A 36-year-old doctor from Brazil is the latest victim of COVID-19 reinfection and experts say that her second bout was more severe than the first. The unnamed woman initially developed the disease in March, and after 24 days, her symptoms disappeared. However, 12 weeks after she first fell ill, the symptoms returned.

According to Newsweek, a lung scan showed she developed pneumonia the second time around, indicating a stronger inflammatory response than her previous illness. The case was published in the Journal of Medical Virology and the authors said there was a “strong probability” that the two episodes were caused by different strains of the virus. “There is no guarantee of prolonged immunity,” the team wrote.

Over two dozen reinfections from COVID-19 have been reported worldwide, but experts are worried that the number of cases has been underreported. To confirm a true reinfection, doctors would have to check the genetic code of each round of illness to make sure they are distinct from each other. There have also been other reports that people who are infected with the virus the second time around suffer more, perhaps because their immune systems have not sufficiently recovered to gear up for the fight.

According to The Guardian, the cases of reinfection mean that having COVID-19 doesn’t guarantee immunity, and opens the door to other questions as to the how contagious are victims the second time around, and why did they fall victim twice?

“It’s really hard to find a pattern right now,” said Dr. Akiko Iwasaki, an immunologist at the Yale School of Medicine. “Essentially every case is different.”

A case study published The Lancet this month revealed that a 25-year-old man in Washoe County, Nevada, suffered two episodes of COVID-19, one in April and the second at the beginning of June. Genetic testing showed that the viruses that caused his two bouts with COVID-19 were different, so it was not a case of one prolonged illness. In fact, the second time around he also had a worst outcome from the disease.

Researchers said that while infection with SARS-CoV-2, the virus that causes COVID-19, does elicits an immune response, we do not know why some people are still susceptible to infection.

According to Mic, our lack of knowledge about the novel coronavirus makes it difficult to predict how the human immune system will respond even if antibodies are present to combat the disease. The Nevada incident was not isolated as another distinct case of reinfection was noted in South America. That person was reportedly sicker the second time around, too.

Iwasaki told NPR that there “are many reasons why a person might get sicker the second time around. She said they may have been exposed to higher levels of the virus or the immune system response from the first infection might be weaker, adding that at this point, “this is all very speculative.”