BACKGROUND: SARS-CoV-2 IgG antibody measurements can be used to estimate the proportion of a population exposed or infected and may be informative about the risk of future infection . Previous estimates of the duration of antibody responses vary .

METHODS: We present 6 months of data from a longitudinal seroprevalence study of 3276 UK healthcare workers (HCWs). Serial measurements of SARS-CoV-2 anti-nucleocapsid and anti-spike IgG were obtained . Interval censored survival analysis was used to investigate the duration of detectable responses . Additionally, Bayesian mixed linear models were used to investigate anti-nucleocapsid waning .

RESULTS: Anti-spike IgG levels remained stably detected after a positive result, e.g., in 94% (95% credibility interval, CrI , 91-96 %) of HCWs at 180 days . Anti-nucleocapsid IgG levels rose to a peak at 24 (95% credibility interval, CrI 19-31) days post first PCR-positive test, before beginning to fall . Considering 452 anti-nucleocapsid seropositive HCWs over a median of 121 days from their maximum positive IgG titre, the mean estimated antibody half-life was 85 (95% CrI , 81-90) days . Higher maximum observed anti-nucleocapsid titres were associated with longer estimated antibody half-lives . Increasing age, Asian ethnicity and prior self-reported symptoms were independently associated with higher maximum anti-nucleocapsid levels and increasing age and a positive PCR test undertaken for symptoms with longer anti-nucleocapsid half-lives .

CONCLUSION: SARS-CoV-2 anti-nucleocapsid antibodies wane within months, and faster in younger adults and those without symptoms . However, anti-spike IgG remains stably detected . Ongoing longitudinal studies are required to track the long-term duration of antibody levels and their association with immunity to SARS-CoV-2 reinfection.