It is possible to catch two Covid variants at the same time, experts are warning after seeing a double infection in a 90-year-old woman who became sick with the Alpha and Beta types first identified in the UK and South Africa.
The woman, who died in March 2021 in Belgium, had not been vaccinated.
Her doctors suspect she contracted the infections from two different people.
They believe it is the first documented case of its kind and, although rare, similar dual infections are happening.
Her case is being discussed at this year's European Congress on Clinical Microbiology & Infectious Diseases.
In January 2021, scientists in Brazil reported that two people had been simultaneously infected with two types of coronavirus, one of them a variant of concern called Gamma.
Researchers from Portugal, meanwhile, recently treated a 17-year-old who appeared to have caught a second type of Covid while still recovering from a different, pre-existing Covid infection.
The 90-year-old, who was infected with the two "variants of concern" - the most worrying new versions of coronavirus that experts are tracking - had been admitted to hospital after experiencing some falls, but later developed worsening respiratory symptoms.
Laboratory tests on samples taken when she was admitted revealed she had Covid-19, caused by two different mutated versions of the pandemic virus, simultaneously - Alpha and Beta.
Lead researcher Dr Anne Vankeerberghen, from the OLV hospital in Aalst, Belgium, said: "Both these variants were circulating in Belgium at the time, so it is likely that the lady was co-infected with different viruses from two different people. Unfortunately, we don't know how she became infected.
"She was a lady who lived alone, but she got a lot of helpers coming in to care for her.
"Whether the co-infection of the two variants of concern played a role in the fast deterioration of the patient is difficult to say."
Viruses constantly evolve by mutating as they replicate. This creates new versions or variants.
Covid has undergone some important changes that may give it an advantage - for example, by increasing its ability to replicate or dodge some of our existing immunity from past infection or vaccination.
The most concerning ones are being closely monitored by scientists and are called variants of concern.
Currently, in the UK, it is the Delta variant that is spreading the most.
Experts are confident that existing vaccines offer good protection against it.
Scientists are designing new Covid vaccines that will be an even better match for new variants, and could be used as boosters.
Prof Lawrence Young, an expert in virology at the University of Warwick, said: "Detecting two dominant variants of concern in a single person is not a surprise - these could have been passed on by a single infected individual, or by contact with multiple infected people."
He said more studies were needed to determine whether such infections in any way compromise the efficacy of vaccination, or make for a worse case of Covid-19.