Challenge question! Imagine the following situation. You receive a viral vector coronavirus vaccine that uses a specific adenovirus and your immune system mounts a strong immune response. At the same time, your friend receives an RNA coronavirus vaccine and also has a strong immune response. One year later a new coronavirus variant emerges and neither of the original vaccines protect people from this new variant.

A new viral vector vaccine is developed to the variant using the same adenovirus as before. In clinical trials, it is found that the new vaccine protects people that originally received the RNA coronavirus vaccine that your friend had but does not protect people that originally received the viral vector coronavirus vaccine that you had.

Question: Can you think of an explanation for why the new vaccine didn’t protect you, but did protect your friend?