Ostadgavahi, Ali Toloue, et al. “Heterologous Immunization With Covishield and Pfizer Vaccines Against SARS-CoV-2 Elicits a Robust Humoral Immune Response”. The Journal of Infection in Developing Countries, vol. 15, no. 05, May 2021, pp. 653-6, doi:10.3855/jidc.15368.