Antibodies to the new coronavirus may last only two to three months, especially in people who never showed symptoms.
The strength and duration of immunity after infection are key issues for ‘shield immunity’18 and for informing decisions on how and when to ease physical distancing restrictions19,20. Previous studies have shown that circulating antibodies against SARS-CoV or MERS-CoV last for at least 1 year21,22. Sustained IgG levels were maintained for more than 2 years after SARS-CoV infection23,24. Antibody responses in individuals with laboratory-confirmed MERS-CoV infection lasted for at least 34 months after the outbreak25. Recently, several studies characterizing adaptive immune responses to SARS-CoV-2 infection have reported that most COVID-19 convalescent individuals have detectable neutralizing antibodies, which correlate with the numbers of virus-specific T cells26,27,28,29. In this study, we observed that IgG levels and neutralizing antibodies in a high proportion of individuals who recovered from SARS-CoV-2 infection start to decrease within 2–3 months after infection. In another analysis of the dynamics of neutralizing antibody titers in eight convalescent patients with COVID-19, four patients showed decreased neutralizing antibodies approximately 6–7 weeks after illness onset30. One mathematical model also suggests a short duration of immunity after SARS-CoV-2 infection31. Together, these data might indicate the risks of using COVID-19 ‘immunity passports’ and support the prolongation of public health interventions, including social distancing, hygiene, isolation of high-risk groups and widespread testing. Additional longitudinal serological studies profiling more symptomatic and asymptomatic individuals are urgently needed to determine the duration of antibody-mediated immunity. In addition, low levels of anti-viral IgG in asymptomatic patients, who might be more likely to become seronegative, further support the need for timely serosurvey to study the true infection rate.
Long, Q., Tang, X., Shi, Q. et al. "Clinical and immunological assessment of asymptomatic SARS-CoV-2 infections." Nature Medicine
One thing to keep in mind is that you don't necessarily want antibodies active in your bloodstream for years to come, as it could lead to autoimmune disorders. Rather, you probably want your memory cells to be able to respond effectively to re-infection. This study only looked at the presence of antibodies, as studying the preparedness of memory cells is a lot more difficult (I think you'd pretty much need to have subjects who had been infected before and who have since recovered, and to see what happens to them upon reinfection, and that would be hard to monitor unless you infected them yourself as part of the experiment).