After a decade of planning, postcards will start landing in mailboxes across the country Thursday inviting people to respond to the 2020 Census. But the sudden spread of coronavirus, which on Wednesday reached pandemic levels, has sent the Census Bureau scrambling to figure out how to conduct an accurate once-in-a-decade count of people who may be displaced, incapacitated or reluctant to interact face-to-face.

“I think it’s a fair assessment to say that this has now become the bureau’s most significant challenge for 2020,” said Arturo Vargas, chief executive of NALEO Educational Fund, a nonprofit that advocates for Latinos participating in the American political process.

“I am concerned that the bureau is going to have real problems convincing their workers to go out and knock on doors at a time when people are being advised to keep your distance, don’t talk to people,” he said. “Even though the bureau will say ‘We have plans for any scenario,’ I don’t think this was something anyone was expecting.”

Preparing for health emergencies is built into census contingency planning, but the timing of the coronavirus’s arrival in the United States is particularly bad for the 2020 count, which is scheduled to be completed this summer so the new data can be reported to the president by the end of the year. Census data is used to determine $1.5 trillion in federal funding, along with congressional representation and redistricting.

As the virus spreads, Vargas said directives to practice social distancing could clash with bureau plans such as sending employees out to concerts and street fairs to urge participation or conducting a three-day count of the nation’s homeless population scheduled for March 30 to April 1.

As colleges and universities suspend in-person learning, students may be on the move or back at their parents’ house at the precise time when they should be counted, setting up the possibility that they may be missed or double-counted. Students who spend most of the year living away from their families are supposed to be counted in the setting where they spend the school year.

And, if the epidemic lasts more than a month or two, it could affect hundreds of thousands of employees hired to go door-to-door to count households that haven’t responded online, by mail, or by phone.

Michael Cook, a bureau spokesman, said Wednesday that the census has assembled an internal task force to deal with coronavirus. “We are currently assessing the situation and will monitor the situation and take steps as necessary,” he said, adding that the bureau is following guidance from the Centers for Disease Control and the Department of Health and Human Services.

The bureau released a statement Wednesday afternoon saying that if it needs to delay or discontinue nonresponse follow-up visits in some places, it will make sure those areas are counted in another way.

The bureau is currently conducting fieldwork for some of its non-decennial surveys by phone in areas hit by the virus, and some meetings with partners around the country have shifted from in-person to phone and teleconference, the statement said.

The bureau has faced significant challenges in the run-up to the count, which takes place April 1. These include a lack of adequate funding earlier in the cycle, which caused it to significantly reduce testing for the survey and a protracted legal battle over the government’s unsuccessful attempt to add a citizenship question to the form.

The fact that for the first time a large portion of respondents are expected to fill out the survey online may help mitigate illness-related concerns about face-to-face contact. Cook said the bureau will emphasize that households that do so can avoid an in-person visit from an enumerator.

The bureau has a $2 billion contingency budget for unexpected issues, which Cook said could potentially be used for hiring additional workers, mailing out more fliers, or creating advertisements that specifically mention the coronavirus and remind the public they can respond remotely.

The bureau’s $500 million advertising campaign was finalized before people knew about coronavirus. Cook said no assessment has been made of how much such adjustments would cost.

Terri Ann Lowenthal, a former staff director of the House census oversight subcommittee, said contingency plans might also include extending the door-knocking period and expanding the bureau’s use of administrative records and statistical imputation to fill in information for households from which the census takers can’t collect answers directly.

But such adjustments aren’t ideal because they could affect the quality of the data, she said.

The bureau has met its goal of recruiting 2.67 million people for up to 500,000 temporary positions, but much of the recruiting was done before the coronavirus hit. Cook said he has not heard of any potential census takers expressing concern about going to households.

“If there’s guidance from public authorities that negatively affect our operations, we will adjust,” Cook said. “If we need to discontinue any particular part of our operations we will adapt our operations to ensure we get a complete and accurate count.”

At this point, the homeless count is still being planned, and universities and college students and their families are being asked to fill out the forms as if the students were still attending classes in person, Cook said, adding, “In the coming days as we monitor the situation and our plans adapt and change, we’ll be able to give more details.”