Means and methods for producing mammalian viruses, the method comprising infecting a culture of immortalized human cells with a virus, incubating the culture infected with virus to propagate the virus under conditions that permit growth of the virus, and to form a virus-containing medium, and removing the virus-containing medium. The viruses can be harvested and be used for the production of vaccines. Advantages include that human cells of the present invention can be cultured under defined serum-free conditions and the cells show improved capability for propagating virus. Methods are provided for producing, in cultured human cells, influenza virus and vaccines derived thereof. This method eliminates the necessity of using whole chicken embryos for the production of Influenza vaccines. The method also provides for the continuous or batch-wise removal of culture media. As such, the present invention allows the large-scale continuous production of viruses to a high titer.