When the Covid-19 pandemic was just beginning last March, among the first steps the federal government took was to require that small and medium-sized companies give their employees two weeks of paid emergency sick leave for reasons related to the coronavirus. Firms with fewer than 500 employees were offered a tax credit to be reimbursed for the expense, dollar for dollar.

But that requirement was allowed to expire on Dec. 31, 2020, even as hospitalizations and deaths from the disease reached new highs. The last-minute Coronavirus Response and Relief Supplemental Appropriations Act of 2021, signed into law on Dec. 27, instead made offering paid sick leave voluntary on the part of employers. As a result, millions of primarily low-income employees in service-sector jobs such as retail, child care and food preparation are unable to take paid days off if they have symptoms, test positive or need to quarantine after exposure to Covid-19.

The US Centers for Disease Control and Prevention strongly recommends that anyone with any symptoms stay home — and workplaces around the country caution employees to do the same. So was Congress not convinced that paid sick leave could make a difference in containing the pandemic?

In fact, it can. We analyzed this question by investigating what occurred after cities and states mandated paid sick leave in years past, and after the Families First Coronavirus Response Act passed in the spring. To each data set we conducted what’s called “causal inference” — a state-of-the art statistical method that triangulates evidence from multiple sources — to test what effect giving workers access to paid sick leave has on the spread of contagious diseases.

The findings from our three studies are very clear, but they shouldn’t be surprising.

First, employees without paid sick leave are generally those in low-wage jobs — and thus the least able to afford to lose a paycheck. Others are afraid of getting laid off, particularly in precarious economic times like now. When such workers gain access to sick leave coverage, we find that they become more likely to call in sick and stay home when they have symptoms of illness.

Second, paid sick leave makes workers less likely to spread viruses among coworkers, customers and even fellow commuters on trains and buses. Using seasonal flu data from the CDC, we find that in the first year after a state-mandated paid sick leave policy, rates of influenza-like illnesses decreased by a significant 11 percent. Likewise, using daily coronavirus test result data from every US state, we find that the FFCRA’s emergency sick leave provisions reduced the number of new cases by about 400 per day per state in its first weeks after enactment.

But apparently this overwhelming evidence was not enough for Congress to renew paid sick leave.

To be sure, some will always see any type of paid leave mandate as burdensome regulation. But sick days prevent outbreaks from spreading within a workplace. Further, research we did before the pandemic struck showed no evidence that sick pay mandates hurt job growth or wages.

The incoming Biden administration should make the renewal of paid sick leave a top priority and make larger firms eligible for reimbursement tax credits. The Congressional Budget Office estimated that its paid sick leave tax credit would cost $8.7 billion in 2020; but as of October, the IRS had processed only about $300 million in employer tax credits. In the meantime, the federal government has spent trillions of dollars on economic relief packages for consumers and companies in response to the coronavirus. Providing taxpayer funding for paid sick leave is not only effective at preventing infections, it is very cheap in comparison.

An earlier version of this article appeared in The Hill.

This article is part of Reset: The Science of Crisis & Recovery, an ongoing series exploring how the world is navigating the coronavirus pandemic, its consequences and the way forward. Reset is supported by a grant from the Alfred P. Sloan Foundation.