Migration, both internal and international, is an extensive ongoing process and has become an international issue affecting every country in the world . Internal migration appears as a massive phenomenon and continues to increase sharply (Fig. 1), exceeding international migration . Globally, there are about 763 million internal migrants compared to 281 million international migrants .
According to the International Organization for Migration (IOM), international migrants are ‘individuals who have moved across an international border away from their place of residence, regardless of their legal status, the voluntary nature of movement and/or the causes for the movement’ whereas internal migrants are individuals who have changed their residence within the boundaries of their own country [3–5].
Internal migration movements can be temporary or permanent and include those who have been displaced from their habitual place of residence such as internally displaced persons, as well as persons who decide to move to a new place, such as in the case of rural–urban migration .
Rural–urban migration also referred to in this study as internal migration, in Sub-Saharan Africa countries, particularly Ghana and Nigeria, has been the main reason for the current intensive urbanization [6, 7]. The internal migration patterns (Fig. 2) in both countries have tended to be more rural-urban, thus, from the North to the South, since the second half of the 20th century [6, 8]. Statistical data from Ghana have shown that out of about 80% of internal migrants, 70% settle in the urban areas and cities . Like in Ghana, 60% of internal migrants live in the cities in Nigeria .
Notable among other factors, the move to urban areas are linked to better opportunities for work, education, better life standards, and also to escape turbulent political instabilities or environmental disasters [10, 11].
While this form of migration brings advantages to many , it also has negative an impact on health outcomes . Growing evidence has shown that, people who migrate from the rural areas to the urban cities in search of better opportunities, have increased vulnerability to ill health due to the difference between their health profiles, values and beliefs, and those in the host population . Generally, they perceive worse health than majority host populations [14, 15].
Couple with the high health risk individuals who migrate and move through new environments face , they go through unpleasant and difficult conditions towards accessing and utilizing healthcare services [17, 18]. As a result, these vulnerable migrant populations compromise their health by living unhealthy lifestyles and seeking alternative assistance for their ill health which expose them to poor health conditions .
There have been several recommendations and resolutions to ensure equal and easy access to health care particularly for migrant populations [20–22]. For instance, the United Nations set targets under the Sustainable Development Goal (SDGs) 3, that seek to promote good health and wellbeing for all populations . However, these set targets and resolutions on promoting good health and ensuring equitable access to health care services for all population remains elusive [13, 24, 25].
Growing evidence has shown huge health disparities in terms of access and utilization of healthcare services particularly among people who move from the rural areas to the urban areas [21, 22].
The aim of this study is to examine access and use of healthcare services among rural-urban migrants, who are also referred to in this study as internal migrants, in Ghana and Nigeria. These two countries are chosen for the study because, the two countries also share similar history in terms of policies and health system structures in addressing healthcare access problems .
Access to health care services is mostly regarded as one of the indicators of equity in health care provision  and central to health system’s performance worldwide [28, 29].
The study will be guided by dimensions to healthcare access developed by Levesque and Colleagues . According to Levesque and Colleagues (2013), access to health care services is categorized into broad dimensions such as geographical, economic or social aspects. In their view, access to health care is mainly an attribute of services, determined by the availability, price and quality of health resources, goods and services, and demand side which is determined by the ability pay and uses available healthcare services . The framework (annex 1) is comprised of five dimensions of accessibility of care (approachability, acceptability, availability and accommodation, affordability, appropriateness) and five corresponding abilities of patients and populations to access care (ability to perceive, ability to seek, ability to reach, ability to pay, ability to engage).
According to the access dimension by Levesque and Colleagues , availability is the opportunity to easily access health care when needed. It includes considerations concerning the organizational aspects of healthcare delivery, such as opening hours and appointment systems. It can also be measured using indicators such as the numbers of doctors or hospital beds per capita. Also, early contributors like Mooney  suggested from a health economic perspective that the availability of services may be measured in terms of the costs to individuals of obtaining care. Similarly to Gulliford  proposition, if services are available, then an opportunity exists to obtain medical care when the services are available but these may be limited by other barriers such as financial, organizational, social, cultural issues, and other socio-economic factors.
Affordability is determined by the individual’s ability to pay for services without financial hardship and mainly based on coverage decisions , and also by out-of-pocket (OOP) expenses . The services may be available but care seekers may not be able to pay for the healthcare services.
Acceptability is perceived as the willingness to seek healthcare services which is influenced by individual perceptions of the quality of healthcare services and various subjective considerations related to cultural expectations, religious orientations . In addition, Sekhon et al., Colleagues  in their study also proposed acceptability as “the extent to which people delivering or receiving a healthcare intervention consider it to be appropriate, based on anticipated or experienced cognitive and emotional responses to the intervention”.
Levesque elaborates more on acceptability by including approachability, ‘the ability to identify healthcare services that can be reached and the ability to perceive health needs, which is determined by factors such as health literacy or beliefs’ .
On the ability side, it is determined by the financial capabilities of patients and the ability to pay for out-of-pocket expenses for healthcare services determined by the range of health services and products covered by the compulsory or voluntary health insurance and the share of the cost covered by the healthcare system.