(Bloomberg) -- Financial incentives and other nudges by local governments and employers have failed to increase Covid-19 vaccinations among Americans who are hesitant about getting the shot, a new study shows. 

What’s more, financial incentives and “negative messages” actually decreased vaccination rates among some groups, underscoring fears about a public backlash, according to the paper circulated by the National Bureau of Economic Research.

The pace of Covid-19 vaccinations climbed rapidly earlier this year as availability increased, with millions of adults getting the jab each day. However, that pace has slowed sharply. In the last week in the U.S., an average of about 800,000 doses per day were administered. 

States and cities including California and Chicago have offered gift cards to encourage people to get the jab, while employers have offered a range of incentives including cash bonuses. Several states have run lotteries for people who get vaccinated, with prizes of $1 million or more. The NBER paper, based on surveys in Contra Costa County in California, found that public health messages did have a positive impact on vaccination intentions but no real impact on actual vaccination rates. 

“Reaching a goal of very high vaccination rates likely requires much stronger policy levers, such as employer rules or government mandates,” wrote the authors, who include Tom Chang and Mireille Jacobson of the University of Southern California. 

Read More: Vaccine Lotteries Didn’t Move the Needle on Covid Inoculations

©2021 Bloomberg L.P.