Levels of antibodies that are enough to protect a person against various strains of Covid-19 are different, and cannot be viewed as a reliable criterion when deciding whether vaccination is needed or not, said Yekaterina Stepanova, a Russian infectious diseases specialist.

"As preliminary research shows, various strains require various levels of antibodies. For example, protection against the South African strain requires a level of over 1,000 BAU/ml. The level for the Delta strain is presumed to be even higher, at 1,500 BAU/ml," Yekaterina said.

"Any level of antibodies shows that a person had been exposed to the infection. IgM antibodies in high numbers illustrate an acute phase of Covid-19. However, serum diagnostics cannot be 100% precise, and should not be viewed as an ultimate reason to skip vaccination," she added.

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Also, Yekaterina dismissed rumours about a certain "dangerous" level of antibodies when vaccination is not advised.

"Covid patients are being treated with donor plasma, boosting the number of protective antibodies to speed up recovery.  So, there has been no evidence supporting this theory," she said.

"The same applies to the theory of antibody-dependent enhancement (ADE) of the infection, according to which a virus can find its way into a cell quicker when it merges with antibodies. Again, the success of plasma therapy proves this is not possible for the time being," she said.