If an influenza pandemic were to hit us tomorrow, who would need the most help? Obvious answers include children, the elderly, and people who are already ill. We expect them to be at higher risk than healthy adults—but that isn’t always the case.

What about the poor? Depending on who you ask, intuitions vary: some people assume influenza does not discriminate by social class and that everyone is at risk. Others might guess that conditions that go hand-in-hand with poverty (like poor access to healthcare or crowded living quarters) create a higher level of risk. But everyone is guessing, because evidence on this question has been surprisingly difficult to pin down.

While many studies have analyzed the risks of flu on a country or county level, the city-level is where we’d really be able to compare strong gradients in wealth to risks from the pandemic. That’s what a recent paper in Proceedings of the National Academy of Sciences of the United States of America (PNAS) does: it compares census data dating from the 1918 flu pandemic in Chicago to health records from the time. “We had this great data,” says Madhura Rane, one of the authors of the paper, “and we thought it would be interesting to see this association on a small spatial scale.” She and her fellow authors found evidence that poverty made a difference in that pandemic.

One difficulty the researchers faced was that the census data did not include Chicagoans’ degree of wealth or poverty. The closest datapoints were literacy levels, homeownership, and unemployment, so the authors used these as proxies. Their analysis showed that all of these factors made a difference: the risk of dying from influenza increased with higher levels of illiteracy, unemployment, and population density. Homeownership was associated with a reduction in risk.

Nick Wilson is a public health researcher at the University of Otago who has looked for similar associations between poverty and the 1918 pandemic, but never found any. He found his own results surprising “given that nearly all infectious diseases show such gradients,” he told Ars. According to him, the Chicago research is “the most scientifically rigorous study to date on socioeconomic gradients and the 1918 pandemic.”

Why have previous studies not found the same result? “There are limits with data quality from 1918 for some studies,” according to Wilson. And in some settings, social disparities may have been smaller. For instance, in New Zealand in 1918, the social class gradient might not have been that steep, “so that it might need a very large study with fine-grained data to detect.”

Rane hopes that the evidence might help to inform policymakers and public health officials. With the swine flu pandemic in 2009, she says, health departments faced the problem of not knowing who should be first in line for the vaccine.

Wilson thinks it’s reasonable to conclude that both future research and public health responses should focus on social disparities. Vaccines are always in short supply in a crisis, and priority groups for vaccination are currently health workers, first responders, people with pre-existing conditions, and high-risk age groups “with such groups being prioritized ahead of any groups defined by social class,” he says. But we’d certainly need more evidence before findings like these work their way into policy.

Right now, all we know about is Chicago in 1918. A clearer picture will emerge as researchers look not only at other cities, but also other pandemics. Finding the data for this is partly a matter of luck. “It’s not easy to do because you need to have the information available to you,” says Rane, “but when it is [available], I think it’s worth looking.” What drives the difference is also not clear: the recent PNAS paper can tell us about the correlations, but it can’t pin down what exactly about being poor made people more likely to die.

Still, studying 1918 is one of the best ways to equip ourselves to cope with future disaster. Influenza epidemics will keep coming at us, says Wilson, “typically several times a century.” We need to understand how they worked in the past to prepare properly for the next one.

PNAS, 2016. DOI: 10.1073/pnas.1612838113 (About DOIs).