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Young and healthy people should be prepared to wait their turn for immunization, experts warned this week. The World Health Organization’s chief scientist suggested that the delay could last well over a year for some among the young and healthy.

“People tend to think, ah, on the first of January or the first of April, I’m going to get a vaccine and then things will be back to normal,” Soumya Swaminathan said in an online WHO question-and-answer session on Wednesday. “It’s not going to work like that."

“There will be a lot of guidance coming out, but I think an average person, a healthy, young person, might have to wait until 2022 to get a vaccine,” she said.

Young people can get sick and die of covid-19, the disease caused by the novel coronavirus, and can spread it. But evidence suggests they are less likely to suffer serious complications than older people or those with health problems.

With an unprecedented global demand for a vaccine, governments and international organizations such as the WHO will have to work to ensure that people most at risk get priority. Health-care workers and others on the front lines may go first, followed by the elderly or sick.

The remaining healthy, young people waiting for a return to normal life may end up at the back of the line.

Vaccines are going to be available in the initial years in too small quantities to vaccinate the seven billion people we have across the globe today,” Robin Nandy, the chief of immunization at UNICEF, said in an interview. “Vaccines will arrive in dribs and drabs.”

Those providing essential services including health care and education should be among the first vaccinated, he said. “We have to live with the pandemic for a while, so we need these systems to continue."

Children, often the target of mass immunization programs, may have to sit out the initial rounds of vaccines, in part because few vaccine candidates have been fully tested on people under 18.

Confusing matters further, some scientists have warned that early vaccines may be effective only half the time, which would mean that more doses are needed. The U.S. Food and Drug Administration has said a vaccine must be at least 50 percent effective to win approval in the United States, though it hopes for a higher efficiency.

More than 150 countries have joined the WHO’s covid-19 Vaccines Global Access Facility, or Covax, initiative, a cooperative arrangement in which nations combine forces to invest in vaccine development and agree to allocate the first doses based on population and then by level of risk.

The initiative has said its goal is to have 2 billion doses of safe vaccines to deliver by the end of 2021 — a goal that could mean one dose for roughly a quarter of the world.

Countries that are pursuing their own vaccines will also have to prioritize. In China, early doses of one vaccines have gone to soldiers in the People’s Liberation Army, which helped develop it.

Russia, which this week approved a second vaccine, has said that doctors and teachers may be the first to get vaccinated, though President Vladimir Putin said one of his adult daughters had participated in a preliminary trial.

The United States, which has not joined the Covax initiative, has instead pumped billions of dollars into its own vaccine development program dubbed Operation Warp Speed. So far, seven vaccine candidates are being pursued.

Projections show that the United States could have an estimated 700 million doses of a vaccine by spring. But even with record-breaking speed, it is likely that the average person may have to wait some time longer.

In an updated document released Tuesday, the U.S. Centers for Disease Control and Prevention said that with limited supplies, it may ask some groups of people to not get the vaccine at first. It said it was working on a way to “distribute these limited vaccines in a fair, ethical, and transparent way.”

In a framework released this month, the National Academies of Sciences, Engineering and Medicine said 10 million to 15 million doses of a vaccine may be available initially, which may cover only 3 to 5 percent of the U.S. population.

The NASEM guidance also offered a tiered approach for how a vaccine should be distributed, with health-care workers and first responders in first place and the elderly or vulnerable in second. Essential workers, including teachers and school staff, would be next, alongside people in homeless shelters and prisons.

Under the guidance, children and young adults would receive the vaccine in a fourth tier of immunizations, before it is passed on to the remaining population.

The CDC said that a covid-19 vaccine may not be recommended for children when it first becomes available in the United States, because the vaccine has not been tested on them.

“Only non-pregnant adults” have participated in early trials so far, the CDC said on its website. “However, clinical trials continue to expand those recruited to participate. The groups recommended to receive the vaccines could change in the future.”

Though the time scale for widespread vaccine use may be longer than many expect, other therapeutic drugs are likely to have helped lower the mortality rate, and improved social distancing practices can prevent infection.

Speaking on Wednesday, Swaminathan reiterated the WHO’s stance against allowing the virus to spread unchecked in an effort to reach herd immunity, emphasizing that the concept should be discussed only within the context of a vaccine.

Once we have a vaccine, we can aim to have population immunity — herd immunity — because you’ll need to vaccinate at least 70 percent of people, have them protected, to really break the transmission,” she said.

This report has been updated.