A man from Germany (62) was, against medical advice, vaccinated 217 times against covid, the doctors said, as reported by the BBC.
The bizarre case was documented in the journal "The Lancet Infectious Diseases".
Vaccines were purchased and given privately within 29 months.
The man appears to have suffered no ill effects, say researchers from the University of Erlangen-Nuremberg.
"We found out about his case from newspaper articles," said Dr. Kilian Šober, from the university's microbiology department.
"Then we contacted him and invited him to undergo various tests in Erlangen. He was very interested in it," Schober added.
The man provided fresh blood and saliva samples. The researchers also tested some of his frozen blood samples that had been stored in recent years.
"We were able to take blood samples when the man received an additional vaccine during the study at his own insistence. We were able to use these samples to determine exactly how the immune system reacts to the vaccination," Shober said.
Evidence for 130 stabbings was collected by the public prosecutor of the city of Magdeburg, who launched an investigation into the alleged fraud, but no criminal charges were filed.
Covid vaccines cannot cause infection, but they can teach the body how to fight the disease.
MRNA vaccines work by showing the body's cells part of the virus's genetic code. The immune system should then recognize and know how to fight against covid if it is really faced.
Schober worried that hyperstimulating the immune system with repeated doses might tire certain cells, but the researchers found no evidence of that in the 62-year-old. There were also no signs that he had ever been infected with covid.
"It is important that we do not support hyper-vaccination as a strategy to enhance adaptive immunity," the researchers said, according to the BBC.
The results of their tests on the 62-year-old were insufficient to draw long-term conclusions, nor recommendations for the general public.
"Current research suggests that three-dose vaccination, along with regular booster shots for vulnerable groups, remains the best approach. There is no indication that more vaccines are needed," the university's website states.
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