Many folks in the US who are aspiring to be technical writers have absolutely no technical or scientific skills - at least the ones I have been interviewing here in New England. I prefer to hire folks who actually have some technical background, such as in software development, project management, science, engineering, etc. Barring that, formal training in technical writing is preferred (as opposed to journalism). In my own experience as a senior tech writer and manager, it's very difficult for a journalism major to be technical enough for what I need (they need to be able to read and write code snippets, understand basic technical concepts and the software development lifecycle, and know how to gather information from their more technical team members, then convey that information appropriately). They also need to be able to learn complex software and technical concepts very quickly. Because of this I tend to lean toward people who have a programming background of some kind.
My personal background is first as a scientist, then as a programmer, then a combination proposal and technical writer. Haviing more technical jobs initially REALLY helped me to be a strong tech writer.