The US state of Washington is offering "joints for jabs," in the latest innovative bid to get more Americans vaccinated against Covid-19 as inoculation rates continue to slow.

Cannabis dispensaries in the northwestern state -- where recreational marijuana sales were legalized in 2012 -- will be allowed to offer one free, pre-rolled joint to anyone aged 21 or above who gets vaccinated at an in-store clinic.

The move follows a decision last month to allow bars and other liquor licensees to provide one free alcoholic drink to adults in Washington state who receive their jabs within a six-week "window."

Just over half of Washington state adults (54 percent) have received at least one dose of a Covid-19 vaccine so far, according to recent health department figures.

But the pace of inoculations has slowed down across much of the country in recent weeks, leading to a flurry of measures to encourage more uptake.

Several states including California and Ohio have operated vaccine "lotteries" offering cash prizes or college scholarships to randomly selected vaccine recipients.

Other inducements have included free sports tickets, airline flights and beers, while an Arizona cannabis dispensary has already experimented with giving free products to those receiving jabs.

Washington state's temporary "joints for jabs" exemption -- which was requested by cannabis retailers, with participation optional for each vendor -- will run until July 12.

President Joe Biden has set a target of getting at least one vaccine shot into the arms of 70 percent of American adults by July 4 -- the nation's Independence Day.

According to the latest Centers for Disease Control and Prevention data, the current figure stands at 63.7 percent.