de Salut in Tarragona, Spain, said in a statement, “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 adds that “nevertheless, both previous infection with other variants and vaccination do seem to partially protect against severe disease and hospitalization in those with Omicron.”

How likely am I to catch COVID again?

Before November 2021, fewer than 1% of all cases recorded in the U.K. were reinfections. But after Omicron swept through the country, this figure shot up to 11%, indicating that a previous infection with the Alpha or Delta variants was unlikely to offer strong protection against the more contagious Omicron variant.

But while reinfection after catching one of the previous variants is quite likely, reinfection of Omicron is far rarer.

Early data out of the U.K. indicate that some people have been infected with both strains of the Omicron variant, first BA.1 and then its subvariant BA.2. Researchers at Denmark’s infectious disease authority Statens Serum Institut counted the numbers in February 2021 and found that while double infections are possible, they are very rare.

The Danish study tested 1.8 million cases of COVID-19 that occurred during the initial Omicron wave between late November 2021 and mid-February 2022. It registered only 187 COVID reinfections, which occurred between 20 and 60 days apart from each other. Of those reinfections, the researchers found only 47 were reinfections with the BA.1 and BA.2 variants of Omicron, which mostly occurred in young unvaccinated individuals with mild disease not resulting in hospitalization or death.

While 47 in 1.8 million is a very low ratio, the data may change the longer it is collected.

Monica Gandhi, a professor of medicine at the University of California, told DW, Germany’s international broadcaster, that Omicron reinfections could become more common than the Danish study suggests. Gandhi notes that antibodies provided through infection or a booster vaccine may last only around four months, and as protection wanes, people may become vulnerable again.