(CNN) Covid-19 vaccination record cards can easily be forged, and US law enforcers are keenly aware.
For weeks, the FBI has warned the public against making, selling or encouraging printouts of fake versions of the Centers for Disease Control and Prevention's white record card.
And more than 40 state attorneys general last month warned social media and online shopping platforms to crack down on sales of blank or fraudulently completed cards.
So, what's prompting the warnings?
The Biden administration says the federal government won't require Americans to carry proof of vaccination. Florida has even passed a law prohibiting businesses from requiring proof.
But elsewhere, businesses, venues and events are increasingly offering special access or treats to those who can show this evidence, if not flat-out requiring proof for service.
Many initiatives are underway to create tamper-resistant printed or digital vaccine passports, like one used in New York. Such passports involve checking user-entered data against state or other immunization registries with the user's permission, meaning a user would have pre-verified evidence to show a business.
But at this moment in much of the country, the CDC card might be the only record a business can expect to see on the spot.
As for the inducements: A few sports teams are letting fans with vaccination records sit in vaccinated-only sections, where social distancing is not enforced. Krispy Kreme and White Castle and others are offering freebies.
Airlines are considering vaccine verifications for international travel, as are cruise ships for their voyages. The European Union has said it will eventually let Americans in for nonessential travel again if they are vaccinated.
And some employers are requiring vaccination after the Equal Employment Opportunity Commission said they could if they allow religious and medical exemptions. Some colleges also are requiring inoculation for access to campus.
Not everyone wants to get vaccinated. Combine that with a desire to keep up with rising documentation requirements as society opens up, you begin to see the temptations to fake them.
About 26% of US adults queried in a CNN survey last month said they would not try to get a Covid-19 vaccine.
A different poll, from the Kaiser Family Foundation in April, found 13% of adult respondents saying they definitely wouldn't get a Covid-19 vaccine, and a further 6% saying they would get it only if required.
Crimes associated with making or using fake vaccination record cards include wrongfully using government seals, the FBI said in its warning.
That's because fake cards often use the CDC and Health and Human Services seals seen on the real ones. That's punishable by up to five years in prison and a fine.
In Northern California, a bar owner was arrested Tuesday on suspicion of selling fake Covid-19 vaccination cards.
He was charged with felonies including forgery of a government seal and identity theft of Pfizer, CVS and the CDC, a district attorney's office said.
For employees required to get vaccinated to work, those that are unvaccinated might stand to lose their jobs.
But in other situations -- such as an unvaccinated worker being caught using fake documentation to voluntarily reenter the workplace instead of working from home, or an unvaccinated college student caught using a fake to be on campus -- it stands to reason that lying could jeopardize a person's status as an employee or student.
An unvaccinated person using a fake vaccination card to access a business, venue or workplace set aside for vaccinated people presents at least some risk to both groups, said Dr. Michael Mina, assistant professor of epidemiology and immunology at the Harvard T.H. School of Public Health.
-- Risk to vaccinated people: In general, risk to vaccinated people would be low in many venues. There are a few key caveats.
First, some places requiring vaccination may be relaxed on social distancing. So, a vaccine card faker who is infected could have an up-close chance to pass on the virus. Covid-19 vaccines are very effective and the chances of a vaccinated person becoming sick are low, but the vaccines are not 100% effective in preventing infection.
Second, risks could rise for vaccinated elderly people as time advances, because generally, the immune systems of the elderly tend to be worse at retaining protective immunity, Mina said.
No one is yet sure how long Covid-19 immunity will last across age groups. But Mina said the following scenario could be dangerous: An unvaccinated person using a fake vaccination record to access or work at a nursing home. "Even if everyone else in that place is vaccinated, there may be vulnerable people where immunity has waned or they never took to (the vaccine) well," Mina said.
-- Risk to the unvaccinated person: If all other attendees were vaccinated, the chances of one unvaccinated faker picking up the disease would be low.
But again, vaccination is no guarantee that the inoculated person can't carry small amounts of the virus. And we're talking again about a venue where people might not be socially distancing.
"We have seen plenty of papers now ... that suggest that even as a vaccinated person, you can still have the virus grow in you" to a small degree, and not enough to make the person sick, Mina said.
"If I was not vaccinated, I would not want to be next to someone who is vaccinated and positive," Mina said.
For both vaccinated and unvaccinated people, risks increase if more than one faker shows up.
"If there are too many people doing that, there are no safe places anymore," Mina said.
More than 20 initiatives -- including those driven by tech companies, health care providers and other businesses -- are working on vaccine passport systems, said Mary Beth Kurilo, senior director of health informatics for the American Immunization Registry System.
"I think a lot of these initiatives are just getting off the ground and still in development, and I think we're all learning in real-time about them as they come online," she said.
One group, the Vaccination Credential Initiative -- which includes IBM, Microsoft, Salesforce, Oracle, Mayo Clinic and the Commons Project, a non-profit with a vaccine passport app working with some airlines -- is playing a role in developing US standards for digital health passes, including its approach to data privacy. Members of the non-profit will be required to not collect or store user data.
True CDC Covid-19 vaccination record cards still matter now -- they're the one consistent record showing someone has had a vaccine, Kurilo said.
And because not everyone has cell phones, those cards -- or other offline methods, such as ensuring that printed, pre-verified vaccination passports are available -- will matter in the future, to ensure equity, she said.