The chances of immunity against SARS-CoV-2 after recovery from COVID-19 are very high.
Current studies on immunity after past disease (as of May 27, 2020)
According to a study conducted at Mount Sinai Hospital in New York on more than 1,300 recovered COVID-19 patients, over 99% of them have produced IgG antibodies. The number of antibodies produced was independent of age, sex, and severity of the disease. The ELISA test used also detects antibodies against the spike protein, which is our test’s target antigen. (https://www.medrxiv.org/content/10.1101/2020.04.30.20085613v1).
With the help of this test, Mount Sinai Hospital also identified potential plasma donors, whose plasma can be used to treat severely ill COVID-19 patients. (https://www.medrxiv.org/content/10.1101/2020.05.20.20102236v1.full.pdf+html)
Another case study also reports the positive effects of using these convalescent plasmas. (https://jamanetwork.com/journals/jama/fullarticle/2763983)
In a recent study, published in the peer-reviewed journal Cell, 13 of 14 patients that recovered from COVID-19 showed antibodies against the spike protein of the novel coronavirus. The sera of these 13 patients showed anti-viral effects in so-called neutralization assays. This type of cellular assay is considered to be proof that the patients’ antibodies can neutralize the novel coronavirus. (https://www.cell.com/immunity/pdf/S1074-7613(20)30181-3.pdf)
Studies at the French Institut Pasteur came to comparable results. From 160 previously positively tested clinic employees, antibodies against the spike protein could be detected in 159 persons after an average of 24 days. Also in this study, the function of the antibodies was checked in neutralization tests and confirmed in 98% of the samples. (https://www.medrxiv.org/content/10.1101/2020.05.19.20101832v2.full.pdf)
First data of a clinical Phase I/II study for the development of a SARS-CoV-2 vaccine shows a clear correlation between the production of IgG antibodies against SARS-CoV-2, their neutralizing effect, and the protection against new infections in NHPs. (https://www.biorxiv.org/content/10.1101/2020.04.17.046375v1.full.pdf).
Early reports from South Korea regarding the recurrence of SARS-CoV-2 infection are currently investigated by the Korea Centers for Disease Control and Prevention.So far (as of May 27, 2020), no cultivable virus could be isolated from any of the 108 swabs that were again PCR-positive (after interim negative PCR tests). Only virus fragments were detected by PCR. Of the 790 people who were in close contact with the patients, none was infected with SARS-CoV-2. (https://www.cdc.go.kr/board/board.es?mid=a30402000000&bid=0030)
As of today ( May 27, 2020), there is no case known to us in which a secondary infection has been proven by several valid test systems.