Young "vaccine chasers" with sleeping bags, warm hoodies and folding chairs are lining up for hours outside Los Angeles inoculation centers in the hope of obtaining coronavirus jabs from leftover vials that would otherwise end up in the trash.

Los Angeles county, where Covid-19 cases have surged this winter, is currently only vaccinating frontline medical workers and people aged 65 and over.

Even among those groups, appointments are extremely difficult to obtain, with Pfizer-BioNTech and Moderna shots in limited supply.

But a small number of patients fail to show up as scheduled each day, and this is exactly what the "vaccine chasers" are counting on.

"If this vaccine is just going to end up in the trash, then what good does that do to anyone?" asked Elaine Loh, who was outside a community health center in working-class south Los Angeles, where some have been waiting almost 18 hours.

"If we are young enough and healthy enough to spend the night to come and get it, we might as well."

Also waiting outside the Kedren Community Health Center, Adam -- who did not give his last name -- said he was happy to wait for hours because "it's not like anything is open right now, so I would be sleeping."

"I think anyone would feel the same thing. If they throw it out, why waste it, you know?" he added.

- Don't waste a drop -

Success is far from guaranteed, and the county's health authority has not formally sanctioned giving leftover Covid vaccines to people who do not meet the current criteria.

"Officially there is no wait list, there is no queue, there is no standby," said Jerry Abraham, a doctor who oversees the Kedren center vaccination program for a non-profit nongovernmental organization.

But "once we open it, we have to use it all within six hours," he told AFP.

"There are times when the appointment line is down and we have expiring doses of vaccine. And I refuse to let a drop go to waste. Nothing.

"No vaccine will ever be discarded if it's given to us here at Kedren."

At times when doses risk going unused because patients have not arrived or the day is ending, quickly searching for candidates to receive them poses a difficult challenge -- making the informal standby lines a blessing.

Still, only one to three percent of those with appointments fail to show, and it can be "heartbreaking" to watch hopeful young people arriving at 2:00 am and leaving hours later without a shot, Abraham said.

"I can't guarantee them anything."