More than 10 months into his presidency and 20 months into the pandemic, Joe Biden finds himself in a difficult position. He’s trying to lead the “battle against COVID-19” in the age of politics.

It’s not going as well as the administration had hoped or projected. Average daily deaths from COVID-19 crossed the 2,200 mark in September, the highest total since early spring. More than 60 million eligible Americans are still unvaccinated. Meanwhile, elected officials and government agencies remain locked in partisan disputes over practically every aspect of the pandemic response.

These are deflating facts given the months of masking and social distancing, the travel bans, the shutdown orders, the dozens of bills batted around Congress, the disparate undertakings of two polar-opposite presidents and the surge of vaccine mandates that followed FDA approval.

Despite all we’ve tried, the pandemic is more than 600 days old and still very far from over.

The future remains uncertain, fueling the nation’s anxiety and unhappiness. Though the U.S. death count has receded in recent weeks, public health experts can’t be sure what the winter (and upcoming holiday gatherings) will bring.

Amid the confusion and fear and stubborn partisanship, President Biden faces difficult questions about how to lead a divided nation in a time of continued crisis.

For answers on where to go from here, let’s start with three facts that scientists know for sure.

First, scientists know that herd immunity against COVID-19 will occur only when a sufficient portion of the population becomes immune to the disease. And they know there are only two ways for immunity to occur: vaccination or infection.

Second, scientists know that the threshold for herd immunity has increased significantly throughout the pandemic due to the now ubiquitous and highly transmissible delta variant. As such, health experts estimate that 90% of the U.S. population will need to either be vaccinated or infected to reach that threshold.

Third, scientists know that unvaccinated people are at significantly heightened risk of dying compared to individuals who’ve been vaccinated. CDC data show that unvaccinated people are 6.1 times more likely to test positive for COVID-19 than fully vaccinated people and 11.3 times more likely to die from it.

In light of these facts, the White House’s mission is crystal clear: vaccinate, vaccinate, vaccinate.

The challenge now is expediting herd immunity through vaccination without letting politics further undermine the effort.

While private enterprises were stepping up, the Biden administration was tiptoeing and taking politically measured half steps. At first, Biden publicly encouraged American industry to impose mandates on workers but refused, for months, to weigh in on the possibility of a federal mandate. Only when the courts began upholding mandates and polls showed majority support for vaccine requirements, did the Biden administration take notice and act.

In September, the president announced that all businesses with more than 100 employees must require workers to be fully vaccinated or get tested for the virus weekly. It wasn’t until November that Biden finalized the details of the vaccine mandate and set a firm date for implementation: Jan. 4, 2022.

The role of the commander in chief extends far beyond the presidential powers bestowed upon him. Our nation’s greatest presidents have provided moral leadership in times of crisis, guiding citizens with clarity and honesty. Biden has an opportunity to stand up for science and rebuild the nation’s trust, so we can move forward together. Now is the time for that leadership.

Robert Pearl is a clinical professor of plastic surgery at the Stanford University School of Medicine and is on the faculty of the Stanford Graduate School of Business. He wrote this for The Fulcrum.