The deputy director of the Pan American Health Organisation (PAHO), Jarbas Barbosa, warned on Wednesday that fake Covid-19 vaccines were being sold in Argentina, Brazil and Mexico.
Barbosa warned it was a major "problem" for health and police authorities across Latin America.
Quizzed about a report by The Wall Street Journal reporting on the sale of fraudulent vaccines from a Pfizer laboratory in Mexico, Barbosa said: "Yes, we have received information from Mexico, Argentina and Brazil that some doses have been offered through social networks."
"Illegal markets offer vaccines that are probably counterfeit, they are not the real vaccine, or perhaps they are being stolen from a health cenrer and no-one can ensure that they were stored correctly," Barbosa said at a press conference.
"So it is clearly a problem, not only for the health authorities, but also for the police, to identify this criminal activity," he said.
Barbosa insisted that only vaccines administered by trusted health authorities should be used because only these are guaranteed to be "safe and effective" and to have been preserved in proper conditions.
"Therefore, it is very important to refuse to buy any type of vaccine that is offered through social networks and the Internet," he urged. "This is a risk to your health."
The Wall Street Journal reported Wednesday that Pfizer is investigating with the Mexican authorities reports that false vaccines were being sold.
The newspaper noted that the United States, Mexico and other countries have detected "dozens of websites" that fraudulently claim to sell doses of Pfizer's jab and another from the US pharmaceutical firm Moderna.