Belgian medical researchers confirmed Sunday that an unvaccinated 90-year-old woman who died in March is the first documented fatality from two simultaneous COVID-19 variants.

The woman became sick with the Alpha and Beta variants. Alpha was first detected in the U.K., while Beta was first detected in South Africa. The researchers said they don't know how the patient became infected and that she may have contracted the variants from two different people.

"This is one of the first documented cases of co-infection with two variants of concern of SARS-CoV-2," Dr. Anne Vankeerberghen, a molecular biologist and author of the study released by the European Congress of Clinical Microbiology and Infectious Diseases, said in a statement.

After the patient suffered a series of falls, she was sent to a hospital near Brussels. She was later tested for the coronavirus and did not show symptoms. The woman "rapidly developed worsening respiratory symptoms and died five days later," according to a statement.

Vankeerberghen said it is "difficult to say" to what extent the two variants played in the quick deterioration of the woman's health.

Both the Alpha and Beta variants have circulated in Belgium.

While rare, the discovery is evidence that unvaccinated people can be susceptible to dual infections. There is still more research needed to understand the extent of having two variants simultaneously.

"The evidence so far does not suggest that infection with more than one variant leads to more severe disease," Maitreyi Shivkumar, a senior lecturer in molecular biology at De Montfort University in Leicester, England, wrote in March in The Conversation.

Researchers can't fully confirm if this is the lone case of two variants contributing to a patient's death. There were two similar cases in Brazil in January but the results have not been published in a scientific journal.

Coronavirus