“If the goal is to return life to some semblance of what it was like pre-pandemic, I don’t see that happening in 2021,” Vivek Murthy, a surgeon general under President Obama, said during a virtual event organized by cruise operator Carnival Corp. and the World Travel & Tourism Council.

More than 160 efforts are underway to develop vaccines for COVID-19, according to the World Health Organization, with about 25 being tested on humans in clinical trails. A handful of vaccines—including those developed by the University of Oxford with AstroZeneca, Moderna Therapeutics with the National Institutes of Health, and several Chinese entities—have advanced to the Phase III stage of being tested on thousands of patients.

But Murthy, who previously led U.S. responses to the Ebola and Zika outbreaks, cautioned that “even in an optimistic scenario, it would likely be closer to mid-2021 that we would see vaccines starting to be distributed at scale.”

That distribution will bring its own challenges, including the need to overcome Americans’ growing anti-vaccine sentiment of recent years and their new skepticism about the vaccines being developed for COVID-19. So even if a final vaccine is 100% effective—“which would be extremely rare,” Murthy pointed out—at least 70% of the population would need to be vaccinated in order to reach “herd immunity” against the coronavirus.

“Is there a chance we could do this all and vaccinate 70% of the population by the end of 2021? I think it would take the best vaccination campaign that we’ve ever assembled in the history of the world to do that,” Murthy said. “Could it be done? Well, there’s a first time for everything.”

It was one of the more bluntly grim timelines offered by public health experts in recent days, as U.S. infections of COVID-19 surge in the face of various city and state efforts to reopen. Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, last week acknowledged that there is “so far essentially no end in sight” to the pandemic and that it would never be fully eradicated.

But he was also slightly more optimistic than Murthy about the speed of a return to normal. “With a combination of good public health measures, a degree of global herd immunity, and a good vaccine, which I do hope and feel cautiously optimistic that we will get…we will get control of this,” Fauci told the TB Alliance during a virtual interview last week. “Whether it’s this year or next year, I’m not certain.”

Murthy’s prediction was probably not welcome news to the hosts of the Global Scientific Summit, where he spoke on Tuesday. The event was put together by the World Travel & Tourism Council (WTTC), a travel industry trade group, and Carnival, a company that’s desperately hoping that the cruise business somehow manages to resume normal operations before 2022.

CEO Arnold Donald said earlier this month that he is confident Carnival can survive “into late next year, even in a zero-revenue scenario.” But his company and its competitors are also mustering panels of public health experts to reassure passengers that it will be safe to board cruise ships long before then. (If, that is, governments allow it. Earlier this month, the U.S. Centers for Disease Control and Prevention extended its “no sail” order for cruise companies for a second time, barring them from resuming passenger operations in U.S. waters until at least Sept. 30.)

Other travel executives have warned that they expect the pandemic to continue hurting their industry until 2022, or longer. Earlier this month, Delta CEO Ed Bastian warned that “it could be two years or more before we see a sustainable recovery” in air travel.

Murthy and other speakers at Carnival’s virtual event, which was cohosted by Donald and WTTC CEO Gloria Guevara, did not spend much time addressing cruises specifically. Donald ended the program by saying that he had wanted to keep the event to “just science, and not make it promotional for cruise or travel,” but added that many of the scientists who spoke during the event were working with Carnival on its health and safety standards.

“Wear your mask, wash your hands, stay safe, get tested,” he concluded, “and the world will be a better place.”