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As health officials around the world scramble to fight the coronavirus pandemic, many questions about its effect on the immune system remain unanswered.

Vaccines for treatment are still in preliminary testing stages. But even if the research goes well, a vaccine available to the general public is still more than a year away.

What is known definitively, is that the virus causes only mild or moderate symptoms that clear up in a few weeks. Older adults and people with other illnesses, however, are more vulnerable to the virus and more severe symptoms and are at higher risk of death.

Studies suggest that otherwise healthy patients who recover from COVID-19 are likely to be immune from catching the virus again.

Through a process of “immune memory,” the immune system of recovered patients will cause antibodies to quash a pathogen from infecting the body again if it is detected.

Still, some microbes are not as easily detected, and immunity against these viruses can wane in a few months. Which begs the question: can people become infected with the coronavirus twice? At this early stage of research, no consensus has been reached.

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In South Korea earlier this week, 51 people who recovered from the coronavirus were reported to have tested positive again.

Jeong Eun-kyeong, director-general for the Korea Centers for Disease Control and Prevention (KCDC), said the virus likely was reactivated instead of the people being reinfected once they left, South Korea's Yonhap News Agency reported.

The group of people came from the epicenter of South Korea's coronavirus outbreak in Daegu.

But the question remained whether these people had recovered and were reinfected when they came, or, in fact, had not completely recovered when they were released from quarantine.

A preliminary study from China, published in MedRxiv, proposed that survivors of COVID-19 might, in some cases, have undetectable levels of protective antibodies against other strains of the virus.

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Regardless, blood transfusions of recovered COVID-19 patients to newly-infected patients have proven effective in helping the latter produce more antibodies needed to purge the virus.

Fox News’ Travis Fedschun and The Associated Press contributed to this report.