For a study, the first step determined the degree of protection after infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was to determine the duration of immunity. Such knowledge was scarce, particularly among the general public. Up to 7 months after the onset of symptoms, the researchers looked at changes in immunoglobulin isotype seropositivity and immunoglobulin G (IgG) binding strength of SARS-CoV-2–specific serum antibodies in a countrywide population. Participants were chosen from prospective representative serological research in the Netherlands based on IgG seroconversion to the SARS-CoV-2 spike S1 protein (N=353) and up to three consecutive serum samples per seroconverted participant (N=738). The concentrations of immunoglobulin M (IgM), immunoglobulin A (IgA), and IgG antibodies to S1, as well as the rise in IgG avidity, were measured in proportion to the period since the onset of clinical symptoms. While SARS-CoV-2–specific IgM and IgA antibodies dropped rapidly after the first month, specific IgG antibodies were still present in 92% (95% CI, 89% –95%) of the subjects after 7 months. The time it took for IgG antibodies to decline by twofold was 158 days (95% CI 136–189 days). When compared to asymptomatic people or people with very minor upper respiratory complaints, people reporting substantial symptoms were able to maintain higher concentrations for longer. Similarly, sick people’s avidity of IgG antibodies increased more rapidly over time than people with mild or no symptoms (P=.022). IgG antibodies specific for SARS-CoV-2 remain and grow in avidity over time, indicating immunological maturation. These findings indicate the formation of immunological memory against SARS-CoV-2, providing insight into population protection for those who have not been vaccinated.