Since the discovery of the vaccine by Edward Jenner, which led to the eradication of smallpox, populations have been subjected to vaccination in order to prevent diseases as common and relatively mild viral infections as measles and as severe as polio. Generally, vaccination programs in both developed and developing countries include vaccines for the same illnesses but differ when it comes to their chemical formulation. For instance, thimerosal, a mercury-containing preservative, is still used in vaccines offered in developing countries and it offers risks to the health of the population – a matter that has been discussed in the last two decades (Sykes et al., 2014). However, little has been done in regard to this different composition of vaccines in these two large groups of countries that have pronounced distinct socioeconomic status. In this essay, I discussed the reasons why this dissimilarity of vaccine formulation still continues to exist and I provide an ethical analysis using justice theories concerning this matter.Thimerosal is one of the most commonly used preservatives in vaccines since the start of their commercialization in the 1930s, when protection against microorganisms started to be taken seriously. This preservative has been almost completely removed from vaccines in the United States and the European Union, due to a reformulation of the vaccines and to safety studies made in humans. Nevertheless, in developing countries, vaccines provided to the population still contain thimerosal (Sykes et al., 2014), mainly because these countries utilize multi-dose vaccines that are more susceptible to bacterial or fungal contamination and therefore, require the use of preservatives, as opposed to individual doses used in developed countries. Single-dose vaccines are more expensive than multi-dose when it comes to both production and storage. Thus, low- and middle-income countries face a challenge to afford the best vaccines in the market.The effects and the impact of mercury over the health of millions of children in developing world has been shown in a series of papers published, which associate the application of the thimerosal-containing vaccine in infants with neurological problems, as tic and attention deficit hyperactivity disorders (Geier et al., 2017). Children are particularly vulnerable to these damages since they are still developing their neuronal and immune systems during the first years of their lives (Geier et al., 2015) when they receive a considerable amount of mercury through the vaccination programs usually established (Dórea, 2017). Despite it being clear that mercury is harmful to human health, even in the levels present in vaccines (Geier et al., 2015), governmental and health authorities seem to still ignore this issue. By integrating theories of justice into the ethics behind the different formulation of vaccines in countries with distinct developmental status, it is easier to determine what can be considered unfair and, as a result, think about ways to make this issue fairer and ethical.It is widely known that cumulative doses of mercury are health-damaging. Also, most cultures would usually agree that giving people no alternative apart from receiving a product that contains mercury in a concentration sufficient to cause health problems is unethical.When looking at this matter with a deontological approach, one could say that giving populations mercury-containing vaccine would not be considered wrong per se. Although many studies point it out as harmful, health organs – such as the World Health Organization – affirm there is no problem in using this vaccine. As long as these vaccines carrying thimerosal are not prohibited by international health organizations and authorities, deontology would say that there should be no problem if they are still being used in developing countries because people should have the right of getting vaccinated, hence this chemical formulation difference is irrelevant. On the other hand, if we adopt a utilitarian point of view, in which the greatest good for the greatest number of people should be achieved, one would claim that everyone should be vaccinated. The long-term end result for the two types of vaccines described is not the same, even though both provide immunization, one is seen as harmful to the individual’s health and the other one is not. Since utilitarianism focuses on the outcome rather than the action, this ethical theory would suggest that thimerosal-free vaccines should be given to all. However, again, would it be appropriate to prohibit the use of mercury-containing vaccines in developing countries where even the access to vaccines is difficult? Where any vaccine can be life-saving? This decision could certainly bring an even more uncertain fate to the well-being of people if banning the use of these vaccines when there is no other alternative. Perhaps one would weigh the probability of being affected by a disease against the malicious effects of mercury and choose the one which would generate the best outcome for most people.Another theory of justice that can be used to address this issue is egalitarianism, in which equality is sought. From an egalitarian point of view, each person should be able to get vaccines and should be treated equally despite their nationality, geographic location, economic status, race, age, gender or religion. If we intend to end or prevent diseases without causing any avoidable damage to health, countries administrations need to make sure that people in every nation have access to safe formulations of vaccines. That would imply however that public health worldwide would ideally achieve this equality in countries with diverse economies. In less developed countries, pharmaceutical companies own the power with not much of a market competition, and prices for mercury-free and single-dose vaccines thus remain high, making it impossible for these countries to acquire them. Therefore egalitarians would push for efforts to be done by first world countries authorities to help developing countries in acquiring thimerosal-free vaccines, by providing financial support or stimulating research and development in their industries, so to give them the access to cheaper mercury-free vaccines. That being so, it is important to take these matters into account when thinking about public health policies in low- and middle-income countries in order to end the differences in vaccine formulation and treat people equally as those from developed countries. Since vaccination is an important subject to be discussed as it implicates in dealing with millions of lives worldwide, especially children’s, care should be taken so to make a public health decision that is just to people with different access to health care.