The 90-year-old woman, who was unvaccinated, was admitted to the OLV hospital in the city of Aalst after a series of falls in March and tested positive for Covid-19 the same day.
While her oxygen levels were initially good, her condition declined rapidly and she died five days later.
It was later discovered that she was carrying both the Alpha strain and the Beta variant.
“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,” said molecular biologist Anne Vankeerberghen from the OLV hospital who led the research.
“Unfortunately we don’t know how she became infected.”
Ms Vankeerberghen said it’s difficult to assess whether the co-infection played a role in the fast deterioration of the patient.
She added that while there had been “no other published cases” of similar co-infections, the “phenomenon is probably underestimated”.
The research, which has not yet been submitted to a medical journal for publication, is being presented at a European congress on microbiology and infectious diseases.
A study published in the journal Nature found that just a single dose of the Pfizer or AstraZeneca vaccine produced just 10 per cent protection against the variant first identified in India.
Researchers conducted laboratory experiments on blood samples from people who had received their first dose but not the second.
After a single dose, just 10 per cent of those samples had developed antibodies that neutralised the delta variant.
However, after two doses 95 per cent of samples had developed neutralising antibodies against the Delta variant.