There are three different types of Influenza viruses: A, B, and C. Influenza A viruses and Influenza B viruses cause the annual influenza epidemics and irregularly occurring pandemics. The seasonal Influenza vaccine protects against both Influenza A and Influenza B.
Because Influenza C viruses also infects humans, but its effects are much less severe. Influenza C infections cause a mild respiratory illness, no fever and the virus was not thought to cause epidemics. That’s why the Influenza vaccine does not contain Influenza C. It is not a cause for concern and therefore the Influenza C virus is hardly the subject of scientific studies.
 of Influenza C. On a national scale, thousands of children were ill with acute respiratory symptoms.
Scientists in Finland then tried to understand how much impact the Influenza C virus might have and tested about 900 soldiers. Infection with influenza C virus was detected in 38 of 892 soldiers. The virus usually caused a mild upper respiratory tract infection. Most typical clinical features of influenza C virus infection were cough, rhinitis and hoarseness. Infections were usually mild but were severe in some cases.
Therefore, Influenza C virus can cause an epidemic, can lead to severe infections and it can mutate.
 Matzusaki et al: A nationwide epidemic of influenza C virus infection in Japan in 2004 in Journal of Clinical Microbiology - 2007
 Kauppila et al: Influenza C virus infection in military recruits-symptoms and clinical manifestation in Journal of Medical Virology - 2014