– A top White House adviser starkly warned Trump administration officials in late January that the coronavirus crisis could cost the United States trillions of dollars and put millions of Americans at risk of illness or death.

The warning, written in a memo by Peter Navarro, President Donald Trump’s trade adviser, is the highest-level alert known to have circulated inside the West Wing as the administration was taking its first substantive steps to confront a crisis that had already consumed China’s leaders and would go on to upend life in Europe and the United States.

“The lack of immune protection or an existing cure or vaccine would leave Americans defenseless in the case of a full-blown coronavirus outbreak on U.S. soil,” Navarro’s memo said. “This lack of protection elevates the risk of the coronavirus evolving into a full-blown pandemic, imperiling the lives of millions of Americans.”

Dated Jan. 29, it came during a period when Trump was playing down the risks to the United States, and he would later go on to say that no one could have predicted such a devastating outcome.

Navarro said in the memo that the administration faced a choice about how aggressive to be in containing an outbreak, saying the human and economic costs would be relatively low if it turned out to be a problem along the lines of a seasonal flu.

But he went on to emphasize that the “risk of a worst-case pandemic scenario should not be overlooked” given the information coming from China.

In one worst-case scenario cited in the memo, more than a half-million Americans could die.

The memo was dated the same day that Trump named a White House task force to deal with the threat, and as the administration was weighing whether to bar some travelers from China, an option being pushed by Navarro.

Trump would approve the limits on travel from China the next day, though it would be weeks before he began taking more aggressive steps to head off spread of the virus.

Questions about Trump’s handling of crisis, especially in its early days when he suggested it was being used by Democrats to undercut his re-election prospects, are likely to define his presidency. Navarro’s memo is evidence that some in the upper ranks of the administration had at least considered the possibility of the outbreak turning into something far more serious than Trump was acknowledging publicly.

Neither Navarro nor White House representatives responded to requests for comment.

The memo, which was reviewed by the New York Times, was sent from Navarro to the National Security Council and then distributed to several officials across the administration, people familiar with the events said. It reached a number of top officials as well as aides to Mick Mulvaney, then the acting chief of staff, they said, but it was unclear whether Trump saw it.

In the memo, Navarro cautioned that it was “unlikely the introduction of the coronavirus into the U.S. population in significant numbers will mimic a ‘seasonal flu’ event with relatively low contagion and mortality rates.” He noted the history of pandemic flus and suggested the chances were elevated for one after the new pathogen had developed in China.

“This historical precedent alone should be sufficient to prove the need to take aggressive action to contain the outbreak,” he wrote, going on to say the early estimates of how easily the virus was spreading supported the possibility that the risks were even greater than the history of flu pandemics suggested.