Various systems exist for the robust production of recombinant proteins. However, only a few systems are optimal for human vaccine protein production. Plant-based transient protein expression systems offer an advantageous alternative to costly mammalian cell culture-based systems and can perform posttranslational modifications due to the presence of an endomembrane system that is largely similar to that of the animal cell. Technological advances in expression vectors for transient expression in the last two decades have produced new plant expression systems with the flexibility and speed that cannot be matched by those based on mammalian or insect cell culture. The rapid and high-level protein production capability of transient expression systems makes them the optimal system to quickly and versatilely develop and produce vaccines against viruses such as 2019-nCoV that have sudden and unpredictable outbreaks. Here, expression of antiviral subunit vaccines in Nicotiana benthamiana plants via transient expression is demonstrated.