The novel coronavirus (2019-nCoV; official name: SARS-CoV-2), which belongs to the betacoronavirus genus, causes the virus infection known as COVID-19 (coronavirus disease 2019) [1-4]. The infection may manifest as a severe respiratory syndrome [2, 3], and is the cause of the global pandemic (WHO, 11 March 2020) with more than 250,000 fatalities worldwide (as of 12 May 2020).
SARS-CoV-2 is very easily transmitted by airborne droplet infection, mainly by the upper respiratory tract. The spike structure (S) on the surface of the virus shows a very high affinity to the human ACE2 (angiotensin-converting enzyme 2) receptor by which the virus can enter the cells. 
I) Early direct detection using PCR
For early diagnosis, even a short time after the infection, the polymerase chain reaction (PCR) method is used. The genetic material of the virus is directly detected from the obtained patient material (oral, nasal, throat swab) with this method. By detecting highly specific gene sequences of SARS-CoV-2, the PCR is very specific. False-positive results can thus be nearly excluded. However, in rare cases, depending on the type of sampling, storage and processing, false-negative results can occur.
II) Indirect detection of antibodies to obtain immune status
This test measures the immune reaction of infected persons. According to current knowledge, the antibodies that are formed against the N or S protein of SARS-CoV-2 can be detected already a few days after the first symptoms appear.
Significance of IgM detection
IgM antibodies can be detected in the blood approximately 3 to 5 days after the appearance of symptoms and therefore point towards an early stage of the infection. The probability that the infected person still carries the virus and is therefore contagious to other people is very high. This is especially significant as there are also courses of the infection free of the typical symptoms, and these carriers of the virus are currently not identified. In case of a positive IgM antibody test in particular, a PCR should be performed so that the carrier of the acute virus can be isolated if applicable.
Significance of IgG detection
IgG antibodies are formed with a slight delay and can be detected approx. 5 to 7 days after the symptoms first appeared . Even if they are measured relatively early in some patients, they increase only towards the end of the infection and are detectable in the blood for a longer period, despite the virus having been eliminated already.
IgG and IgA antibodies are formed at a similar point in time. However, IgG antibody concentrations are detectable for a longer period which is why IgG antibodies are preferred for serological monitoring studies. [8, 9]
It is assumed that there is at least a temporary immunity against SARS-CoV-2 following an infection, in line with what is known of similar viral infections. There is no scientific data on this as yet, however [10-14]. Immunity after past infection is thus the focus of currently running studies.