9 September 2021. Vietnam’s Ministry of Health has allowed the use of the Pfizer-BioNTech COVID-19 vaccine as the second dose for people who already received the Moderna vaccine and vice versa due to a supply shortage.  https://tuoitrenews.vn/news/society/20210909/vietnams-health-ministry-approves-mixing-moderna-pfizerbiontech-vaccines-due-to-short-supply/63015.html