Middle-aged and elderly COVID-19 patients have far higher levels of anti-spike antibodies — which, again, increase infectivity — than younger patients.

Wu, F., Wang, A., Liu, M. et al. “Neutralizing Antibody Responses to SARS-CoV-2 in a COVID-19 Recovered Patient Cohort and Their Implications. MedRxiv, vol. 20 April 2020. https://doi.org/10.1101/2020.03.30.20047365.