It's "likely" people will need a third dose of Pfizer's COVID-19 vaccine, according to the CEO of the pharmaceutical company.
In a panel discussion that aired on Thursday (local time), Albert Bourla said it was likely a booster shot would need to be administered six to 12 months after the first two.
He also said it's possible that people will need to be immunised against COVID-19 annually, like the flu jab.
"There are vaccines that are like polio that one dose is enough… and there are vaccines like flu that you need every year," Bourla said.
"The COVID virus looks more like the influenza virus than the polio virus."
Research shows the Pfizer vaccine is more than 90 percent effective at preventing COVID-19 six months after receiving the second dose - but more data is needed to see if immunisation extends beyond that.
Pfizer and its German partner BioNTech began studying a third dose of their vaccine in late February.
The booster shot is aimed at protecting against future variants, which may be better at evading antibodies from the vaccine than earlier strains.
About 144 volunteers will be given the third dose, mostly those who participated in the vaccine's early-stage testing in the US last year.
Earlier on Thursday (local time), Dr David Kessler, the Biden administration's chief science officer of the COVID response, said Americans should expect to receive booster shots - especially as variants continue to spread.
"We are studying the durability of the antibody response," he said during testimony before the House Select Subcommittee on the COVID-19 Response.
"It seems strong, but there is some waning of that... I think for planning purposes, planning purposes only, I think we should expect that we may have to boost."
