Some bosses are hoping that cold, hard cash may convince holdouts to get a COVID-19 vaccine.

On Wednesday, Vanguard became the latest American employer to offer a $1,000 bonus to employees who get vaccinated.

“We are offering a vaccine incentive for crew who provide COVID-19 vaccination proof. The incentive recognizes crew who have taken the time to protect themselves, each other, and our communities by being vaccinated,” said spokesman Charles Kurtz. Employees will have until Oct. 1 to get vaccinated.

The Malvern-based investment giant follows companies such as Kroger, Walmart and JBS and cities in New York, Louisiana and California, which offered bonuses, prizes and even scholarships to vaccine recipients.

And there’s research to back up using economics to nudge behavior.

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Wharton and Penn’s Behavior Change for Good (BCFG) initiative this year released findings from two research studies aimed at increasing vaccine adoption.

In one study with Walmart, Wharton and Penn tested which reminder messages encouraged Walmart’s 700,000 pharmacy patients to come in and get a flu vaccine.

The researchers found that simple text message reminders that a flu shot was “waiting” or “reserved” boosted vaccine rates by up to 11%. The “waiting for you” messages were sent to patients 72 hours and 24 hours before their vaccine appointment.

In another study with two health systems (Penn Medicine and Geisinger), Wharton researchers also found text messages that a flu shot was “waiting” or “reserved” proved effective.

The results can be adapted to encourage COVID-19 vaccinations, according to Wharton professor Katy Milkman, author of the new book How to Change.

“While the vaccines are different, we face the same behavioral challenges in encouraging uptake of the flu and COVID-19 vaccinations. Rather than convincing the skeptics, we focused on encouraging people who want the vaccine to actually receive it,” said Milkman, BCFG co-director and lead author on both studies, in a statement.

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Among those sitting on the fence is Peter Gold, Philadelphia-based vice chair of the Global Interdependence Center. “Yes, incentives do work. But these should be coupled with other COVID-related benefits,” Gold said. He said he believes in benefits such as free testing, free transportation to vaccination and testing sites, contact tracing programs, and check-ups months after vaccinations.

Other companies are now putting workers’ jobs on the line.

The nation’s largest nursing home operator told workers this week that COVID-19 vaccinations are compulsory if they want to keep their jobs. Genesis Healthcare, headquartered in Kennett Square, gives employees until Aug. 23 to get their first shot.

”To succeed against the delta variant is going to require much higher vaccination rates,” said chief medical officer Richard Feifer. “Our tactics in the fight have to change.”

The mandate at Genesis Healthcare, which has 70,000 employees at nearly 400 nursing homes and senior communities, is similar to by New Jersey Gov. Phil Murphy’s announcement that all workers in elder-care and nursing-home settings must be vaccinated by Sept. 7.

It’s legal for companies to mandate vaccines, with three caveats, said Amy Traub, chair of the BakerHostetler law firm’s labor and employment practice.

The first is a medical exemption, say, an allergy or contraindication to components of the vaccine; second, religious beliefs or observances; and third, city or state law regarding pregnancy or other related medical conditions.

Nationally, 16% of companies are now mandating COVID-19 vaccines for current employees, up from 15% in February; 14% of employers mandate vaccines for new hires only, according to a July survey by the Blank Rome law firm.