The possibility of re-infection could critically affect attempts to use herd immunity as a strategy to fight the disease. Those who have had the virus once will develop some immunity. It is rare to get an infectious disease again.
Last month, Japanese authorities said a woman had the virus and has later been declared virus-free but she was tested positive again.
A man in his 70's was also tested positive on February 14 when he was on the Diamond Princess cruise ship. After a recovery period, he was tested negative. But, he was tested positive again on March 13.
These emerging cases left scientists confused there is some evidence in the scientific literature for persistent infections of animal coronaviruses. These can be mainly found in bats.
Based on other viral infections once a person has had the infection they will generally be immune and would not get it again. There is normally some short term immunity with diseases even if there is no long term immunity.
Previous studies on the SARS disease have shown that SARS-specific antibodies remained at the same level in a patient's blood for about 2 years. Then, during the third year after infection antibody levels tended to drop very steeply. This suggests that reinfection is possible after 3 years. Scientists need an antibody test for the infection to give a clear answer to this dilemma.