The spread of SARS-CoV-2 poses one of the greatest challenges to humankind in recent history. According to current estimates (as of July 23, 2020) from Johns Hopkins University [1], there are over 15 million known infections and about 624,000 deaths worldwide in connection with the COVID-19 pandemic.

In the beginning, most COVID-19 infections occurred in countries of the Global North (eg, the United States, Spain, Italy, etc); however, the focus of the pandemic is now shifting toward countries of the Global South (eg, Brazil, India, South Africa, Peru, Chile, and Pakistan). Although only 5% of all confirmed cases (approximately 769,000 as of July 23, 2020) come from African countries, it can be assumed that the number of cases in this region will continue to increase with potentially serious consequences due to limited medical resources [2]. Many African as well as some South Asian countries have the lowest income in the world, with a large proportion of the population living in precarious housing situations often referred to as “slums,” “informal settlements,” or “deprived areas.” According to United Nations estimates, about 1 billion people worldwide currently live in slums [3].

The socioeconomic situation of a person or a group may influence the course of COVID-19. Previous analyses in the United States have shown that socioeconomically disadvantaged groups are at more risk, since they are more frequently affected by the comorbidities that lead to a severe course of COVID-19 compared to the rest of the population [4]. These risk factors are cardiovascular diseases, high blood pressure, diabetes, as well as previous diseases or damage to the lungs [5]. For example, relative to the total population, Black individuals are more likely to experience a severe course of COVID-19 than their White counterparts [6] (the terminology used here—“Black” and “White”—corresponds to that used by Garg et al [6]). Similar distributions of serious cases have been reported in other countries, such as the United Kingdom [7].

Since many studies have shown that slum dwellers are socioeconomically disadvantaged (eg, [8,9]), we need to assess what we know about this group of people in order to determine the risk posed by the virus. We must also examine what we do not know and what we should know.

There is much to be said about the fact that the COVID-19 threat is particularly prevalent in low-income countries, particularly the poorer parts of the population; inhabitants of slums could be especially impacted by the pandemic [10]. There are several reasons for this.

Firstly, social distancing, which is currently being implemented on a large scale across the world, is a physical impossibility in slums due to the high density of buildings and persons per dwelling [11,12]. Furthermore, we know that residents of low- and middle-income countries have increased risks for respiratory infections due to elevated levels of air pollution [13].

As Dahab et al [10] pointed out, if the pandemic enters the slums, their occupants could be much more threatened by severe disease outcomes due to the higher transmissibility of the disease, higher infection-to-case ratios, and higher case fatality. They also demonstrate which measures should be taken, which are realistic (eg, shielding at different levels like households, streets, or blocks), and which are not (eg, tracking of patients) to protect underprivileged and underserved areas [10].

Corburn et al [14] discussed how slums and informal settlements are poorly prepared to manage the pandemic and offered suggestions to minimize the risk of the virus. This is not only a matter of treating the inhabitants of informal settlements on a level equal to the rest of the population but also involves providing them with special support in order to adequately counter the risks associated with their living conditions [15].

In addition to the studies mentioned above, the pandemic highlights very clearly how limited our knowledge of the living conditions of slums is. We outline below three areas where information is lacking.

The pandemic unequivocally underlines that we know too little about this vulnerable part of the world’s population, their living conditions, state of health, and thus their inclusion in COVID-19 risk groups. In order to initiate appropriate countermeasures to contain the pandemic, adequate information is necessary.

As mentioned above, researchers have already developed proposals to prevent or contain the spread of COVID-19 in slums in a very concrete way [10,14]. Beyond this, however, we think that the following long-term measures are necessary:

Although these measures are of a longer-term nature and cannot be achieved during the pandemic, they do indicate a way of making visible again the part of the world population that is currently invisible.