As governors across the U.S. consider when exactly to reopen state economies amid the coronavirus outbreak, a former White House health official warns that large gatherings likely could not safely resume until at least the fall of 2021.

Dr. Ezekiel Emanuel, who served as special advisor for health policy to the director of the White House Office of Management and Budget during the Obama administration, told a recent New York Times panel that “restarting the economy has to be done in stages... Larger gatherings — conferences, concerts, sporting events — ... I think those things will be the last to return. Realistically we’re talking fall 2021 at the earliest.”

Dr. Emanuel, who is currently Vice Provost for Global Initiatives and chair of the Department of Medical Ethics and Health Policy at the University of Pennsylvania, added that the overall experience is “going to be this roller coaster, up and down. The question is: When it goes up, can we do better testing and contact tracing so that we can focus on particular people and isolate them and not have to reimpose shelter-in-place for everyone as we did before?”

View photos A crowd at the Coachella Valley Music And Arts Festival on April 15, 2017, in Indio, California. (Photo: VALERIE MACON/AFP via Getty Images) More

‘Too soon to rest on our laurels’

Emanuel, who has also cast doubts on the effectiveness of the use of an anti-malarial drug against the coronavirus due to the lack of sufficient data, is not alone in advising caution.

Dr. Tom Tsai, an assistant professor at the Harvard Global Health Institute, also stressed the need to think about other safeguards in place before reopening the economy.

[See also: Ex-Obama adviser Dr. Emanuel explains what it will take for America to return to ‘full normalcy’]

“A lot of these early signs are encouraging … but it’s still too soon to rest on our laurels,” Tsai said on Yahoo Finance’s The Ticker. “The data from New York showing that yes, we may be at the peak, but it’s not a rapid fall off the peak back to normal.”

View photos Coronavirus cases are still on the rise. (David Foster/Yahoo Finance) More

Tsai advised testing and social distancing to continue until there is “definitive data” that the pandemic is over.

“There are going to be multiple recurrences that occur throughout the summer,” Tsai said, adding that “we still have to keep our eye on the ball and be vigilant as those cases resurface because the data from the other countries have shown that once we lift these quarantine measures, then the chances of community transmission increase.”

According to a separate study by Harvard — unaffiliated with Tsai — coronavirus cases could surge again during the winter. To prevent that, the authors wrote, “prolonged or intermittent social distancing may be necessary into 2022” unless “critical care capacity is increased substantially or a treatment or vaccine becomes available.”

View photos People try to keep a social distance while they enjoy a sunny day at Central Park, as the outbreak of coronavirus disease (COVID-19) continues, in the Manhattan borough of New York City, New York, U.S., April 6, 2020. REUTERS/Eduardo Munoz More