Technically it's easy, Moderna already has multi target vaccines (for other diseases), one with 7 different targets. It can be as easy as literally creating 7 different vaccine liquids and then mixing them all in a single vial.
The problem is deciding what to put in, what ratio, possible side effects, etc... Lots of combinations which can mean lots of expensive and slow clinical trials.
Technically it's easy, Moderna already has multi target vaccines (for other diseases), one with 7 different targets. It can be as easy as literally creating 7 different vaccine liquids and then mixing them all in a single vial.
The problem is deciding what to put in, what ratio, possible side effects, etc... Lots of combinations which can mean lots of expensive and slow clinical trials.