The 90-year-old woman, who was unvaccinated, contracted both the U.K. and South African COVID strains, which are respectively the Alpha and Beta COVID variants. Currently, the Delta variant has shown increasing dominance among new coronavirus cases.

Reuters points to remarks broadcast by local Belgian news station VRT made by experts from the European Congress on Clinical Microbiology & Infectious Diseases (ECCMID), which discussed the rare case over the weekend.

One particularly strange aspect of the case is that the patient may have been infected by the two strains by two different people. Elderly, with two versions of the coronavirus raging simultaneously, the woman died in less than a week after first becoming sick.

“Both [Alpha and Beta] variants were circulating in Belgium” at the time the woman fell sick, said molecular biologist Anne Vankeerberghen of the OLV hospital in Aalst on VRT. “It is therefore probable that this woman was infected by two different people with two variants of the virus. Unfortunately, we do not know how this infection happened.”

The peculiar (and potentially singular) case highlights how much public health officials still have left to learn about how COVID variants affect the body, and how they may even work in tandem as the novel coronavirus continues to mutate amid the global vaccination campaign.

It also underscores the slow COVID vaccine rollout many European nations grappled with in the early days of rolling out the jabs, a dynamic that has drastically improved in the past month with the EU now overtaking the U.S. in the vaccine push.