generally speaking, every time you conduct a study in which patients are involved, an ethical approval is needed. On the other hand, if your research is observational and/or retrospective, you shouldn't find ethical obstacles and therefore you should receive approval in a very short time. Just to be sure, it is preferable to insert an ethical disclosure in every paper that includes patients, specifying that you strictly observed the Helsinki Declaration.
Sometimes such study may be ethical but it's publication may raise some ethical dilemma in terms of stigma relating to such reporting.
However, it seems you plan to use secondary data like hospital records: In that case, I think you might simply seek for ethical guidance and waiver from IRB.
This was a original study utilizing government data. I think researcher must need to have proper approval to conduct study from appropriate authority. Otherwise the study will loose authentication.
Ethical approval should be taken for all studies involving human being whether it is hospital based or field based. It reflects our comittment to human welfare
Attention to ethical issues can facilitate the effective planning, implementation, and growth of a variety of public health programs and research activities.
Please find a review article by Steven S Coughlin
"Ethical issues in epidemiologic research and public health practice"
It says: "Progress in medical care and disease prevention depends upon an understanding not only of physiological and pathological processes but also of the social, cultural, economic, and other environmental determinants of health, including the effects of the healthcare system and other social institutions. Producing that understanding requires performing research involving human subjects. Such research should be carried out only by, or strictly supervised by, suitably qualifi ed and experienced investigators under accepted ethical guidelines."