A woman was found to be infected with two separate strains of Covid-19 at the same time, doctors have revealed.
One was the British Kent variant and the other the South African one.
The 90 year old Belgian woman died within five days of her symptoms worsening.
The case is believed to be among the first of its kind and has implications for how the viruses interact in an infected person.
The alarming phenomenon could generate deadlier variants that dodge vaccines and spread more easily.
Dr Anne Vankeerberghen, a molecular biologist at OLV Hospital in Aalst, Belgium, said: "Both these variants were circulating in Belgium at the time.
"So it is likely the lady was co-infected with different viruses from two different people. Unfortunately, we don't know how she became infected."
The 90 year old was admitted on 3 March - and tested positive that day. She lived alone and received nursing care at home.
She had no major underlying health problems - and had not been vaccinated.
A throat swab showed she had caught the 'Kent strain' of south east England - and another first detected in South Africa.
Initially, the patient was breathing well and had no signs of respiratory distress. But she declined quickly - and died five days later.
Dr Vankeerberghen said: "This is one of the first documented cases of co-infection with two SARS-CoV-2 variants of concern."
Their presence was confirmed by PCR (polymerase chain reaction) testing on a second respiratory sample through genome sequencing.
All viruses, including SARS-CoV-2 that causes Covid, mutate.
In the pandemic different types are split into two groups - VOCs (variants of concern) and VOIs (variants of interest).
It has helped the World Health Organisation (WHO) prioritise monitoring and research.
Dr Vankeerberghen said: "Whether the co-infection of the two variants of concern played a role in the fast deterioration of the patient is difficult to say. Up to now, there have been no other published cases.
"However, the global occurrence of this phenomenon is probably underestimated due to limited testing for variants of concern and the lack of a simple way to identify co-infections with whole genome sequencing."
The Kent variant, known as B.1.1.7; Alpha, was reported to the WHO by the UK government in December.
It took over from other Covid strains circulating in the region in weeks and has since reached more than 50 countries - including Belgium.
Around the same time the South African mutation (B.1.351; Beta) began spreading fast through three provinces.
It has now been identified in at least 40 countries - including Belgium.
In January scientists in Brazil reported two people had been infected simultaneously with two different strains.
They were the Brazilian variant (B.1.1.28) and a second (VUI-NP13L) first discovered in Rio Grande do Sul.
Both patients, in their 30s, made a full recovery. The cases are yet to be published in a scientific journal.
Prof Keith Neal, an infectious diseases expert at Nottingham, said if there's a lot of transmission "you can pick up two different viruses around the same time."
But when one strain is predominant, that is unlikely. Previous research has found people can catch different strains of flu simultaneously.
The Belgian woman's case was reported at the virtual European Congress of Clinical Microbiology & Infectious Diseases.
