As the delta variant of the coronavirus sweeps across the United States, a growing number of colleges and universities are requiring proof of COVID-19 vaccination for students to attend in-person classes. But the mandatory requirement has opened the door for those opposed to getting the vaccine to cheat the system, according to interviews with students, education and law enforcement officials.

Both faculty and students at dozens of schools interviewed by The Associated Press say they are concerned about how easy it is to get fake vaccine cards.

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Across the internet, a cottage industry has sprung up to accommodate people who say they won’t get vaccinated for either personal or religious reasons.

An Instagram account with the username "vaccinationcards" sells laminated COVID-19 vaccination cards for $25 each.

A user on the encrypted messaging app, Telegram, offers "COVID-19 Vaccine Cards Certificates," for as much as $200 apiece. "This is our own way of saving as many people as we possibly can from the poisonous vaccine," reads the seller’s message, viewed by at least 11,000 app users.

An increasing number of inquiries to these sites and similar ones appear to be from those who are trying to get fake vaccination cards for college.

A Reddit user commented on a thread about falsifying COVID-19 vaccination cards, saying, in part, "I need one, too, for college. I refuse to be a guinea pig."

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On Twitter, one user with more than 70,000 followers tweeted, "My daughter bought 2 fake ID’s online for $50 while in college. Shipped from China. Anyone have the link for vaccine cards?"

According to a tally by The Chronicle of Higher Education, at least 664 colleges and universities now require proof of COVID-19 inoculations. The process to confirm vaccination at many schools can be as simple as uploading a picture of the vaccine card to the student’s portal.

In Nashville, Vanderbilt University places a hold on a student’s course registration until their vaccine record has been verified unless they have an approved medical accommodation or religious exemption.

The University of Michigan says it has a system in place to confirm employee and student vaccinations. A spokesman for the college told the AP the school has not encountered any problems so far with students forging their COVID-19 vaccination record cards.

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But Benjamin Mason Meier, a global health policy professor at the University of North Carolina at Chapel Hill, questions how institutions can verify those records.

"The United States, unlike most countries which have electronic systems in place, is basing its vaccination on a flimsy paper card," he said.

Meier tweeted last week that he spoke with several students who were worried about the accessibility of fraudulent vaccine cards and that they knew a fellow student who had submitted one to the university.

"There needs to be policies in place for accountability to make sure that every student is operating in the collective interest of the entire campus," he said.

In a statement to the AP, UNC said the institution conducts periodic verification of documents and that lying about vaccination status or falsifying documents is a violation of the university’s COVID-19 community standards and may result in disciplinary action.

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"It’s important to note that UNC-Chapel Hill has not found any instances of a student uploading a fake vaccine card. Those claims are simply hearsay at this point," the school said.

But other university staff and faculty have expressed their concern over the alleged forgery of vaccine cards. Rebecca Williams, a research associate at UNC's Lineberger Comprehensive Cancer Center and Center for Health Promotion and Disease Prevention, said while she is concerned by these claims, she isn’t surprised.

"This is why I think the development of a reliable national digital vaccine passport app is very important for the sake of all the organizations and businesses that want to require proof of vaccination for employees, students, or business patrons," Williams said.

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The AP spoke with several students across the country who did not want to be identified but said they were also aware of attempts to obtain fake cards.

Some school officials do acknowledge that it’s impossible to have a foolproof system.

"As with anything that potentially requires a certification, there is the possibility for an individual to falsify documentation," said Michael Uhlenkamp, a spokesman for the chancellor's office at California State University. The school system, which is the largest in the nation, oversees about 486,000 students each year on 23 campuses.