A 90-year-old woman died after becoming infected with two different strains of COVID-19, revealing another risk in the fight against the disease, Belgian researchers found.

In the first peer-reviewed analysis of an infection with multiple strains, scientists found the woman had contracted both the alpha variant, which first surfaced in the U.K., and the beta strain, first found in South Africa. The infections probably came from separate people, according to a report published Saturday and presented at the European Congress of Clinical Microbiology & Infectious Diseases.

The woman was admitted to a Belgian hospital in March after a number of falls, and tested positive for COVID-19 the same day. She lived alone, receiving nursing care at home, and hadn’t been vaccinated. Her respiratory symptoms rapidly worsened and she died five days later. When her respiratory samples were tested for variants of concern, both strains were found in two tests. The researchers couldn’t say whether the co-infection played a role in her rapid deterioration.

The idea of multiple infections isn’t completely new. In January, Brazilian scientists reported two cases of COVID-19 co-infection, but the study hasn’t yet been released in a scientific journal. Researchers have also previously found evidence of people becoming infected with multiple strains of influenza. The cases suggest co-infection might be more common than currently known.

“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,” said Anne Vankeerberghen, the lead author of the study and a molecular biologist from OLV Hospital in Aalst, Belgium. “Being alert to co-infections remains crucial.”

Such instances also raise questions over how much protection vaccines can provide. With the rapidly spreading delta variant now the dominant strain in many places, including the U.K., drugmakers are rushing to test their shots against variants and create new versions that could provide a better defense. Countries are also mulling whether to offer booster shots this winter to guard against diminishing responses from vaccines.

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