The research project, completed by the Munich Clinic Schwabing, found that the concentration of antibodies in the blood decreases rapidly in the months following an infection.
The findings dampen hope of long-lasting immunity for those who have beaten the virus.
The findings give credence to reports out of South Korea and China that some people had been infected with the virus despite having previously recovered from it.
The project looked at patients who had recovered from the virus from January onwards.
It found that while neutralising antibodies were formed in the blood during the time in which the person was infected, these declined in the following months.
“In four of the nine patients, we see falling neutralising antibodies in a very special test that can only be carried out in a high-security laboratory,” said Clemens Wendtner, chief physician of the local clinic for infectious diseases.
“The extent to which this has an impact on long-term immunity and vaccination strategies is still speculative, but must be monitored critically as it progresses.”
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The researchers said more work needed to be done to determine the extent to which these declining antibodies could prevent further infection.
A similar study in China found that antibodies decreased sharply two months after the initial infection, while those who did not develop symptoms from the virus were more likely to accrue fewer antibodies than those who fell sick.