Criminal organizations and individuals claiming access to Covid-19 vaccines have contacted authorities in dozens of countries hoping they will sign illegitimate contracts for millions of dollars, according to documents and people familiar with the attempts.

Countries whose national, regional or tribal governments were approached include the Netherlands, Latvia, France, Israel, the Czech Republic, Austria, Argentina, Colombia, Brazil, Canada and Spain, according to the people familiar with the matter.

Some of the governments were offered either vaccines from Pfizer Inc. and partner BioNTech SE, Johnson & Johnson as well as AstraZeneca PLC, according to the people and copies of illegitimate contracts and letters of intent reviewed by The Wall Street Journal. Some negotiated detailed contracts, before calling off the talks and realizing the offers were scams, the people said.

France doesn’t negotiate directly with distributors of vaccines but gets them through the European Commission, a spokeswoman for French President Emmanuel Macron said. Brazil didn’t end up buying vaccines that were offered by third-party brokers, President Jair Bolsonaro’s office said, although it confirmed there were talks. Latvia’s vaccines are purchased within the contracts signed by the European Commission and vaccine manufacturers, said a country representative.

Offers from people and companies to Austrian public agencies or the chancellery that appeared to be serious turned out to be dubious and were referred to authorities, an Austrian government spokesman said. Colombia received proposals from vendors who later dropped them after being pressed by the government to prove they were working with drugmakers, according to Germán Escobar, chief of staff for the Health Ministry. He said the country ultimately negotiated contracts directly with manufacturers. Meanwhile, Argentina only signed contracts directly with vaccine makers, a government spokesman said.

Investigators at the U.S. Department of Homeland Security have identified between 50 and 75 scammers, businesses and brokers looking for prospective governments willing to buy, the people said.

It couldn’t be determined if any offers have been successful, and the activity is believed to be happening in developing nations where Covid-19 vaccine supply remains limited, according to the people. Some countries have completed investigations, although others continue, according to Pfizer and government officials familiar with the investigations.

Many governments are desperate for vaccines because of limited supply. International law-enforcement officials and a security expert said they are concerned that limited vaccine supply in some countries could prompt government officials to unwittingly sign bogus contracts.

U.S. Immigration and Customs Enforcement, through Homeland Security Investigations’ National Intellectual Property Rights Coordination Center, and its partnerships with pharmaceutical companies, has more than a dozen investigations into vaccine contract-related crimes and is investigating vaccine-contract fraud in the U.S. and overseas, a spokeswoman said.

Lining up for Covid-19 shots last month in Brazil’s Rio de Janeiro state. Brazil says it didn’t end up buying vaccines offered by third-party brokers.

Photo: Maria Magdalena Arrellaga/Bloomberg News

Interpol, the international police organization, is planning to issue an alert in the coming days to governments warning them about potential scams, in hopes of soliciting information to aid ongoing investigations, a spokeswoman said.

Vaccine makers said they still only sell their vaccines directly to governments, so there are no intermediaries public officials should be working through. European Union member governments and regional or local authorities cannot purchase vaccines directly from any supplier.

Pfizer, which developed a vaccine with BioNTech, is aware of 86 fraudulent offers to governments in 45 countries involving the shot, said Lev Kubiak, head of Pfizer’s security. The company shares information on fraud it gleans with law enforcement and governments, as well as rival vaccine makers, he said.

As the Delta variant sweeps the globe, scientists are learning more about why new versions of the coronavirus spread faster, and what this could mean for vaccine efforts. The spike protein, which gives the virus its unmistakable shape, may hold the key. Illustration: Nick Collingwood/WSJ The Wall Street Journal Interactive Edition

“We’re worried that eventually, at one point or another, some government may become fooled,” he said. “Some of those countries that are most desperate are at greatest risk.”

A Johnson & Johnson spokesman referred to an April statement on its website that says it is aware of fraudulent campaigns that offer its vaccine for sale, and said no private companies are authorized to sell or distribute the shot on behalf of the drugmaker.

An AstraZeneca spokeswoman said that there is no private-sector supply, sale or distribution of the company’s vaccine and that if someone offers private vaccines, they are likely counterfeit, so should be refused and reported to local health authorities.

After Covid-19 vaccine distribution began, groups and individuals fraudulently claiming to possess or have access to shots started reaching out to government officials with bogus offers of doses, the people familiar with the efforts said.

