Covid Q&A: Is It OK to mix different vaccines?

In hopes of making this very confusing time just a little less so, each week Bloomberg Prognosis is picking one question sent in by readers and putting it to an expert in the field. This week's question comes to us from Thomas in Havana, Cuba. Thomas is a senior in good health living in a nation that has developed its own vaccine. He asks: 

I can get a vaccine here in Cuba, but it doesn't use messenger RNA technology like Pfizer and Moderna. I would like to get that shot and then get one from Pfizer or Moderna later. Is there any risk?