The virus that causes mumps in humans is composed of a protein outer shell containing a core of DNA. The virus that causes the flu in humans is composed of a protein outer shell containing a core of RNA. Vaccines that are effective against mumps work efficiently year after year. Vaccines that work against the flu must be revised and changed every year. What accounts for the difference in the effectiveness of these two vaccines?

Flu vaccines need to be revised and changed every year because the flu virus constantly mutates. Flu viruses can change in two ways: drift or shift. Antigenic drift occurs when genes of the virus slowly change during replication until the immune system no longer recognizes it. On the other hand, antigenic shift occurs when a major change in the virus happens. This usually leads to a totally different kind of virus. This is what happened during the H1N1 scare in 2009.

DNA viruses like mumps are less prone to mutation due to the presence of DNA polymerase. RNA viruses are more prone to mutation because they undergo reverse transcription to simulate the role of DNA.