The Chinese researchers followed 37 covids infected - from a group of a total of 178 people - with no apparent symptoms.

The researchers could see that the asymptomatic participants had lower antibody levels during the acute phase of the disease than the group with covid symptoms.

Eight weeks after leaving the hospital, levels began to fall in just over 90 percent of asymptomatic patients.

Hans-Gustaf Ljunggren does not think the result is entirely unexpected.

- Probably there is a relatively large group infected without developing more severe disease. It is likely that many of these will not develop tangible antibody responses in blood, or they will develop antibody responses that become more short-lived, he says.  

What does this mean for those who think they have been ill in covid-19 but have mild symptoms and receive a negative antibody test?

- Some of these may have produced antibodies locally, but they have not reached the blood, or they have reached the blood but the response sounded off as a result of a weak reaction.

A positive response can also lead to reflection.

- We don't know how long these individuals will keep their antibodies. In mild covid-19, they may fade or disappear within a few months. The longer we have antibodies the better, at the same time it is important that we gain more knowledge about antibody reactions at covid-19.

Hans-Gustaf Ljunggren, together with colleagues at Karolinska Institutet and Karolinska University Hospital, recently conducted a study on covid-19 and T-cells. The study has not yet been published in a peer reviewed journal, but is available to other researchers.

- In a limited material, we found that many of the patients with asymptomatic or mild disease develop a T-cell immunity, even among some who did not develop an antibody response. We do not know how strong this T-cell immunity is or how long it remains. What we know from previous coronavirus infections is that T cells appear to provide significant protection against the infection for a long time.

After the SARS epidemic in 2003, many patients had antibody responses up to two or three years after the infection, according to Hans-Gustaf Ljunggren. In many, T-cell immunity remains.

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