A sudden increase in severe influenza has been registered in the Czech Republic since the end of 2012, with 264 cases requiring intensive care, including 51 deaths. Most patients had at least one risk factor. Severe influenza in patients with obesity, smoking and/or haematological disorders including haematological cancers was more frequent than in the pre-pandemic period. The seasonal influenza vaccination status of the cases indicates indirect efficiency of the current vaccine in preventing severe influenza.