Researchers found no evidence the vaccine lotteries promoted by many states were effective and said more research is needed on alternative strategies to boost inoculation rates.
Rhode Island, Maine, Connecticut and Massachusetts, all among the top five most vaccinated states, are battling the worst coronavirus outbreaks in the country.
The U.K. reported the highest number of daily Covid-19 cases since the pandemic began on Wednesday and experts believe omicron could already be the dominant variant in London.
The mRNA shots were around 20% effective at preventing infection with delta in males over age 65 months five through seven after completing vaccination.