Boxes containing Pfizer-BioNTech vaccine doses in Spain earlier this year.

Photo: Jon Nazca/Reuters

In late April, the U.S. Federal Bureau of Investigation sent an advisory to healthcare providers and companies warning them of attempts by criminal organizations posing as legitimate vaccine makers and third-party intermediaries offering the shots. The advisory said these criminals claimed they procure millions of doses and asked for half the purchase to be paid upfront through wire transfers to overseas bank accounts.

Israeli police investigated several cases that the country’s Ministry of Health was involved in, said Ronny Berkovitz, director, division of enforcement and inspection, at Israel’s Ministry of Health. He declined to elaborate.

In the Netherlands, the Ministry of Health, Welfare and Sport confirmed it received fraudulent offers but didn’t accept them, and has completed contracts through the European Commission, a government spokesman said. One offer came by mail on Feb. 21, offering five million vials of the Pfizer-BioNTech vaccine, the spokesman said.

Canada’s national police issued a bulletin in February warning that officials at different levels of government across the country had received unsolicited offers of Covid-19 vaccines for sale. Police said the emailed offers could be attempted frauds, offers to sell stolen vaccine doses or attempts to test or breach security.

The Canadian government’s indigenous-services department said it received a request in February from a small community called the James Smith Cree Nation, located in the prairie province of Saskatchewan, about purchasing AstraZeneca vaccines through a source outside the federal government. A spokeswoman for the department said officials followed up with vaccine manufacturers, “who confirmed that these offers are not legitimate.” The department contacted police given concern about the legitimacy of the offers, the spokeswoman said.

Wally Burns, chief of the James Smith Cree Nation, couldn’t be reached for comment. Chief Burns told the Saskatoon StarPhoenix newspaper in March that he believed the offer was legitimate and that it could help people in Canada.

The Canadian indigenous-services department issued an information sheet earlier this year that said it was aware of unsolicited offers to sell about 300 million doses of AstraZeneca and other Covid-19 vaccines to indigenous communities, municipal and provincial governments and federal government departments.

The Royal Canadian Mounted Police declined to comment on whether it was investigating fraudulent vaccine offers.

Seniors waited last winter for doses in Padova, Italy. Prosecutors across the country have opened probes after regional authorities reported receiving offers to sell them vaccines.

Photo: Roberto Silvino/Zuma Press

In Italy, regional governments carried out vaccination campaigns. In February, authorities of the northern region of Emilia-Romagna said they received a number of offers from individuals and companies saying they could import millions of AstraZeneca shots. In the same month, the northeastern Veneto region, where Venice is located, said it received offers for the Pfizer-BioNTech vaccine, including one for 15 million doses and another for 12 million doses.

Across Italy, many prosecutors opened investigations after regional authorities disclosed that they had received the offers, said a person familiar with the Italian investigations. A few of the probes are continuing, while some were folded, the person said.

Several letters of intent sent to government agencies in Argentina, Austria and elsewhere and viewed by the Journal show that offers have ranged from one million doses to at least 20 million doses. They provide step-by-step instructions on how countries should put funds in escrow accounts and say they would be able to verify authenticity directly with vaccine manufacturers.

According to a letter reviewed by the Journal and sent to an unidentified government agency in March, the agency was offered 20 million Pfizer-BioNTech doses for $751 million, with the letter outlining the steps to the apparent purchaser and mentioning a phone call with the letter’s recipient. “We are all aware of the failure of the direct route to the manufacturers via central governments for immediate procurement,” the letter stated.

Covid-19 vaccine fraud schemes have become more sophisticated compared with earlier in the pandemic, when it was easier to identify mistakes in illegitimate contracts, said Mr. Kubiak of Pfizer.

Scammers are now using company executives’ names on phony contracts and, in some cases, saying they are working with legitimate companies in logistics and the import-export businesses, Mr. Kubiak said.

He said legitimate companies have approached Pfizer asking to help with a country’s supposed vaccine deal only to be told by the drugmaker about fraudulent offers. He also said Pfizer learned of a phony offer only after hearing from a country’s embassy that had been contacted by that country’s health ministry, which he declined to name.

—James Marson contributed to this article.

Write to Jared S. Hopkins at jared.hopkins@wsj.com, Kim Mackrael at kim.mackrael@wsj.com and Giovanni Legorano at giovanni.legorano@wsj.com